I have a 1987 Audi 4000CS Quattro with 145000 miles on it. The car currently runs very rough, with the following symptoms:
1) Idle warbles between 800 and 2000 rpm
2) car shudders and lurches, as if it isn't getting enough fuel, when coasting downhill or accelerating uphill. It is roughest when I step on the accelerator or suddenly release the accelerator with the car still in gear.
I had the O2 sensor and the idle valve replaced, as well as the cold start valve. The only known issue with the car is a hole in the muffler - my guess is that somehow the hole in the muffler is distorting whatever sensors are in the exhaust system?
I am considering replacing the muffler, but wanted to have a better sense that it is causing this problem.
Thanks for any advice, Audi fans!