Not really a problem ... more of a story...

My girlfriend's 740 had problems such that under heavy throttle, sometimes ALL acceleration would go away. I noticed once when trying to get on the freeway, but around town, I never noticed any problems.

Anyway, the problem re-occurred yesterday and she was pretty spooked by it. I was going to poke around with it a little bit last night, and we decided to take it out for a test drive. We picked up our friend John (who knows more about these things than I) and went for a drive. Turns out, the problem was easily reproducable if the throttle was more than 3/4 of the way open and the RPM's were above about 3200 or so. VERY reproducable. In any gear. If you let it upshift, it'd have power again until about 3200 (approx, it wouldn't be at EXACTLY 3200 every time, but around there).

Odd, no? When the acceleration went away, the turbo boost would drop down to negative boost as well. Lifting the throttle would actually INCREASE acceleration. It was dark, but we didn't see any billowing clouds of black smoke or anything... The engine wasn't ROUGH or anything when it was bogged down ... there just wasn't any power. So it didn't really seem like a misfire.


So, we decided to check the air cleaner. She hasn't had the car for very long, and we hadn't had this open yet. A badly air cleaner would cause problems if the engine were breathing harder. As soon as we had the top of the air box off, we faced the horror of the disintegrating sound-proofing foam. ACK! Seriously, it was peeling off the top of the box in big strips. There were chunks actually MISSING. Very very scary.

Removed the top of the box (and the MAF sensor) ... shook it around a little bit and some bits of foam fell out of the tube leading to the MAF. We removed the MAF from the airbox lid and about 30% of the surface area was obstructed by foam which had become embedded in the mesh. AIEEE!

We removed the screen and brushed it clean ... gently cleaned the MAF body with compressed air and re-assembled. The problem is now gone.

The theory is that the excess vacuum caused by the obstructed MAF was causing the intake tube leading to the turbo to collapse. Once it collapsed, you'd have to lift out of the throttle to get the vacuum to release enough to let the tube open back up to its full width. But that's just the theory.

Moral of the story: Get rid of that airbox foam. BLEAGH!

--Mitch