[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!