Well, this is a long story, like many other owners, my 92 400E had rough idle problem since the first day I bought the car. Rough idle only happened during the first 20 seconds after I started the car, with warm engine the idle smoothed out. When engine temperature was low, in summer, it shook violently for about 10 seconds. In winter, it developed a kind of miss fire, it also shook, but not as violent as in summer time. This problem had been annoying me for a whole year, I felt embarrassed when I had passengers in my car.
I replaced rotors/caps, fuel pressure regulator, spark plugs, air filter, cleaned MAF sensor, measured fuel pressure, belt and tensioner, the problem was still there. Later I even started to question motor mounts, timing chain and fuel filter, I already got the parts, was ready to take several days vacation for the project.
Lately Michael, a 500E owner, suggest me to try fuel addictive BG 44K, however in Canada, BG 44K can only be obtained through mail order. I then decided to try some equivalent fuel system cleaner, I bought two bottles of Pennzoil complete fuel system cleaner($14 each). I added one bottle to the fuel tank and filled the tank yesterday, then last night I put the transmission into gear "B", drove it for about 5 minutes, I kept engine rpm between 4000-5500(60-70 kmph). Today, for the first time, the rough idle disappeared, I could only feel slight vibration from the steering wheel after I start the car. I am so happy, guess it can be much better if I finish 2 treatments, I now love my 400E twice as much as before.
THANK YOU! Michael.