I have recently become the proud owner of an immaculately looked after 1991 Audi 200 Turbo 10v. I have had the car serviced and tuned. Recently I have noticed a slight hesitation on pull away, irregular idling speeds, hesitation when trying to accelerate up a hill (esp. in third gear) when the revs are appr. 2500 RPM and black smoke coming out of the exhaust pipe when the engin is under load and busy accelerating. The agents, unfortunately, list the possible problems as the most expensive parts (naturally) without being able to pin it down for me to one or two specific parts which may be the culprits. Any help or suggestions would be of great help.