928 Engine FAQ (Frequently Asked Questions)

compiled by www.rennlist.com

from its archives database

SECTION 1: Time Belt
Maintenance
Replacement
Won't Turn
Belt Tensioning
Tension Light 
Tension Light II
Adjusting Sensor
Harmonic Balancer
Slack Timing Belt

SECTION 2:Water Pump
Water Pumps

SECTION 3:Motor Mounts
Inspection
Replacement
Replacement II

SECTION 4:Coolant
Leaks & Flushing
Recommendations
Coolant Leak
Coolant Level Sensor

SECTION 5: Heater Valve
Replacement

Section 6: Thermostat
Operating Temperature
Thermostat Overview
Do I need one?

SECTION 7: Torque Converter
Explanation

SECTION 8: Idle
Adjusting Idle

SECTION 9: Back Firing

SECTION 10: Oil
Oil Changes

Section 11: Miscellaneous
Motorvac
Ramblings


SECTION 1: Time Belt
Maintenance
Replacement
Won't Turn
Belt Tensioning
Tension Light 
Tension Light II
Adjusting Sensor
Harmonic Balancer
Slack Timing Belt

Timing Belt-Maintenance

The 928 has a timing belt that should be changed along with the water Pump every 50K miles.  The 16 valve engines pre -1985 are non interference engines and will not damage the engine if the belt breaks.  I have never seen one really break, but the teeth do get chewed up causing the belt to slip accomplishing the same end result.  The 1985 and up engines were 32 Valve engines and are interference engines and will bend the valves if a timing belt slips of breaks.  The 32 valve engines have a warning light that senses changes in tension and works pretty well to detect a belt that is beginning to stretch.  Some mechanics not understanding the system will over ride the belt warning light by grounding it. - with very bad outcomes.

Overall the belt is a very reliable system and if proper maintenance is followed works great.  But unlike a chain the belt is affected by age.  An old belt will fail even with very low mileage.  So I would suggest a new belt every 5 to 7 years regardless of mileage.  Changing the belt, pulleys and water pump runs around a grand in a typical shop.

As far as the double clutch it is a very reliable system and actually better than the newer S4 single disc system.  The DEVEK White Car runs a dual disc system behind high owered engine as a the preferred set up.  The dual disc system should hold up for 100K of normal spirited driving with no problems.  You will have more problems sooner with the clutch hydraulic system than The actual clutch.

The transmission was changed in 1985 to a Borg Warner.  This transmission is much smoother and does not experience premature syncro wear.  The disadvantage is the high differential ratio that was imposed to get better gas mileage to meet federal specs.  The older Porsche transmission is still a good transmission and should go 100+K with no problems if it is not abused.

Myself, I am installing an 84 transmission in my 86 car as a conversion so I can get the lower gear ratios and the better Limited Slip Differential.

I hope this helps clear up some of the urban myths that are circulating out there.  The 944 is a very reliable and fun car to drive, but it is not in the class of a 928.  The 928 is a world class car that still ranks very close to one the best production performance cars ever.  For power and handling there are very few competitors.

Dan B

Self Tensioning and Re-Tensioning

In a question dated: 5/27/99

I am in the process of routine maintence on my 1981 928 5 speed. Speedo upgade from 85mph unit, A/C service and conversion, alignment, etc. The timing belt was replaced last July, (about 3000 miles ago) before I purchased the car. I asked the service shop to re-tension the timing belt. Much to my surprise, I was told the belt was "self tensioning" in the 1981 and would not require re-tensioning. This does not seem correct based on the many posts to the list. I am concerned as this seems to be the time to service. The shop is an independant but extremely well respected, selling pristine used porsche products and with a porsche car racing prep program. Shop manuals seem to support the need to re-tension. Suggestions? Advice?

Also, I am advised Porsche has contacted all present and past dealers to identify and acquire 928 parts. I was in the process of tracing down 928 parts not purchased by the new dealer, when a dealership changed ownership last year only to be advised that the new dealer had been pushed by Posche to go back to the former dealer acquire the entire lot. Any one else hearing the same?

Jon Deily
1981 928 5 speed
Pacificblaumetallic

Response(s)

In a response dated: 5/27/99

The belts are NOT self tensioning. There is a belt tensioner in the belt system, but it only accounts for a small amount of stretching. If never serviced, the belt will eventually fail (lose teeth or part). In either case, the result can be from "just disabled" to "needing new valves".

Merry motoring, Ed.

Replacement

In a question dated: 7/10/99

Should the belt and pump ALWAYS be changed together, even if the car is tight? (I understand the cost savings in labor by having them both done at the same time, just wondering from a mechanical point of view)

Is there a way a mechanic can actually tell if a timing belt needs to be replaced or is it always based on the warning light/age? (Especially if the age of the current belt is not documented)

Thanks,

Dave
86 928S

In a response dated: 7/10/99

Dear Dave:
Very good questions. The water pump doe not have to be replaced when the Tbelt is replaced, but it's common practice to do it since it takes only a little more time and money to do the pump too. In an ideal world, the pump should last a least two belt changes.

If you do not know the belts history, it's best to be safe than sorry. A relatively new belt may still have a film on it that looks like talc. IMO, when in doubt - rip it out, and put in a new one.

Merry motoring, Ed.

Timing Belt Won't Turn

In a question dated: 9/15/99

I recently had the timing belt and water pump replaced on my '79 at the shop. They also replaced a couple pulleys. I was 300 miles to go before the scheduled belt retension when yesterday while driving, the car made a brief whirring sound and died. All systems at the time were normal, including temperature and oil pressure.

Towed to the shop and their initial diagnosis indicated that the crank was turning on start but the timing belt is not moving. The belt teeth at the crank are sheared off, preventing it from turning the cams. Further investigation reveals no siezing of the oil pump, water pump or any thing else for that matter - whew!

The shop called Mark Anderson who indicated he has not encountered such a situation before. They are now in the process of installing a new belt.

Could it have been just a defective belt or possibly something else?

All suggestions welcome.

Randy Venier
San Diego, CA

In a follow-up dated: 9/17/99

One of the things I suspected was that a later MY rounded-tooth style belt had been inadvertently installed last time, but turned out not to be the case.

Apparently the belt had weakened at a bend in it during a probable lengthy storage at the Porsche parts warehouse in Reno. I guess sitting in a hot dry place for years will take its toll on a drive belt. A hard acceleration caused the crank teeth to strip the four inch weakened section down to the belt's metal core, preventing the belt from turning since there was now nothing to grab onto
the crank.

Total cost to me for the replacement - $0.00 without any hassles at all. I'm back on the road in time for the weekend.

Another reason why I recommend Dieter's to anyone in San Diego.

Randy Venier
San Diego, CA

Belt Tensioner

In a question dated: 10/4/99

Bill asked -
What is the reason for the seemingly hyper complex belt tensioner on
our car? 

In a response dated: 10/4/99

The belt on the 928 is reputedly the longest toothed belt ever used on a production auto. It drives either two or four overhead camshafts, the oil pump and the water pump, and several other pieces of mechanism. We are not noted for taking it easy on our engines, so it gets to see rapid acceleration, lots of gearshifts, and high RPM operation.

