I had the chance to work on my 91 560SEL yesterday ( on the rough idle)...I cleaned the throotle body and plate with cleaner and removed the plugs. To my surprise, all my plugs were black, and 3 of them had heavy carbon deposits on them..I mean real bad. I could nt imagine my car would run at all with what I have
seen. Since I did not ahve replacement plugs I decided to clean them up with carb cleaner and scrape off the carbon deposits. I installed them back, and the rough idle went away. I have ordered new set of plugs.
I had used the BG 44K fuel system cleaner a few weeks back and I am wokdering whether it might have conributed to the heavy carbon deposits on the plugs?
My car is running fine now. I will observe it it holds up.
One more problem that I need to resolve is it stalls randomly at any speed. I am not sure if this is related to the plugs problem or not. I will open up my fuel pump relay and verify open solder as some other posters have suggested...