RET

The engine of my'98 A8 Quattro stalls intermittently in neutral or in gear at very low throttle (idle or barely above). The engine quits immediately and smoothly (no shudder, shake, misfire, etc. - it is as if the ignition is turned off); and engine can be immediately restarted as if nothing was wrong.

This problem might be happening but go unnoticed at normal driving speeds. If the engine RPM is maintained above approximately 1100 RPM by riding the accelerator pedal while stopped, the engine quits/hesitates abruptly and the tachometer drops momentarily, but the engine does not stop and quickly and smoothly resumes speed. This momentary hesitation is without any shaking, backfire, or rattle.

A local well recommended shop read the diagnostic codes and all of the engine related codes showed no problem, so they were not inclined to work on it. I have called the local dealer as a last resort and left detailed messages for the service folks.... they seem reluctant to call back.

I can't be the only person who has this failure mode. Any help would be appreciated.

Thanks!

silverd2

If there are no codes, the arrow is pointing toward a failing fuel pump ... maybe ... it's the only failure that can shut off an engine, which throws no code of its own.

RET

Thanks for the quick reply.

I have no service documentation for this car, so I apologize for my ignorance...

How can I check fuel pressure? My previous inspection did not turn up any likely ports to attach a gauge.

How do I access the fuel pump? I suspect that I can find it, but is there any special technique for access... how much of the car has to be dismantled?

Is there a relay in the circuit, and if so, where is it located? My owner's manual is pretty vague about the electrics.

Any chance this has anything to do with the alarm system?

Can you recommend a complete service manual, including wiring diagrams? I have read references to the Bentley manual, but there was some indication that important items were missing.

Thanks again!

audinutt

If the tach is dropping out, it could also be the engine speed sensor also known as the crank position sensor.

If the fuel pump hasn't been done it's on borrowed time.

Do you have access to VCDS? can you scan for fault codes?

-Richard

RET

No fault codes for engine.

Does a failing CPS cause the symptoms described?

audinutt

It can, but not usually.
Usually it will not restart once hot.

I've poured a bottle of water down on one before to get the car restarted and verify it was the issue.

-Richard

rbjones17

Like SilverD2 said, CPS will cause a fault code. Also CPS is likely to be an all or nothing problem. I once had a loose wire to the CPS (behind the firewall on the drivers side) and you could hear the fuel pump kick off or on when you rattled the wire. If you have a CPS problem, you won't hear the fuel pump engage after turning on the key. This slow steady dying sounds like fuel to me (like SilverD2 said).

Google search or youtube search for fuel pump replacement audi a8 d2 and you'll find some diy's on that. It would appear to be a messy pain in the *** to replace if its your first time, like most things on the D2. Good luck.

I see that you asked how to check the fuel pressure. Can't help you there. However, if the car idles well but stalls when you need power, then it is pointing to a lack of air or fuel, the typical culprits of which are fuel filter or fuel pump.

RBJ

RET

Thanks for the responses. I will follow up on the youtube reference regarding the fuel pump. I doubt that the fuel filter causes this problem as the car has plenty of power and runs smoothly when accelerating and at extended cruise.

Any specific help with the following would still be appreciated:

How can I check fuel pressure?
Is there a relay in the fuel pump circuit, and if so, where is it located?
Any chance this has something to do with the alarm system, and is there an easy way to defeat the system temporarily?
Any recommendations for a complete service manual, including wiring diagrams?

Thanks!

the_duke

Sign up for the linked forum and then you can access this link. Good stuff in there.

A8 Parts Forum

RET

Hello All,


This is a cross-post FYI in the hope that it might be useful to someone...


The problem is resolved: it was a bad mass air flow sensor.


I had changed relays and cleaned the recommended connections (none of which were corroded...); all with no change to the stalling problem.

The car had been taken to four service shops, including the Audi dealer: the first place could not even read the diagnostic codes; the second could read the OBD codes, but the errors were minor and not engine related, so the shop declined to work further on the car since the problem had not been identified by the OBD errors; a third shop could read the codes also and thought that maybe the ECM might be at fault since no hard error was shown, but had no way of confirming this; finally the dealership tested the car, found no significant fault codes, and gave the opinion that the fuel pump was responsible and needed replacement.

The diagnosis of a faulty fuel pump was determined by measuring the current, per the WSM, drawn by the fuel pump and was reported to be 3 amps (not the nominal 8...). I pointed out that the car accelerated well and had good power climbing hills at speed, and asked that the dealer guarantee the results (i,e,, if the new pump at $1800.00 did not cure the stall, I would not be charged for it...) they declined, of course. FWIW, later that day I measured the current to the fuel pump at 7.9 amps...

The momentary losses of power at low RPM were similar to the symptoms of the failing MAF sensor on my '89 928 GT Porsche, so I unplugged the MAF on the Audi and the stalling problem went away (note that the gear selector indicator on the instrument panel displayed in error mode). The car even drove well.

A new Bosch MAF sensor cleared the error and seems to have solved the stalling problem.

Bob