The rather conservative Porsche recommendations say to replace the belt every 60K miles or every five years. Few of us actually drive our 928s 12K miles per year, so we don't get 60 K miles before the belt should be replaced. Unfortunately, this was/is often ignored, so some belts have broken due to age.

Actually, the tensioner might be considered to be too simple. While the bimetallic belville springs (washers) compensate for temperature changes, there is no automatic adjustment for wear and stretching, other than a simple spring. The oil, piston, valves, etc., in the tensioner are apparently there only to damp fluctuations in the belt, and to help compensate for temperature changes, as the oil is not pressurized. It would have been easy to incorporate a take-up mechanism using engine oil pressure and a ratchet mechanism. But, if the tension were automatically adjusted, very, very few owners would ever have the belt checked, much less replaced. By requiring a periodic manual adjustment, triggered by the tension warning lamp, Porsche helped to ensure that the belt would be visually examined every couple of years.

The early belt replacement intervals are partially a result of using mileage as a replacement determinate and ignoring time, and partially a result of the fear factor due to the high cost of failure on the later cars.

The water pump replacement recommendation is often made because the replacement labor is essentially identical to that of the toothed belt. The water pump is not more prone to failure than those on any other make.

Bottom line: I believe that the belt system, including the tensioner, works well, and is reliable. If the belt is properly tensioned every couple of years, and replaced every four or five years, and the tensioner is properly serviced annually, you are not at all likely to have a problem.

Wally Plumley
928 Specialists

Trouble Shooting Tension Light

In a question dated: 4/23/00

John K wrote:
Could you elaborate on why adding oil is one step in trouble shooting the tension light?

In a response dated: 4/24/00

There is a lot of variation in loading on the timing belt, which leads to flutter and jerking of the belt. This jerking and fluttering can cause the "switch" in the tensioner to lose ground contact, sending a signal to the warning system computer that the belt is loose. The tensioner oil appears to serve two main functions - damping these fluctuations, and transferring heat from the engine block to the bi-metallic washers that serve as the spring in the tensioner so that they compensate for engine expansion due to heat. Failure of either of these functions could lead to false belt tension warnings.

To fill the tensioner on a later car with two nipples: Get a trigger-type pump oil can. Fill it with 90 wt oil. (One tech sheet from Porsche says to use engine oil, but I prefer to go with the heavier 90 wt called out in the shop manual - I think that it will give better dampening.) Get a foot-long piece of clear plastic tubing that fits on both the spout of the oil can and the nipple of the tensioner. Remove the rubber caps and open both bleed nipples.  Attach the tube and oil can to the inner-most (upper) nipple. Pump oil slowly (that's the only way it will go in!) into the tensioner until you get a solid stream from the outer-most (lower) nipple.  Close both nipples and re-install the rubber caps. Check the engine below the tensioner for oil leakage for the next week or two.

If you add oil to the tensioner and it promptly leaks out, you need a tensioner rebuild. The rubber cap gets hard and brittle with heat and age.

Wally Plumley
928 Specialists

Tension Light II
 
Oil Pans and Tension Sensors

In a message dated: 7/17/99

It is normal (but not desirable!) for the oil pan bolts on a 928 engine to loosen, allowing oil to leak. These small bolts can and should be snugged up with a 10mm socket and/or wrench to stop the oil leaks. If you overtighten them, you will make the leaks worse by crushing the gaskets and possibly even squeezing it out of the joint. Proper torque is "thumb and one finger on a 1/4" ratchet".

The belt tension sensor on the later engines is a very simple switch. As long as there is enough belt tension to keep a stout coil spring on the tensioner compressed, the switch maintains a ground connection. When the spring is allowed to relax, the switch loses ground, and the warning control unit turns on the warning light. If you are getting a warning light and want to check your system, there is a quick and easy test. Find the wire that plugs into the timing cover right behind the oil dipstick. Unplug it and crank the engine - the timing light should come on within seconds. Shut the engine off, and connect the wire to a good engine ground with a small jumper wire. Crank the engine. If the light comes on, there is an open in the wire between the connector and the warning control unit, or the warning control unit is bad. If the light doesn't come on, either there is a loose connection at the connector or the connector just inside the timing cover, or you have a real problem with belt tension.

Warning! Do NOT attempt to cure a belt tension problem by simply tightening the belt! If the problem is a loose sensor connection, or a bad tensioner boot, or an empty tensioner, you can over-tighten the belt. This will destroy your pulleys, guides and idlers! Use the tension gauge!

There will be a document available for 928 Owners Club members that details how to replace the timing belt, rebuild the tensioner and adjust the tension Real Soon Now.

Wally Plumley
928 Specialists

Adjusting Timing Belt Sensor

In a message dated:

Look just behind the oil dipstick tube on the front of the timing belt 
cover, and find a single electrical wire that goes into a small round plug. This is the belt tension warning light switch. Unplug it, and make sure that the loose wire can NOT get in the accessory belts. Crank the engine. Within a couple of minutes, the belt tension warning light should come on. If it doesn't, someone has shorted the circuit to ground to bypass the warning system, or has otherwise killed the warning system. If the warning light does come on, cut the engine off, and reconnect the wire. Recrank the engine, and see if the warning light comes on.

Your buddy's warning light may have come on because there was no oil in the tensioner, etc.

If you have joined the 928 Owners Club, there is an article on the web site that shows the tensioner and the switch.

Wally Plumley
928 Specialists

or visit John's Pirtle Membersite for a step by step write-up....
http://members.rennlist.com/pirtle/tbelt.html.

Removal of Harmonic Balancer

In a message dated: 11/18/00

I'm doing a timming belt change and can't get the harmonic balancer off. Yes I have taken the 27MM bolt out of the crank and the pullys off.  They were also tough to get off and I noted a little bit of rust on the crank. The big three have been very helpful and do not recommend a puller since it may ruin the balancer (the book calls it a vibration dampner). I'm considering using a puller and if it damages it I will have to replaceit.

Does anyone have any other suggestions? 

Thanks,
SWS
87 Gurards red/black Gemballa mods
preventative maintenance weekend

In a response dated: 11/18/00

Mine slid right off. So I am guessing you are right. I would start with a lot of wd-40 or the like and soak it good for a couple of hours, lots of tapping with a hammer in all directions. I bet that at that point it won't take much of a tug with a puller to remove it. I do remember something about the damper and it's dislike for pullers a while back so be careful and don't pull too hard.

Mark Grasser
1978 928 5-speed
Guards red/black/tan sheepskins

In a message dated: 11/18/00

If it doesn't bother you to pay $300 for a used one or $700 for a new one, then go for it.

I, on the other hand, would recommend spraying penetrating oil on it,
and letting it soak for a few days, and then trying to work it off of there,
without a puller.

wk

In a response dated: 11/18/00

The outer ring of the damper is a press fit on the rubber. Put a puller on the outer ring and you are likely to just pull it off the rubber ($300 used,
$700 new)

-Garey Casey

In a response dated: 11/18/00

Hey guys, a puller could be used depending upon where one does the pulling. If the puller can reach around and contact the center hub, it would be OK. If I were faced with the problem, I would think about drilling and tapping into some part of the center hub in two places opposite each other. Then pull on the bolts. 

