I have an Audi A4 1.8t Quattro and after warming up, any time at Idle (900rpms) the engine will drop to about 4 - 500rpms then back up. It is intermittent, only thing i can think of right now is dirty TB.


Also wondering if any body has seen or is selling parts of a CAT BACK, i have a down/test pipe and the flex/front pipe i have is stock so i can't compensate for the 2" difference.

 

I've been having a very random, intermittent bouncing idle problem too. Based on the searching I've done it could be a dirty/misaligned throttle body, crapped out pre-cat O2 sensor, vac/boost leak, MAF sensor, among other random things I've ruled out the vac leak and misaligned TB so far, this weekend i'm going to clean it and see if that helps. I've also been chasing a bank 1 lean code that comes around about once a month and stays for a day or two, so i'm leaning towards O2 sensor now. Its like Pandora's box lol

 

idk what year you are, but a ATW or AWM wouldnt idle weird because of a MAF. i had this problem at first too, then i ripped out my entire vacuum system EGR and PCV, then it stopped. so my guess is vacuum leak

 

Check for codes, just cause you don't have a cel doesn't mean there isn't codes. My guess would be vac leak/MAF I had the same issue in the summer, cleaned the throttle body, checked vac lines, and ended up being the MAF

 

What csf says about codes is true, so do check that. I think it would be worth unplugging your MAF (electrically) - if things run better than the MAF is dirty and/or bad. Vacuum leak is fairly likely, as is the primary O2 sensor. It shouldn't be a catalytic converter because that would make the car suck before warming up, too, and it would also make the car slower.