Engine idles high, about 1200-1300 rpm when warmed up. Idle is normal when cold. Replaced the idle control module when I encountered a rough, calloping idle. Idle control module solved that problem, now I have the high idle, warm engine problem. Tried another idle control module from another car, problem remains the same.

 

Your engine uses an idle control valve. It's likely this unit is your problem. It is located right in front of the air cleaner canister (top front of the engine). I own a 1986 560SL and a 1986 420SEL that both use this same design idle control valve, and have had to replace it on both cars. Symptoms are fast idle when engine is warm, just as you describe. I would remove it (easy job), thoroughly clean it out, but to no avail. There is an electric unit as part of it that I believe was the failure. The price is $237.73 on thebenzbin.com, or you can find one right now on ebay used for about 45 bucks - depends on what you want - new or used. I bought them through my local Carquest auto parts store for $125.00.

Hope this helps!

 

Test idle control valve per the repair manual and it tests OK, function check shows that it is functioning. Any other thoughts?

 

How, specifically, did you test it? Too bad you couldn't just find one to "borrow" or swap out yours with another temporarily so you can test it that way. I did that, since I have two cars with the same unit, and this is how I really found out what was wrong.


I'd be interested in the specifics as to how to test the unit otherwise...