The penetrating oil is a good thing to try of course. I would also consider heating it up a bit with a rosebud torch. Not hot enough to hurt the rubber, but just hot enought to get the different metals to move around a bit.

Guess I'm lucky. Mine just pulled right off with the pullers that I was borned wif.
--jer
83sat

In a response dated: 11/18/00

I haven't really done the thing on the damper myself, but one thing that I have found works pretty well in these circumstances is to loosen the bolt,
but not very much - maybe one turn loose. Then pound away on the end of the bolt. The crank doesn't care and the damper just might work itself out. Then loosen it another turn and keep doing it until it comes loose. 
Might mangle the bolt, but the damper will come off - with a little luck.

-Garey Casey

In an updated dated: 11/26/00

Couple of weeks back I asked the list for suggestions removing my
vibration dampner (harmonic balancer). I did get it off by prying from behind, a little help from a puller, lots of WD 40 and patience. I was very careful with the puller since several suggested a puller wasn't recommeded. But the crank had some rust keeping the balancer from sliding off. I cleaned the crank and balancer up and applied a light coat of grease. Everything went back together fine.

Oh and by the way, my tensioner needed rebuilding. The dust boot
crumbled off and it had very little oil in it. If your having your belt
tightened or changed, make sure the tensioner is checked for oil (75-90 gear lube)and that it is working properly.

With this list and the "BIG 3" my car is much more dependable and enjoyable. Thanks again!

SWS
87' 5sp Red/black Gemballa

Slack Timing Belt

In a question dated: 11/21/00

Well, I had more time to pull the covers and got the belt off and all the  belt drive gears. Two things: first the timing was MALIGNED!! The two cam marks were on target, but the crank was lined up on 10 BTD. I don't know if it skipped or was put that was by the last guy. Does anyone know what this would have done to the engine? Also the belt itself was 3-4 mm narrower than new as it seems it's been getting shaved off one side. There are little bits  of belt, mostly in the form of black powder, all over the lower engine area.  Is this normal or did someone previous not do something right?

BTW, Audi uses ground at switches as well in many of their older cars.

Thanks guys,
John

In a response dated: 11/21/00

John,

No, it is NOT normal for the belt to shave itself thinner. Consider yourself fortunate to have caught this now! Find out why it was misalligned / rubbing - look for bent roller guides, etc. I suspect that is a different issue than the10 degree timing issue, but perhaps not...
Good luck!

CFM 80 928S

In a response dated: 11/21/00

The timing should not have damaged anything - the belt being worn on the  edge can be caused by the two cup shaped washers on the crank gear being put on with the curved part toward the belt . Or if the plastic tensioner arm bushings have failed it allows the belt to run off to one side. This condition worn plastic bushings would also ground the tensioner warning light, making that warning system inoperative.

-Jim Bailey

In a response dated: 11/21/00

John-
I just saw the TB on my car do a quick death a few months ago- You should see the pictures of the black rubber powder! Time for TB replacement surgery! And aside from the Water Pump replacement, have the tensioner rebuilt or replaced. My wrench claimed to do this but there was evidence that it was never taken off and inspected, and lacked any oil!!!
Experienced TBer,
AlexM.

SECTION 2:Water Pump
 
Water Pumps

Waterpump expiring?

In a question dated: 12/28/98

Question

I spent some time today trying to track down the cause of a coolant leak on  the drivers side of the engine. All radiator hoses are intact and leak  free. The source of the coolant is difficult to place but is behind the  cambelt covers and under the exhaust manifold. My first thought was a  leaking water pump, but then I decided to carry on looking for other  symptoms. I found caramel (not the tasty kind) in the bottom of the oil  filler housing. I then checked the radiator overflow tank and found it to  be clean. This looks like a blown head gasket to me unless the waterpump is oil cooled and somehow the leaking pump is allowing oil and coolant to mix.  Has anyone had a similar problem? I have never seen a post re head gasket problems so I assume they are normally trouble free. I know the waterpump  will set me back around $900 for parts and labour, any ideas what a head  gasket will cost? I am hoping the problem is the lesser of the two numbers!
 
 Thanks in advance
 John McDermott
 1982 928 AT

In a response dated: 12/28/99

John, I don't know for sure about the brown stuff, but as the seal on the water pump drive pulley goes, it will start leaking, and that is the symptom I saw when mine went. It also sounded a little noisy at the time too. You might hear the bearing rattling above cyl. 1, where the coolant hose exits the engine. The sludge you refer to might be the stuff washed out from behind the timing belt cover since this is where the coolant trickles down, and grime and oil can accumulate there. A danger you may be facing is if the H2O pump bearing is going, it will effect the timing belt tension, and you may risk a slipped belt and big $$$ valve damage.

A couple of points to consider:

When doing the pump, you're best off to do the timing belt as well, but I also did the front engine seals for a very small additional cost, as well as the tensioner and the tension roller arm. DON'T get talked into a new tensioner or arm, these can be rebuilt for you with a few dollars worth of parts. (I replaced the bearings (I kept the rollers) and seals on these puppies myself for <<$50.) I invested one looong weekend (I did all of the above work myself), and about $250 - I think, and my shark runs as good as new. Be warned, this is not a task for the faint at heart though!!

I hope this helps you get you back on the road sooner and with a few bucks left in your pocket.

Happy Holidays to you and all PorscheListees!!

Rob
84 928S Auto Gold/Tan (For Sale)
86 928S Auto Black/Stripped (For Parts)
87 928S4 Auto Blue/Camel (For Speed)
87 VW Westfalia SYNCRO (Four by Four!!)

 ____________________________

SECTION 3:Motor Mounts
Inspection
Replacement
Replacement II

Motor Mount Inspection

In a question dated: 3/8/99

What specifically should one be looking at/for when inspecting motor mounts. Do they progressively go bad, or are they either good or bad? Also, does any one know of a good brand of stripper, primer, paint to use for the intakes and valve covers without having to use some of the aerospace techniques earlier posted? There's got to be an easier way.
Thanks in advance
Adam
Bailey928@aol.com

In a response dated: 3/8/99

Dear Adam:
928 motor mounts wear out very gradually. Basically, the fluid in them evaporates or leaks out with time and use. This causes the mounts to "settle" and compress. When they are completely worn, they allow the metal engine block to come in direct contact with the metal cross member. As all running engines vibrate, this vibration can be felt throughout the car, and makes for a less than smooth running V8.

There are several ways to tell if the mounts are worn. I think the best way is to measure the amount of free space between the mounts and their safety hooks. If the gap between them is large enough to insert your index finger, the mounts are worn. If the space is too narrow to insert your pinky, they are fine. Anywhere in between, they are wearing. YMMV.

Merry motoring, Ed.

Motor Mount Replacement Procedure I

In a question dated: 2/24/99

My motor mounts need to be replaced, but I could not find it in the workshop manuals. Can someone give me the procedure for replacing them?

In a response dated: 2/24/99

David:

Here's the quick version, leaving the engine in (I've just done it, but the engine was out, so somewhat easier)

1. After raising the car to max height and strategically placing your jack stands, support the engine from above with an engine hoist (you'll need to raise the engine later in the procedure).

2. Remove the sway bar-to-body mounts (leave it connected to the shock mounts), allowing the bar to rotate back and down, out of the way.

