My dad's audi 200 turbo, i think early 90's prolly 91 and it is pearl in color, is exhausting black smoke. We have been told it is because it is running rich, it blows this smoke while it is idling and when we accelerate after waiting at a stop light. what is the problem?
We have replaced the oxygen sensor and idle control valve. And spark plugs. when we took the old ones out they were all covered in soot or blackened, then we put the new ones in, the car was runnin fine for a day or less, then quit, he pulled the spark plugs and they were blackened again. What is going on?!?

I would say the car is running rich. A simple check (to start)would be to see if the air filter is plugging off

sounds like waste gate not working if loosing oil turbo is out very costly be sure