RE: 1988 Audi 90 ??
Congratulations on your purchase. The Audi's are well built cars, solid, not too difficult to service for the DIY, but given the age there is going to some maintenance required. Ownerâ€™s manual is a dealer only item but if you search the glove compartments of your local salvage yard you just might find one. The best service manual is going to be the official OEM by Robert Bentley. The idling problem could be the result of a dirty Idle Stabilizer Valve. Remove it, plug one end, spray carb and choke cleaner, plug the other end, shake it, pour the dirt out and repeat a few times.