3. Remove the power steering lines where they attach to the rack (have drip pan ready).

4. Remove the nuts & bolt (8 & 1) that hold the support plate and rack to the cross member. This allows the rack to droop, being "held" by the steering shaft and the tie rods.

5. Remove the central motor mount nuts (1/side) now exposed on the underside of the main engine bay cross member.

6. Still from underneath, remove the 4 bolts that run thru the "ears" of the mount into the block.

7. This next part has got to be a bitch. I've never done it with the engine in place, but what obviously has to happen is the engine has to be raised enough to allow the center bolt of the mount to clear its hole in the main cross member. As an option, disconnect the cross member from the frame, allowing the motor mounts to be extracted that way? In either case, you need the hoist for this.

So I get ya' this far and leave you hangin'...time for another lister to help complete this saga?

Enjoy!

David Lloyd
Farmington, CT
79 Euro 5-spd, track car with 86 motor mounts which hold the 5.0 motor:)

Motor Mounts Replacement II

In a question dated: 8/9/00

Any hints or tips out there for the do-it-yourselfer/home mechanic?

I've searched the standard areas and all I found was general info and "it was easier to take it to the P-mechanic"

Any BTDT advice?

In a response dated: 8/9/00

Replacing the motor mounts is one of the more frustrating tasks that I have under taken on the 928. It is a hard, dirty and expensive job - but it did make a very noticeable difference in both of my cars. Some reasons for frustration:

The nine-volume shop manual doesn't even mention this job, and doesn't have a good illustration of it.

The area is dirty.

You have to take off several unrelated items to replace the mounts.

Fasteners are very tight and very hard to get to.

You have to jack the engine.

The mounts are expensive - $210.60 each for '82 - up.

Some tips:

Support the car securely at a comfortable working height. You will be reaching up into the engine compartment 8", so allow for that. You will need a jack to support the engine - a small bottle jack takes up less room under the car.

Replacing the mounts is easy once the cross-member is out, basically impossible without it being out. Removing the cross-member requires dropping the steering rack and the inner lower control arm mountings. If you plan to replace the front shocks, now is the time.

1)   Jack the car.

2)   I chose to lower the steering rack rather than removing it. You can leave the hoses connected - don't over-stress them. Remove the steering shaft universal joint. There is one nut on top by the shaft that is a pain to hold. Try sticking a flat-bladed screwdriver beside it to hold it.

3)   I chose to remove the lower control arm mounts. This shouldn't screw up your alignment. Removal is pretty obvious. Be careful of the ball joint  boots and the brake lines - it is easy to damage them by letting the arms swing.

4)   Support the engine - be sure to spread the load with a large wooden piece, so you don't damage the crankcase. Drop the cross-member and mounts.  There are a few other pieces that are in the way, such as ground cables,  etc. Remove what you have to.

5)   Replace everything, but first: Snug - don't over-tighten - all of the oil pan bolts; Clean and check everything that you touch, such as ground   cables, etc.

6)   When you get to the steering universal: Pull the plastic plug found on the front of the rack housing just in front of where the shaft goes in. Look inside the hole, find the dimple in the rack, and center it in the hole. This centers the rack. Install the universal so that the steering wheel is straight with the dimple centered. I have probably forgotten something - this is the type of thing that you try to blot out of your memory.

Have fun!
Wally Plumley
928 Specialists

Response(s)

In a message dated:  8/9/00

I am 1.5 days into a motor mount replacement on a 1983S w/ about 140K miles.

Comments: This job is a lot of work so beware if you are thinking about doing it.

After about 10-12 working hours, I just got the old mounts out.  (Some time was  spent doing shifter cup). I took off the exhaust manifolds (and air pump) to get to them, but in retrospect I am not certain I needed to do that.

I spent a lot of time trying to remove the top mount bolt while it was in the car. This is not necessary; I eventually figured out that the metal brackets came off with the four bolts into the oil pan/block area.

I could not find any reference to this job in my manual---if it is there I did not find it.

Must drop steering rack (you can leave ball joints attached) and  cross member  (bolts here may be very stiff so be prepared).

It also took a lot of time to cut off the nuts holding the mounts to the cross member as mine were too tight and frozen to get off w/wrench (nuts rounded) (I used a chisel; too close to fuel to use torch or I am too chicken).

While I did not have any strong evidence that the mounts were bad, they are after all 17 years old so it seemed time to change them (while I was doing shifter cup and some exhaust work).

The old mounts are close to 1/2 inch shorter than the new ones.

My point: this job takes lots of time and is hard on the knuckles so be prepared before you start. If you are not a professional, even if you have all the tools, you would really have to work very quickly or have done it  many times before to finish the entire job in a weekend. If you are way faster than me, more power to ya.

If you are going to do the mounts, my-as-well replace the shifter cup if it hasn't been done. The plastic in the cup gets brittle with age and breaks, and then pops out--but this is well known.

I have no idea yet whether or not the motor mounts will make a big difference as it is not back together. Given the effort involved, I sure hope it will be noticable.

Phk

SECTION 4:Coolant
Leaks & Flushing
Recommendations
Coolant Leak
Coolant Level Sensor

Coolant Leaks & Flushing

In a question dated: 9/21/00

Hello everyone, I am always greatful that I have this list to ask question on many problems. ( even if I take the car to the mechanic, I won't get screwed) On to my latest problem, driving to work this morning, coolant light came on, but temp was between the two marks. Rolling into the parking lot, temp needle climbed rapidly and and car almost died before I parked it. Openning the hood, coolant everywhere in the engine bay. Steam everywhere.

I just had done t-belt, water pump replacement about 1500 miles ago. Scheduled for belt adjustment this week. Where should I start looking? The coolant light came on yesterday so I addeded some this morning, but  temp was within the limits. Is there any thing that I can do before taking it to the mechanic ? Do I dare to drive it home? ( 15 miles) Ahhhh... I really need a reliable commute car. ( I love my car anyhow)

Thanks much in advance.
Jonathan 86S

In a response dated: 9/12/00

Sounds like two possibilities:
The car is overheating because it lost coolant; or,
The car lost coolant because it overheated.

If you are losing coolant, carefully refill the system while looking for leaks. Unfortunately, completely filling a 928 cooling system is not a trivial task, especially in a public parking lot. Fill, burp by squeezing the top hose, fill, crank and keep checking the level until it is warm. Burp by squeezing the hose (Watch out for the fan!) as you do this. Sometimes, pulling the small vent hose on top of the radiator as you fill helps. If you can get it full, and can't see a leak, and it isn't hot, you might try getting it home or to a shop. Have a cell phone handy! You HAVE TO find the leak!

If the car is overheating, probably not a good idea to risk it. The most likely cause for overheating is the thermostat. You can not remove the thermostat to prevent the car from overheating. Do not drive a 928 without a good thermostat in place.

Wally Plumley
928 Specialists

In a response dated: 9/14/00

the same thing happened to me today... problem? a rip in the hose... for you, well it can be a different story on the location. It is wise to look  at the area in where the leak has come out of. First add water/coolant (fill up!!) Have a friend rev the engine. 2-3k rpm. Watch carefully where the water comes out of. This is the area you would need to see. After water sprays, let the engine cool and try to find where the leak is at. (you might have to disassemble some parts)

Then find the hose and replace. Make sure after you have changed the proper hose, 'burp' the radiator (you can do this throught the coolant resevoir located on the upper left of the hood). Burping means let the water flow out until all the air has come out and all that is coming out is water/coolant.

