1990 928 Stalls when hot.

[cbmouse]

I have a 1990 928 that will stall at idle once the vehicle reaches operating temperature or a little above that. I have replaced all seals on the intake and new bushings for the tuning valve in the intake. The car caught fire once when a technician was trying to find a vacuum leak with brake clean. He replaced everything that was toasted. When I got the car and removed the intake I did not see any signs of fire damage except alot of residue from the dry chemical fire exstingiusher. The car ran good for about a month or about 800 miles. Then it started stalling on me at red lights. I noticed it only happened when the vehicle was at or slightly above operating temperature. Anyone have this stalling issue before. When I start the car up after the stall it fires up and stalls immediatly after. The only way I can keep it running is keeping my foot on the throttle. So the RPM's dont drop low enough to stall. I am having a hard time trying to fix this car and am not too familiar with these cars.

[lizard928s]

I would check the flexplate tension first.

[Danglerb]

Ouch, I agree first check the flexplate and crankshaft endplay.

Half measures, single item fixes, often do not work with an old 928. You should have the car thoroughly checked by someone that knows the 928 well and owns a full set of workshop manuals and diagnostic tools.

Don't drive it until you are sure its not a thrust bearing failure.

[cbmouse]

Just curious why you would point to crank walk for a stalls when hot concern? Thanks

[928 Go Go Joe]

Stalling at idle when hot, then not turning over and starting til engine cools down is a sign of TBF. Could be something else, however

Joe

[Danglerb]

Symptoms fit, and thrust bearing failure wrecks the whole motor if not caught very early.

On later model automatic transmission 928's the engine flywheel has a flexplate bolted to it and the torque tube shaft is connected to the flexplate with a pinch clamp. Normally the flexplate provides all the compliance needed for front to back movement, but sometimes the pinch clamp slips, and unfortunately is seems to only slip in the forward direction eventually applying a lot of pressure to the flexplate and from that directly to the crankshaft. The thrust bearing simply is not designed to bear that kind of load for long periods and quickly wears through and WHAM crank meets block. Bits of bearing are all through the motor by then, and damage is often enough that the whole motor must be replaced.

Recommended service is to once or twice a year check the flexplate for flatness and release the tension measuring how much the pinch clamp moves, then checking the endplay of the crank.

[928sg]

Originally Posted by cbmouse

Just curious why you would point to crank walk for a stalls when hot concern? Thanks

This is textbook TBF on a 928.. this is how many new owners find out they need to buy a motor.

[Shark_GTS]

Holy cow!! Start with the simple stuff first.

Best to pull the codes if you can get a shop with a scanner. Look at the o2 sensor. When the car hits 2/3 of it's op temp, it switches to o2 sensor control. If the sensor sig is out of range or nop then you won't idle well at all and get stalling.

Next stop is the MAF. In limp mode the car is not very driveable.

I've never started a TBF car but my guess is that when the oil barrier thins, the friction of the failed bearing is like putting on a band-brake of an auto trans on the crank, therefore stalls the engine.

[lizard928s]

I suggested checking it first as it is easy not that time consuming, and if that is the problem you dont wast money replacing other parts.

As well this should be checked on all the cars, and from the first post it didnt sound like the owner had previously checked it.

[cbmouse]

TBF?

Thanks for all the support. I will let you know what I find.

[lizard928s]

TBF is either Thrust bearing failure (in this case) or Timing belt Failure.

[cbmouse]

I found no signs of damage to the crank sensor. I will put it on the lift thursday and check the flex plate.

I checked fuel pressure a while ago and everything was good.

[ROG100]

Classic TBF as already said - once that is eliminated then move on to the other suggestions.
Checking the flexplate and crank is very simple stuff and needs eliminating from the possibilities ASAP.
Make sure you check the crank end play and report back.
1990 is the worst year for TT failure and TBF in my book 8>(
Heed the advice of Lizard and 928SG - two of the most experianced 928 engine guys I know.

Are you able to check crank end play/float?

Where in the World are you?

[chesireIsugaree]

What's up ROG!? been ages hasn't it? This could be injectors, it could be a fuel pump, it could be a vac leak, the TBF thing is so hardcore. I hope it's not that, poor guy.

[ROG100]

Mase,
Could be a lot of different things - it also could be TBF - I hope not either.
To release the flex plate and check the end float costs nothing and takes 2 hours max.
Once that is out of the way we move on to other culprits.
Using a Hammer would be usefull as well of course.

[cbmouse]

New injectors in the car. I had the intake off and there are no vacuum leaks. I doubt fuel pump because if you have access to the throttle during the stall you can catch it before it dies. Thanks for all the help. Its going up on the lift today. I will post back ASAP!

[cbmouse]

.006-.007" play. I will find some accurate specs. Wouldn't a worn thrust bearing cause some noise??? Plus it dies in park. not just drive. If That means anything.

[lizard928s]

A worn thrust bearing will not always cause noise, and it wont matter what gear it is in if it is TBF.

[Danglerb]

Maybe take off the oil filter and cut it open and check the media for metal.


This is one half of the thrust bearing.



Group picture of thrust bearing half and three sisters.



Thrust bearing still in the block.



Group picture of all the block side bearing halves, thrust bearing in the middle.

[cbmouse]

Found Porsche manual!

Specs for crankshaft axial play is between .004 to .012".

I think I am in the clear, based off these numbers.