could be Mass Air Flow Sensor or need to have purge valve replaced
what exactly is a purge valve for the audi? mechanic said car's running to rich on 1998 Audi A4 Quattro
Purge valve is a small motor unit that pulls gas vapors out of tank then is pushed into your intake. Makes vehicle more fuel efficient- it's used as part of your evaporative emissions system. I've never heard it making the car run more rich- usually uses more fuel and possibly chokes if not working. You should however get a trouble light on dash when your computer recognizes problems with that system. Mass air flow sensor makes more sense as to the problem though just the same you should be getting a trouble light for it.