Let is stay. Then the next morning, when the car is still cool and not turned on, check the water/coolant. Add if necessary.

You can bring this to the repair shop, but it might cause a little more than you expect on the labor.

Good luck!

In a response dated: 9/14/00

Since all of the hoses are basically of the same construction, and are all of the same age - if one goes, the rest won't be far behind. Better to replace all at once. Much cheaper than a new engine, or even a flat-bed to the shop.

Wally

Product Recommendations 
for Flushing Cooling System

In a question dated: 7/28/00

Does anyone have any good suggestions for what to use flush the cooling system and perhaps clean any deposits out? I've heard of using automatic dishwasher soap due to it's low "sudsing".
Just curious.
- David Moody Jr.
'78 5sp
'81 AT

In a response dated: 7/28/00

Mercedes sells a product to do this - Citric Acid Cleaner - part number. A 000 989 10 25. Here are some instructions that were posted on the MB list:

Sokoloff - Len <sokoloff@umd5.umd.edu> wrote:
<Get the coolant out of there and rinse it a few times. Then add some liquid Tide laundry detergent and run it up to temperature to deoil the system. You're supposed to deoil before you use the citric acid whether you need it or not. Rinse the system well a few times. Then dissolve the citric acid in warm water in a bucket and pour it in. Fill the system again, run it up to temperature and rinse very well. Then add your new
coolant and you're good to go for three years. Might consider changing hoses and thermostat now as needed.>

--
Jim Stadter
'83 928 S 5-speed (U.S. spec)
'88 928 S4 5-speed
Lone Star Region PCA (Austin)
928 Owners Club (http://www.928oc.org)

In a response date: 7/28/00

I think that I would prefer to stick to a known brand-name product that has been around for awhile, such as the Prestone cooling system flushes.

Flushing a cooling system at home is a pain in the butt (and after my morning's experience with a flexible sigmoidoscopy, I am a damn expert on those! ), especially if you don't have a way to dispose of the old coolant.  It takes a couple of days to do it thoroughly.

Wally

Heater Valve and Hot Smell in Air Vents
Coolant Leak

In a question dated: 3/30/99

I think I have a problem with my heater valve. I am not sure that this is the correct name, but I'll describe my symptoms. I have noticed for the past several months a slow loss of coolant from the cooling system. I occasionally get a low coolant warning message, I add about a quart of antifreeze, and a month or two later it happens again. I do not see puddles of coolant under the car, but I did notice last December when I had the lower engine cover off to install my new Devek front sway bar that there was some coolant on the cover. It was near the rear of the engine and also had some green dry residue which appeared like dried coolant. Last summer I noticed a few small puddles of coolant on the garage floor located at the rear of the engine. I wasn't sure at the time if it was from the overflow tank for the coolant lines.

Just in the last 2 months I have also noticed a hot smell coming through the ventilation system as soon as the car gets warm. The coolant loss rate has also appeared to have increased. I am hoping that this is a problem with the infamous vacuum operated heater valve that has been mentioned on the list. Do these correlate? There has been some recent discussion about bad smells in the vents so I guess I should also check the plastic "tee" to the canister in the passenger wheel well.

If the heater valve is the likely case, what is the price and how easy is this to change? Can I change it without draining the cooling system?

Thanks for any help and assistance.

Regards,

Curtis Eames - 91 GT
Vancouver, WA

In a response dated: 3/30/99

Curtis,

The plastic valve is a simple vacuum operated "on - off" valve that controls the flow of coolant to the heater core.

What you describe is a leak. If you can't see much of a leak when the car is running, that means that the fluid is evaporating before it leaks to the ground or somewhere else in the chassis. These type of leaks are difficult  to find. But, if you look for places that involve heat you may find the leak.

Some examples are:

a. radiator core or heater core: a small leak can exist for a long time since all the air flow over the core is a great evaporator.

b. engine "V": in-between the heads is a nice trough that is heated. Anything that dribbles down there will evaporate.

c. Head gasket: water leak into the combustion chamber 

From your description you may have a leak in the "engine V" or heater core.  The smell of evaporating antifreeze is probably strong enough to be sucked  in the air intake or to be noticed from the heater core.

I hope this helps.

Rich
'93 GTS
'83S

Coolant Level Sensor Testing?

In a question dated: 11/11/00

I had an intermittant coolant level light that is no longer intermittant.  It is now on constantly. I have forced as much coolant in there as possible and it is still on. I am trying to figure out if it is the sensor or wiring. I disconnected the sensor and the light stays on. I jumpered  the two wires and the light stays on. I am reading voltage on one of the wires but nothing on the other. I thought I would read ground over there.  I am thinking that it is a simple on/off type switch which means that I  should be able to jumper it to make it a complete circuit. Any clues on this??

Thanks
Mike
'87 928 S4 5 spd

also, has anybody removed one lately? It appears to be a tight fit above the tank.

In a response dated: 11/11/00

There are TWO sensors on that circuit - the level switch that you know about, and a pressure switch that you don't know about. It is buried just under the fender, a few inches for the tank, in the pressure line.

Wally Plumley
928 Specialists

In a response dated: 11/11/00

Mike,

I looked at the wiring diagrams and it is an on / off switch operated by a float. The brown wire goes to the MP5 ground point. You should be able to jumper across the switch to test it. I don't know if open or closed is the
full coolant indication.

The problem could be with the ground contact. Probably at the switch itself rather than the ground point.

Of course the problem could also be with what looks like a control unit.

Ken '86.5 928S

Section 5: Heater Valve
Replacement

Heater Valve and Hot Smell in Air Vents
Heater Valve Replacement

In a question dated: 3/30/99

I think I have a problem with my heater valve. I am not sure that this is the correct name, but I'll describe my symptoms. I have noticed for the past several months a slow loss of coolant from the cooling system. I occasionally get a low coolant warning message, I add about a quart of antifreeze, and a month or two later it happens again. I do not see puddles of coolant under the car, but I did notice last December when I had the lower engine cover off to install my new Devek front sway bar that there was some coolant on the cover. It was near the rear of the engine and also had some green dry residue which appeared like dried coolant. Last summer I noticed a few small puddles of coolant on the garage floor located at the rear of the engine. I wasn't sure at the time if it was from the overflow tank for the coolant lines.

Just in the last 2 months I have also noticed a hot smell coming through the ventilation system as soon as the car gets warm. The coolant loss rate has also appeared to have increased. I am hoping that this is a problem with the infamous vacuum operated heater valve that has been mentioned on the list. Do these correlate? There has been some recent discussion about bad smells in the vents so I guess I should also check the plastic "tee" to the canister in the passenger wheel well.

If the heater valve is the likely case, what is the price and how easy is this to change? Can I change it without draining the cooling system?

Thanks for any help and assistance.

Regards,

Curtis Eames - 91 GT
Vancouver, WA

In a response dated: 3/30/99

Curtis,

Here are some pertinent msgs that I've archived.

