I have a 1988 S4 A/T that is fairly pristine but has an odd recurring problem (for the past 2 years) that has never progressed to a major problem. I’ll list all of the symptoms and attributes of the problem along with my insights and anyone with an idea or advice should post it. Here goes:

For the past two years my car has suffered a weird stalling problem. The car just stops running. It has never failed to restart immediately after this happens – and it has happened hundreds of times. My daily commute is only about 10 minutes, mostly at or near idle. This stalling problem only occasionally happens during my commute. It is more likely to happen on a weekend drive that lasts more than my usual 10 minutes. It can happen when idling along in traffic, or even on the highway (although it is usually when I’m idling along at 100km/h and hardly have the throttle depressed). When it happens I can shift into neutral and restart immediately. It will generally happen 2 or 3 times within a 10 minute period and then the car runs fine the rest of the trip.

At first I thought it was an idle stabilizer problem (they tend to stick) and I thought spraying WD40 into the idle stabilizer helped. But the problem comes back.

I have also replaced a few key relays but the problem remains. Several people have suggested my computer is on its way out, which is possible, but does it take more than two years to die? As I said, the problem has occurred hundreds of times and the car never fails to restart immediately (which makes this whole thing almost impossible to trace).

My latest theory is that the problem could be related to the alarm system. The alarm has the ability to turn off the ignition, so could there be some malfunction in the crappy Porsche alarm? Does anyone know how I could bypass the alarm system on this car as a test?

Other possible theories relate to the usual – lack of spark or lack of fuel. Could my electrical system, which I suspect struggles to manufacture enough power to run everything, be slacking off? (that’s why Porsche made so many things run on vacuum in this car). Could the fuel pump simply be shutting down for some reason and then be back to normal once I restart?

Anybody got any troubleshooting suggestions?

 

Swap computers. see if it still happens. There is a guy here that rebuilds the computers, if yours is bad..............I bet he will chime in, or someone will refresh my memory on his name......

 

check the ohms that the fuel pump is drawing, it might be worn and hitting the shutoff pressure turning off then upon restart it releases the pressure and works,
the other possibility is a weak spark

 

Check the MAF. If it's makeing thing too rich, the car can stall before the O2 sensor can correct for it.

 

I would electrically bypass the ignition switch to see if there is a problem in there, and look into electrical issues. It doesn't sound like fuel since it starts so well every time.

 

Wow. This one could be any number of things. I'll suggest 2 possibilities:

First is the ignition switch. Check to see if the electrical connection on the back of the key unit is loose...the archives will turn up some previuos posts along this line. Usually this type of failure is accompanied with additional electrical failures, however. I'm assuming this is not the case since you did not mention any.

Second, check the magnetic crank position sensor. Again check the archives or Greg Nichol's site for more on this, but I recall at least a couple of other people having weird shutoff/restart problems because of this.

Otherwise, I think you are into a good bout of troubleshooting...just after the car fails. I'd like to know several bits of info immediately after the car shuts off (but with everything in "run" mode)...power to fuel pump, output from FI computer, spark conputer, etc. Let's hope you find the problem before having to go down this road.

Regards,
SteveCo

 

An auto that stalls when the car is idling sounds potentially like Thrust Bearing Failure. I hope it isn't (and you did say the car restarts immediately) but this is your worst case scenario and you need to check before driving again. If you haven't had your flex plate checked then you need to no matter what. Do a search on TBF for more advice.

 

When the engine stalls, does the dash light up like a Xmas tree (normal srart up), or give no visual indication until neutral is engaged and the key turned off - then on again?
SteveCo's point on the CPS is one I'd 'second', as well as an archive search . Paul's comment on the flex plate is a 'must' - if not already attended to. Best of luck!

 

I had the idle stabiliser problem. Best way to say if it is that just that is by revving upto lets say 5000 rp/m and let go of the throttle. The stabiliser should pick up. If it’s broke it stalls or drops below idle to slowly come back. Does it also occur when your slowly driving on a very low rpm, or only at idle?
When the engine stalls, is it just that, or do lights go out or come on? Does the engine just die, or quit immediately?
As it occurs after 10 minutes, it is probably something that heats up and then starts playing tricks with you. The suggestions already given are worth investigating. I would also check and/or exchange fuel pump relay. Also check fuel pump(s).

