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.

Hi bigcar,

welcome to the forum.
Start with a search. There is a lot of info in old posts regarding high, low, rough and any other idle you can think of.
There is the idle adjustment valve, that might need cleaning etc.

Regards
Reinhard Kreutzer ('87 420)

Hello RKreutzer,

Thanks much for your reply to my query. Indeed I should have done more archive searching prior to posting my query. In the past couple of days, I have been searching and I have found several items which could be causing my problem. I've already eliminated two of them from consideration and two other are currently on the table for examination. Of special interest right now is the possibility that I have an exhause value in need of adjustment via way of shimming.

Again, thank you.

Bigcar

I explained earlier that my '89 420SEL had suffered a very rough cold idle from day one following my purchase five years ago. The bad idle lasted from startup to near full-hot condition, then the engine leveled out.

I searched your archives and found the BENZ-LGB had written up his experience with a nearly identical problem in a nearly idenical auto, i.e., a 1991 420SEL with 188K miles (my car now has 175K).

About a year ago, I became suspect of the coolant temp. sensor and replaced it. No help. I made other engine adjustments of no help.

BENZ-LGB went through a lot of fixes that did not fix his car. He finally zeroed in on exhause valve adjustment. That ended up fixing the problem.

Yesterday, I removed the right bank valve cover and used a tool I purchased some time ago (from Baum Tools in Florida) to check the valve adjustment. All valves were OK except for the last one at the rear; It was far out of adjustment. Using another tool that I purchased earlier, I removed the rocker arm, then the adjustment shim. I found it to be 5.10mm in thickness. I ordered, and received today, a shim of 3.70mm. I finished installing it a few minutes ago. My problem is completely solved. Cost $6.50 for the shim, $20 for over-night delivery from an LA supplier. I just love it when things work out, don't you?

Now we know why the valves need adjustment once in a while...
Congratulations
Reinhard Kreutzer