--
Jim Stadter
'83 928 S 5-speed (U.S. spec)
'88 928 S4 5-speed
Hill Country Region PCA (Austin)
928 Owners Club (http://www.928oc.com)

>Greg Nichols wrote:

> Message text written by Tom Green
> >Is that 1 cup leaking out AFTER draining the cooling system? Or is
> >it not necessary to drain the system to replace the heater valve?

> Tom,

> The heater valve is high up compared to the rest of the cooling system, so
> there is no need to drain.

> Here's the valve replacement directions, in case you didn't get them
> before:

> 1. Remove air filter. Remove lower air filter housing
> 2. Locate heater bypass valve - right (passenger) side of engine
> compartment, between
> engine and firewall.
> 3. Loosen hose clamps and slide them out of the way. Remove vacuum hose
> from valve actuator
> 4. Place newspapers and catch pan under vehicle and/or position a rag
> underneath the valve
> 5. Remove old valve. Less than one cup of coolant will leak out.
> 6. Insert new valve and reinstall hose clamps and vacuum hose
> 7. Bring engine up to operating temperature and check for leaks. Any
> spilled coolant will be
> evaporated by engine heat.
> 8. Replace air cleaner, and go and enjoy a cool ride on a hot day!

> Tip: Twist hoses to loosen, then carefully use a flat blade screwdriver to
> help slide them off.

> +Greg

Section 6: Thermostat
Operating Temperature
Thermostat Overview
Do I need one?

"Normal" Operating Temperature

In a question 5/26/99

Looking for some general feedback. What is the "normal" operating 
temperature range that I should be running at with regular around town 
driving? I'm tyically seeing 2/3 to 3/4 on the guage.

Rob
'91GT bl/bl

In a response dated: 5/26/99

Rob, 
I run the same. (2/3 to 3/4 guage)
Mike 91GT Black/Black

In a response dated: 5/26/99

Dear Rob:
Now that both fans are operating, the guage reads between 2/3 and 3/4. Back 
when only one fan was working the reading was 3/4 to 4/5 (at the upper 
white mark).
Merry motoring, Ed.

In a response dated: 5/26/99

1/2 plus or minus 1/8 depending on outside air temp...
Ted Childs
87 S4 5sp

Thermostat - Description

In a question dated: 7/25/00

Steve Lewis wrote:

I have drilled the thermo housing with bypass holes( as recommended by several highly experienced Porsche mechanics

If I am not mistaken, the 928 thermostat performs in the opposite manner to "normal" thermostats. It closes when it heats up and lets the engine water go through the radiator. When it is open, some water bypasses the radiator and recircs through the engine. I think your highly experienced Porsche mechanics need some more experience and you need a new thermostat again.

In a response dated: 7/25/00

I posted a couple messages to the list on the thermostat subject in the last year, and one of them pointed out how the operation of the thermostat is different from what one typically sees in automobiles these days. DR mailed me off-list back then (I've reduced the dosage since then, Dave ;-)), letting me know that the way I described the operation wasn't exactly accurate. I'll try again, and maybe some of this will clear up the confusion. The original response way back then was to somebody considering
taking the thermostat out to increase water flow, so the description then was very targeted.

The thermostat in the 928 does two things. If you look inside the water manifold where the thermostat sits, you'll see passages that connect to both radiator hoses and the passages in the block. In normal operation, the thermostat is open to the radiator, and the bypass between the discharge and inlet ports is blocked closed. The thermostat flange that the sealing o-ring wraps around could be drilled to allow coolant flow to the radiator even when the engine is cold, and that sounds like what was done here.

Going to the cold condition, the thermostat normally blocks flow through the radiator, and also allows coolant to bypass between the discharge and inlet ports on the block. This bypass flow eliminates those local hot spots in the block, promoting a nice even warm-up of the block from cold. As the coolant temp comes up, the bypass closes, the radiator outlet opens, and coolant is directed through the radiator. Looking at that function, it would seem that drilling the thermostat would extend the engine's warm-up time if the holes are very large.

Many cars have small holes drilled in that front flange. Primary reason is to allow gas or air bubbles to pass through. My Explorer, for instance, has a very specific instruction to make sure the thermostat vent hole is at the top when installed. If you don't follow that rule, you risk having a gas or steam bubble in the top of the housing where the temp sender is located. With the bubble, gauge readings are false and slow. It's pretty sucky too, because if you lose coolant in that car, you lose accurate gauge reading almost immediately. Anyway, I suppose there might be a case for drilling a vent hole in the 928 thermostat, I just haven't seen the need yet. The system burp procedure following a coolant change is pretty interesting, but once done there doesn't seem to be any problem with bubbles in the water manifold.

I'm open to discussion/criticism of this subject. I just don't want to have my previous partially-correct description propogated as complete. I apologise for causing the confusion.

Best regards,

dr bob

Do I Need A Thermostat?

In a question dated: 11/3/00

I have an 86.5 928S in Orlando and my thermostat is bad. Do I really
need one do to the fact that I am in Florida and the thermostat just holds
fluid in the block for cold starts.

Todd

In a response dated: 11/3/00

Yes.

The 928 thermostat does not work in the same manner as the standard
set-up. You MUST have a thermostat in the engine.

When you install your new thermostat, be sure that the old rubber o-ring 
gasket is full removed from the cavity.

Wally Plumley
928 Specialists

In a response dated: 11/3/00

YES !!!!!! the 928 must have a thermostat or it will overheat !! the
thermostat has two functions one is when it opens it allows circulation
through the radiator . the other is to close the hole in the back of the
housing which allows circulation back into the engine . Run with out a
thermostat and it will overheat badly .

In a response dated: 11/5/00

Some thoughts--

You really --do-- need a thermostat in your 928. As has been previously
mentioned ad mausoleum (same as 'to death'??) the themostat does in fact block recirculating coolanat when the engine is warm.

On to the question of running a thermostat in a conventional car. The misconception about running without and the "water traveling too fast to shed the heat" is just that, a misconception. Faster circulation is one of the factors used in sizing heat exchangers like radiators, and more flow improves the heat transfer.

--BUT-- running your conventional engine without a thermostat will often cause it to overheat. It's just not from the "too fast water" problem often blamed. Consider that the thermostat provides a certain amount of restriction to coolant flow, and that the pressure upstream of the thermostat in the block operates at a pressure higher because of it. Operating with the fluid at a higher pressure cuts down on hot-spot boiling, where the fuid becomes gas momentarily as it passes a particularly hot place in the block. At that point, the heat transfer is diminished significantly as the liquid contact is lost. It becomes a self-feeding problem, as it gets hotter in that area and immediately downstream as the gas pocket grows. Hot spot grows, gas pocket grows, etc. So, adding the restriction, whether it be a thermostat or just a fixed orifice in its place, definitely can contribute to better heat transfer in the block.

Redline makes a coolant supplement called Water Wetter that claims to improve the transfer of heat by cutting down on this gas pockets. They add a little detergent/surfactant to the coolant to help the liquid stick to the walls of the water jackets. Good stuff in my experience.

Back to the guy who was asking about just removing the thermostat--Why not just buy a new one? They aren't expensive (as 928 parts can go...) and the effort to remove the old one is the same as swapping in a new one anyway.

