I have read (in this thread and elsewhere) that a rich engine can cause a plugged cat converter, plus carbon deposits in the engine. How rich does an engine have to be for it to cause this kind of problem (what lamda)? My car (85 190E, 113K miles) had a rough idle, my mechanic just made the mixture richer, plus idle higher (800 in drive on hot engine), to make it smoother. But the car is now giving me about 18-19 mpg (city driving), while it used to give me 22-23 (city) earlier. I am concerned if this rich running will ruin my cat converter and engine.
Any inputs/experiences/advice would help!