The car: 1991 200 Quattro 20V. The symptoms...there are two: when my RPM's hit 1400, they fall back down, then rise, then fall, then rise (if the same throttle level is maintained). If I apply more throttle, the RPM's will rise, roughly from 1400 to 1800 RPM, then regain smoothness from there upward. This is the case whether idling or being driven. There is no roughness from 1800 up, or below 1400. When being driven, the car will "fall on its face" when trying to accelerate in that rev range...bucking is common, then it takes off. The problem is intermittent, but seems to happen after the car has been driven for a bit and has warmed up fully. Essentially, from 1400 to 1800 is the "trouble spot". Also...when driving, my exhaust will become louder, as though I have a throaty exhaust installed (which I don't...it's stock and has no holes/leaks). Frequently, when I turn the car off, then back on again, the problems disappear immediately...but they always return within minutes. Aside from these symptoms, the car runs perfectly...normal idle, normal power (turbo spools well and holds boost), no oil consumption, no vacuum leaks that I can find, etc. I opened the air box today and found a bit of a "mouse nest" inside it...cleaned it out, installed a new air filter and...no difference. The MAF looks excellent, but I haven't tried cleaning it. Anyone ever experienced these weird symptoms or have an idea of what could be causing them? Thanks, guys!

 

I had a 1989 100Q with similar symptoms, turned out to be a vacuum leak from a hose split right along the seam. IIR the hose ran from the Idle control valve at the front of the L5 engine under the manifold to the back of the engine. The split was in the hose hidden under the manifold

 

I highly doubt it's any sort of vacuum issue. The reason I say that is because the issue will temporarily go away when then car is turned off, then back on again...indicating a strictly electrical issue. I unplugged the MAF sensor and have driven the car approximately 30 miles since doing so. Granted...it now drives rather poorly overall, but it is NOT exhibiting the symptoms mentioned above. Based on this fact, I am leaning toward the MAF. Anyone have one to fit my car? I can use a '92-'94 Audi 100 MAF, or one from a '96 or '97 S6...and obviously one from a '91 200 DOHC as well. Thanks!

 

Fixed. Cleaned MAF...runs perfectly. I'd say thanks for all the help, but only one person responded. My hopes for this forum are not as high as they once were.