Another thought-- If the car is just running hot, or if it takes more
than ten minutes of running to overheat, you are chasing a problem other than a stuck thermostat. I posted a cooling system diagnostic diatribe a while back, and can repost it to you if necessary. I know John Pirtle has it immortalized on his 928 Info website (Thanks John!)if you want to look for it. Try <http://members.prestige.net/jpirtle/auto/s4/keepers/cooling\overview.txt>
for the full treatment.

Hope this helps!
dr bob

SECTION 7: Torque Converter
Explanation

Torque Converter Explanation

In a message dated: 8/7/99

I had not planned to participate further in the gearing discussion, but was asked to explain what I said about torque converters.

A standard automatic transmission (including the 928 versions) include one or two planetary gearsets, which can be hydraulically manipulated thru externally contracting brake bands and multiplate clutch packs to give reverse and two, three, four, or five forward gear ratios, with one almost invariably being straight thru, and the rest gear reductions that multiply torque at the expense of speed. In the MY86 928 automatic transmission, first gear is 3.6760 to 1. This is a steel-to-steel mechanical ratio, and does not vary in any way at any time. If you are in first gear with this transmission, you get a torque multiplication of
3.6760, and a speed reduction of 3.6760. (We are ignoring friction losses, etc.)

In the MY86 5-speed, first gear has a ratio of 4.0672 to 1. This is a steel-to-steel ratio, and does not vary in any way at any time. (Contrary to an assertion made earlier, changing the rear end ratio does not make the ratios closer.) If you are in first gear with this transmission, you get a torque multiplication of 4.0672, and a speed reduction of 4.0672. (Again, we are ignoring friction losses, etc.)

This would seem to give the 5-speed a decided advantage - but let's look at the rest of the story. Between the engine and the gear train in the automatic is the torque converter. This is a doughnut-shaped steel tube, containing another doughnut that has been split into a front half and a rear half. The front half is driven by the engine, the rear half drives the transmission input shaft. There are complex fins inside both halves, and the whole thing is full of transmission fluid. If that was all there was to it, this would be a fluid coupling, which only allows slip, but does not multiply torque. Since about 1953, however, the coupling has contained a stator, which is a sort of turbine wheel that is inside of the split torus (fancy name for doughnut). The stator redirects fluid flow between the split halves, and by so doing hydraulically multiplies torque, and, since TANSTAAFL, reduces speed.

The multiplication ratio varies with the intended usage, but is typically between 1.8 to 1 and 2.3 to 1. One key point to this is that the torque multiplication varies from this high figure (let's assume 2 to 1) to essentially zero, depending solely upon the load placed upon the output as compared to the torque input. In other words, if you are cruising the interstate at a steady 100kph, there is very little load, virtually no multiplication, and very little slippage. (Some manufacturers add a lock-up clutch to make the whole thing steel-to-steel, but that is outside of our discussion.)

If you open the throttle as much as possible without downshifting, the increased load will cause torque multiplication and speed reduction. The reduction, equivalent to going down a very small gear, isn't noticeable, since the engine is speeding up.

If you are sitting still in first gear, and floor the throttle, the 5-speed will give you 4.0672 to 1 torque multiplication, period. The automatic, however, will give you a mechanical torque multiplication of 3.6760, times a hydraulic torque multiplication of about 2 to 1, for a total torque multiplication of about 7. This total multiplication falls off as the output of the torque converter gets closer to the input, but at full throttle, there is ALWAYS multiplication. You could probably calculate the multiplication at any given point if you could accurately measure the rpm with little or no load, then at full load. The difference is almost all torque multiplication, with a little slippage.

There is another factor involved in getting performance out of an automatic, and that is the stall speed of the torque converter. A higher stall speed allows the engine to rev higher with the car not moving (stalled converter), thus developing more torque and horsepower. It looks to me as if the MY86 torque converter has a stall speed of 1650 to 2050 rpm in the USA. Everybody else in the world has a stall speed of 2200 to 2600 rpm. There is definitely some performance being left on the table there.

And I think that this is all I have to say about that.

Wally Plumley
928 Specialists

SECTION 8: Idle
Adjusting Idle

Adjusting Idle

In a message dated: 1/2/99

Is there a way to adjust the idle on my 83' 928S without having to take
it to the dealer. I have looked for an idle adjustment and can not find one.
Any help would be greatly appreciated
Thanks
Carl

In a response dated: 1/2/99

First, be grateful that you don't have a LH-Jetronic system :-) Look down in front of the throttle body, you'll find a slotted adjustment screw. The head of the screw is about the size of a nickel. Clockwise to decrease and counter-clockwise is to increase. Use a good tachometer to do the adjustment. Do not rely on your instrumentation's tach. They're known to be inaccurate up to as much as 20%.

Some advice: Before you make the adjustment, make sure that you don't have any "false air" leak on the intake system. That is basically any rubber seal that you can get your hands on.

Tightening the intake boots on air plenum would be a very good start. Be forewarned that this action MIGHT cause the A/F ratio to be thrown off, so be prepared to make some A/F mixture adjustment as well.

Another place of false air leak would be the gigantic o-ring located at the throat of the air flow meter assembly. It's cheap to replace. You need to remove the air box in order to access it.

Finally, the vacuum servos buried within the dash/console is another cause of false air. Given time, the diaphragm within the servo will crack and leak. A common symptom of this problem would be inaccurate temp control of the AC and the ventilation system. There was a long thread on this subject awhile back, so you should be able to get more info from the archive. Good luck.

Vince Yu
'78 928
'83 928S

SECTION 9: Back Firing

Backing Firing - Upon Cranking

In a question dated: 9/13/00

Hi listers,

This afternoon I got a frantic call from my wife telling me the "Beast" just blew up! I'll get to the point... upon inspection, the air cleaner box top was blown into a few pieces, the entire induction system (86.5) was separated from virtually every rubber connector, including the throttle body, and the explosion went off with such force, that we now have two "power bulges" in the hood where the 2 metal air boxes came in contact with it. This all occurred upon an attempted cold start. My wife doesn't even remember hearing the engine crank... it just went off upon turning the key. Fortunately there was no fire. Once the car was towed home, I disabled the fuel and ignition systems and the engine does crank normally, so hopefully I have no internal damage. She said the car was running perfectly when she shut it down this morning at work. My guesstimate of the problem at this point is that I may have an injector that stuck open and drained a considerable amount of fuel into its cylinder, which ignited upon my wife attempting to crank the engine. I would appreciate any advice from anyone out there who has had this problem, or knows of it, or even has any theory on what might have caused it. I'll be out working on the car for a little while now... Please respond directly to me as well as on list, as I have been in digest mode lately.Thanks to all who can shed any light on this.

Bill Swift

In a response dated: 9/13/00

I can think of two possibilities:

1) A spark plug fired while the intake valve for that cylinder was open, igniting the mixture in the cylinder and the manifold. This is called "cross-firing", and can be caused by high resistance in the plug that should have fired at that instant combined with either bad plug wires or wires dressed too closely together for too long a distance, allowing induction to do its thing.

