For the past five years I've owned a 1989 420SEL. The old girl is a real show piece and runs extremely well with one exception that has existed from day one. At start up, and until the engine temp reaches almost full-hot, the engine runs rough at idle. This is not a miss; rather it seems to be runing very rich. I feel certain that the problem is in the fuel system. I have a full set of Mercedes manuals which I have studied over them for hours. My attention is now, perhaps wrongly, centered on the Electrohydro Activator located on the side of the fuel distriblutor. According to the wiring diagrams, this is controlled by the CIS-E unit and to a large extent determines the fuel mixture. My question: am I barking up the wrong tree or could there be a possum in this one. I've had little experience with fuel injected cars and I feel intimidated. I sure can use help.