1998 A8 Quattro Stalling Problem

[RET]

The engine of my '98 A8 Quattro 4.2 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.); and engine can be immediately restarted.

This problem might be happening but unnoticed at normal driving speeds. If the engine RPM is maintained above approximately 1100 RPM 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 with no engine or engine related errors, so they were not inclined to work on the car. I guess they did not want the challenge...

I have done limited diagnostics as I am unfamiliar with Audis in general, do not have the workshop manuals, and have no specific tools for them.

FWIW, I have done the following (car idling in neutral, HVAC on)-

Fuel pressure: I could not locate a test port to test fuel pressure, so the best I could think of was to listen to the pump while the engine ran and stalled. The sound was consistent while the engine ran and continued smoothly with no change for a few seconds after the engine quit.... Fuel pressure might be acceptable...

Vacuum: Checked vacuum lines, replaced left side cylinder head-to-crankcase breather line (had obviously been totally worthless for a long long time...); monitored vacuum during several stalls with vacuum better than 20 inches until after the engine stopped; so no indication of a massive sudden vacuum loss.

Any help or suggestions would be appreciated. Thanks!

[ainarssems]

Unmetered air or dodgy MAF would be my first point of call. I know you already looked for vacuum leaks and replaced breather hose but have you checked MAF measurements in VCDS? There are a lot of hoses by the throttle body, these can crack with age and let unmetered air in leaks are often hard to spot on them. Could also be dodgy or clogged idle control valve.

Have you tried switching AC off and see if it is better or worse?

[moltuae]

Originally Posted by ainarssems

Could also be dodgy or clogged idle control valve.

I developed a vehicle fuel injection control/diagnostic device many years ago that, among other features, had a mode for cleaning injectors and idle air control valves. As I recall, a clogged IAC valve certainly does produce those symptoms. The best way we found to clean the valves was to pulse them back and forth while submersed in a suitable detergent. IIRC, some had 2 terminals, with a simple power on/off operation, while others had 3 terminals; common, open and close. Cleaning was usually about 90% successful.

[MikkiJayne]

Originally Posted by RET

The engine of my '98 A8 Quattro 4.2 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.); and engine can be immediately restarted.

. If the engine RPM is maintained above approximately 1100 RPM 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.

These two points say electrical fault to me. Is it in any way temperature related?

VCDS is going to be pretty important to find this as it can log parameters while the engine is running which will help point to the problem area. If, for example rpms drop to zero then pick up again (which VCDS would show but the tach may not if its very brief) its likely to be the crank position sensor. If it stops logging then the ecu is losing power, etc. A genuine K+CAN version is probably going to be needed too, since the clones often aren't fast enough to log multiple parameters simultaneously.

Where are you? There may be someone local who can help.

[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

[MikkiJayne]

Ah the MAF again, which causes so many bizarre problems without showing any fault codes!

Glad its sorted

[RS6]

Handy to know its MAF