I recently bought a 99 A6 Quottro 2.8L with 77k on it. The car runs and drives perfect, it even idles fine while in park but once I put it into drive it idles rough. Its starting to get annoying and I read that this can be a common problem. How should I start to fix this?
The car I had before this was a 87 Chevy Monte Carlo SS and when it was idling rough I poured about half a can of Seafoam in the carb and it smoothed it right out. I'm not sure if I should use something like that on the Audi though. Any suggestions?

 

The Audi is gonna be a little bit more advanced than your chevy, get the fault codes checked out at autozone and post them back on here, then we can tell you what to look for.

 

I would start by looking for vacuum leaks.

 

Which would have a fault