2) An intake valve stuck open just enough for the fire in the cylinder to blow back into the manifold and ignite the mixture from all of the injectors. This seems somewhat likely because intake valves do tend to stick open, as the valve stem gets just hot enough to build up deposits, but not hot enough to burn them off. However, it seems less likely because the 32-valve engine is definitely an interference engine, and an intake hanging open should cause serious valve-piston collisions.

There is no source of ignition in the manifold, otherwise the normally-perfect mixture in the manifold would ignite much more often. In addition, more fuel from a stuck injector would make the mixture less likely to explode, not more.

As a pedantic aside, "back-firing" is perfectly correct in this context. An explosion in the exhaust is "after-firing", contrary to common usage that refers to all explosions outside the cylinders as "back-firing".

Wally Plumley
928 Specialists

SECTION 10: Oil
Oil Changes

Oil Change - Why So Frequent

In a question dated: 2/8/99

It seems everyone on the list likes to change the oil every 3,000 miles. This is way under the manufacturers' recommended interval (both Porsche and Oil Companys').

Why? Do we know something they don't know? Are we just risk averse, or do we just like to buy lots of expensive Mobil 1 when we don't have any scientific data to support it?

Dan

In a response dated: 2/9/99

Dear fellow sharksters:
The recommended drain interval specified in the owners manual has a disclaimer. If you drive long enough to raise the oil temperature above 100 degrees Celsius (212 degrees Farenheit) and without having to do much "stop and go", and the air is always clean, and the ambient temperature does not fluctuate very much, then you are one of the rare individuals that does not need to worry about the disclaimer.

The interval should be more frequent and at fewer miles or kilometers if the car is driven in harsh conditions. Those harsh conditions could be anything from; Driving in a dusty area, to driving in a manner that does not allow the engine to warm up enough to boil away accumulated moisture in the oil.

Besides distance, the interval is also time dependent. Typically, every three months for folks that drive alot, and every six months for the rest.

Since the typical 928 engine would cost over $10,000 to replace. $50 for an oil and filter change every 3 or 6 months seems like very cheap insurance.

Merry motoring, Ed.
'78 5speed
928OCCM

Section 11: Miscellaneous
Motorvac
Ramblings

Motorvac

In a question dated: 12/29/98

The shop I was just at for the alignment is swearing by a new? process to clean carbon out of the motor. It called MotorVac carbon clean System. It is supposed to make the engine breath easier, raising the manifold vacuum significantly and improving gas mileage and emissions. Has anyone had this performed? It will be on a Shade Tree mechanic episode on Jan 16 @ 5:30 EST and 17th @
9:30am and 5:30pm EST, and 18th at 9:30am EST.

Ken

I've only pulled a dozen or so 928 heads, and have never seen carbon build-up as a problem. Yes, there was a fine layer on top of the pistons and valve heads, but it's not a problem as long as the amount is consistent on all valves / pistons. It gets to be a problem if it builds up more in some cylinders than others, giving different compression in different cylinders of the same engine. It can also cause pre-ignition if there is a large amount built up, but a properly tuned (and operated) 928 shouldn't require any carbon cleaning. The only place carbon buildup could affect manifold pressure would be in the port leading from the exhaust valve to the exhaust manifold, and that would require quite a lot of carbon. I suppose it could also affect manifold pressure if your exhaust pipes were clogged with it, but that scenario is highly unlikely. Carbon usually will not build up to the point of being detrimental to engine performance unless the oil rings or valve guides are really worn, allowing oil into the combustion chamber. At that point you would see a lot of blue smoke from the exhaust. An engine running very rich can build carbon deposits, but again, smoke (black) would be evident at the exhaust pipe.

I'll be watching that program with interest, but a lot of the stuff I have seen on those Saturday / Sunday car programs have the distinct odor of snake oil. For me, they lost all credibility when they started pushing K&N air
filters.

Maybe someone from Mark Anderson's crew could give us insight as to carbon problems in 928 engines (since they dismantle these things day and night.........)

Walt Konecny
88 S4 A/T

In a response dated: 

Hi Ken and list:

Yes, I have had it done to the 81 and the 88 will get it soon. Some may remember that my pal and mechanic, Mo Zahr, just bought one of these machines. Billed to really clean the injectors and more. It smoother out the idle a touch on the 81 and the car does really run well. I'll post again after the 88 gets it.

I think it seems like a decent product, uses chemicals mixed with gasoline, hook up the fuel lines, connect 12 v, pull fuel pump fuse, and start her up. Runs about 30 minutes for a "treatment" and may cost $75-100???

(Mo gives me a bit of a deal)

Marc White
81 and 88 S4
87 project on the rack

Miscellaneous Ramblings

In a message dated: 7/31/99

According to my mail system, I sent this to the list early this morning. I didn't get it - did anyone else?

Comments on recent posts:

>>My cam tensioner took over 30 ml of oil today (probably empty). The seal showed little no evidence of leaking. I'm wondering if the belt will be
tighter now? Or does the oil act as a damper?<<

The oil in the tensioner: a) Lubricates the tensioner. There is a lot of movement in the tensioner. b) Dampens the movement of the tensioner. c) Aids in heat transfer from the block to the bimetallic washers that correct for varying operating temperatures and the resulting expansion and contraction of the engine block.

To fill the tensioner on a 32-valve engine, use a pump oil can and a short length of plastic hose. The fill nozzle is the HIGHER of the two, closest to the right front fender. The other nozzle is the bleed. Keep pressure on the oil as you pump it into the tensioner - it goes in pretty slowly. The instructions for the early model rensioners say to remove both plugs and pour oil in the filler opening (again, the upper one) until it runs out of the bleeder opening. It would be interesting to see if the bleeder nozzles could be used to replace the plugs on an earlier tensioner. All tensioners use SAE 90 gear oil (Shop Manual, Vol I, Page 15-21 and 15-22).

After you fill the tensioner, keep an eye on the area. Leaks usually come from two sources - the rubber boot gets hard and cracks, or the gasket between the block and tensioner leaks.

>>You could be right, but it's going to take a lot more than some "logic" to get me to change my frequent oil change habits. <<

Very significant statement! This will almost certainly be one of those questions that are never settled. My suggestion is that each of us try to take an objective look at the situation, make up our own minds about our own oil change intervals, and NOT try to convert others to our particular religion.

>>I purchased a a/c compressor pump recently. I attempted to install it when I found that the old pump mounts were broken from the engine block. A local shop told me that I would never be able to replace it unless I replace the engine block. My question is can these be welded back on the 
engine? Can you weld on the block or will it damage the engine? I don't want to replace the engine at a cost of $4000!<<

Amen to that! This is an unfortunately common result of running the front of a 928 fashionably low - the a/c compressor is the first thing that hits the ground! Before I replaced the block, I would certainly investigate two ways of attaching the compressor - welding and bolting. Welding would be my first try, but it will be interesting to try to persuade a welder to work in such cramped quarters around such flammable and easily damaged materials. It might be easier in the end to remove the engine from the car, regardless of what method you try. God knows that there are enough WYAITs to justify removal. Bolting might be possible. Grind the block smooth and make a replacement ear that affords

a pad to spread the loads and give a place for the bolts. Try to get to a corner, and wrap around it if possible. It might be better to make a large scab-on plate to spread the loads as much as possible, and to use numerous small fasteners.

Wally Plumley
928 Specialists