1995 C4 stumbles under anything other than light acceleration

[neilpaku]

My 2.8 A6 Quattro wagon stumbles under any acceleration greater than moderate. I suspect a vacuum leak which is causing the ECM to cut the fuel rate but that's just a guess. Spark plugs only have about 1000mi on them so it's not that, and I just cleaned my EGR and replaced the MAF.
Any other possibilities?

[nick1980]

Check plugs again
Valve covers leaking , plugs in oil?etc

[neilpaku]

This unit uses vacuum for hvac servos and cruise control so it's going to be a mission to trace all the leaks. Definitely not plugs as I pulled them all yesterday after about 1000mi from new and they're all good except one which is a bit oiled but still clean enough to fire ok. The symptom is definitely related to power demand, so if I'm in high gear and put my foot down it will stumble, but if I'm in second at the same speed, and have the gas pedal a ways down, it doesn't stumble. Very much seems like vacuum but it's also intermittent.

Given the small size of the hvac servo tubing, I'd be surprised if even if one of them was completely disconnected, I couldn't see it affecting the engine that much but perhaps the ECM has a fairly low tolerance for vacuum leaks. Seems to be worse when the car is warm. From cold, it won't do it until I'm a mile or so down the road. But then occasionally it will not do it when engine is hot too.

[neilpaku]

Better symptom description:
- starts well when cold, does not stumble when cold
- does not start easily when warm (kind of winds into it), stumbles under acceleration when warm usually, but not always. Runs fine at steady speed both in town and on highway

Took another look at plugs - one was lightly oiled but "clean", second was dry burnt oil. Other four were pristine. I will replace the two and see if the warm start improves and/or the warm stumble stops (until they oil up again...).

When I look at that hard-to-start when warm though, I'm suspicious. That is there whether the car stumbles or not. Could it be low fuel pressure? Had a new fuel pump installed 2000mi ago. Not sure if fuel filter was done but I might replace that just to rule it out

[Grainreaper]

Low fuel pressure possibly... because of The hot start description as well as high load miss and stumble,.. I'd be checking fuel pressure and fuel filter condition, could be a leaking pump seal/o-ring bleeding off pressure in the tank

[neilpaku]

Thx I'll order the filter and take the car down to my shop and see what they find

[neilpaku]

After swapping out a few components, I regret to say that the easiest thing to try was the last thing I tried, which was to swap out the plug wires - that fixed it. Still, live and learn and I do now have a clean EGR system, new injectors, fuel filter, plugs, etc. : )