 

Check the engine ground point under the engine airfilter box. It's hidden by the throttle cable pulley wheel. This can cause the problems you mention.

Then check the throttle closed switch.

If there is an airleak into the mainfold, the idle stabilisier system can't cope, it has a limited range.

It is quite possible that your LH ECU is the problem, and '88 car is living on borrowed time as far as these units are concerned ! The idle stabilisier control circuits are in the LH ECU.

Try borrowing a known good unit if you can, as has been already mentioned. If you can get a diagnostic tester on it, then you can actuate the idle stabiliser, and hear if the circuits are working or not. Alternatively I can check your unit out.

See my web site www. jdsporsche.com

Regards

 

Thanks everyone for responding. As I said, this one is hard to troubleshoot since the problem occurs and disappears within moments. Also, since this has happened hundreds of times over a two year period and the results are always the same I'm not inclined to think that something is "just about to go south". Here is some additional info on the problem:

When the car stalls the dash lights up like a Christmas tree. I simply turn off the ignition and then restart. It always starts immediately (maybe with a little more cranking than usual), but it has never failed to restart immediately.

The problem doesn't just occur at idle, it also occurs at very low RPM/very small throttle opening. I've never experienced the stalling while I'm accelerating.

I seem to be able to prevent it from happening by "two-foot" driving - I simply hold the RPMs above about 1100 and the car won't stall.

The suggestion that the engine ground is weak is interesting since my alternator guage never registers above 12V and most of the time sits at 10V (been like that for 5 years). Also, my fuel guage nevers registers full. I always suspected a ground problem there but never dealt with it because it never actually caused a problem.

I realize the computers on these cars have a fixed lifespan and that may become the achilles heel of this great exotic. But I've never seen an electronic component take more than two years to falter and fail. Usually they just go kaput one day and that's that.

Cheers everyone.

 

On the voltmeter and fuel gauge, this is a very common situation. While it may be ground related (and checking grounds is a very important thing on the 928...many posts to this effect on this list), it is "normal" for the voltmeter to read low. This guage can be adjusted to read correctly but requires the instrument cluster to be removed. The low reading fuel gauge could be a kinked vent hose or simply a out-of-calabration gauge. Unfortunatley there is not an easy way to fix this...very common though.

Here's the thread I was referring to on the crank position sensor. Sound very similar to your problem...but not exactly the same.

https://rennlist.com/forums/showthre...osition+sensor

And here's another that speaks to the same sensor (forgive me if I seem to have the same solution for every problem)

https://rennlist.com/forums/showthre...osition+sensor

I think it is worth checking out the crank position sensor and also the wiring/relays related to the spark system. I'll agree that a failing FI brain seem strage with these symptoms, but Mr. Speak is the guru in this regard. If you could swap out the brain it would eliminate this once and for all...

Regards,
SteveCo in St. John's

 

Ok, now you've given a little more detail, it doesn't sound like the idle stabiliser, 'cos it happens off idle.

Could be an earth point.......

I know it's difficult to check things when the engine is playing up, but when the engine dies, does the tach drop instantly to zero, even when the engine is still turning ?

If so, that would confirm a possible crank sensor problem or maybe the EZK ECU (although the EZK is very reliable).

BTW have you released the tension on the flex plate, and checked the crank end float ?

 

Hey John Speake, thanks for the advice. I will look next time to see if the tach goes to zero immediately or not - that's not one of the things I've checked or noticed. I'm not "tech" enough to check flex plates and such, but I do know that my mechanic had the trans out last year (to replace a torque converter bearing) and he would have put everything back together perfectly as he is an ****-retentive perfectionist who specializes in 928s! Would that affect the flex plate? (I 'm afraid to admit I don't know what a flex plate is).

This is a really challenging problem since as I said it happens quickly and goes away just as quickly. Every time it happens I think "great, another opportunity to deduct the cause" but then its over.

Cheers.

 

BTW, one of the things that makes it imperative to check for a (forward) preload on the flex plate and for crank shaft end play is having had the transmission/torque tube serviced and reinstalled.
The recently reported incidents of thrust bearing failure (TBF) were often preceeded by such work.
Cannot hurt to have this checked!
As the stalling problem preceeded the xmission work by a year, I'm not signaling a relationship - just a precautionary monitoring as per the experience shared on the Rennlist.