I have had my '07 S6 for a few years now and just recently have noticed it doing this rev bouncing thing during a cold start once the car has been sitting for a while.
During the cold start routine the revs bounce up about 300-500 rpm at roughly half-second intervals. If I press the gas pedal a bit, it revs fine, but goes right back to bouncing after I take my foot off the pedal. There are no misfires and a VAG-COM scan shows no active CEL.
Once the cold start routine is done, the revs are rock solid and car runs great. Also, if I immediately shift the car into drive after starting it, the rev bouncing stops.
I did have the car smoke tested not long before this started due a CEL that indicated an air leak. We did find some leaks but all of them were repaired except for a very small one seen at the intake manifold (between the upper and lower sections) and this cleared the CEL which hasn’t returned for months. However, now I’m getting this rev bounce thing…
Would a small air leak cause something like this? Anything else I should be checking?