Howdy. I've read previous threads addressing the issue of rough idle, but none of the threads seems to be on point regarding my specific question, rough cold idle. Hopefully one of you will have some suggestions that I can follow and that will help diagnose and fix the problem.

Here is the symptom:

My 1991 420SEL with 188K miles on the odometer suffers from rough cold idle. The engine starts fine, without any problems, but the idle is rough. Not quite to the point where the engine will turn off, but rough enough to be noticeable. There are no pings, backfires or other engine noises, just rough idle. If I put the car in gear to drive off, the car accelerates fine, again no missing, no sputtering, no backfire. When I come to a stop, the car will again idle rough. Once the engine reaches operating temperature, at around 80 degree Centigrade, the problem disappears and the engine then idles smoothly.

The only other symptom that I have noticed is that the "economy gauge" (a glorified vacuum gauge) sits a little to the right when the engine's idle is rough. After the idle gets to normal, the needle moves to the left (most economical) position where it remains until the car starts moving.

What I have done so far:

I tried using injector cleaner. The problem improved ever so slightly, but not 100% normal. I used a German brand cleaner, called "Moly-something" (sorry I forgot the full name). The O2 sensor, the EHA (it was leaking), and the ICV (idle control valve?--a round cylinder that sits on top of the engine, near the front, made by VDO) were recently replaced. All 8 injectors were also recently replaced. The problem, however, remains.

Someone had suggested the fuel distributor may be bad (but that's an expensive item to replace--are there rebuilt ones available? can the fuel distributor be rebuilt?). Someone else had suggested a vacuum leak at the intake manifold, but I have had cars before with an intake manifold vacuum leak and the symptoms were different. Although some sort of vacuum leak makes sense since the problem goes away when the engine is warm. If there is a gap someplace (causing a leak) then the expansion caused by heat may seal the leak, smoothing out the idle.

The car runs fine once it is warm. Fuel mileage is about 19-21 (combined highway/city) and oil consumption is less than a quart every 3K miles.

The problem is no more than annoyance at this point. Unless it gets worse, I can live with it and just write it off as another one of those endearing Mercedes idiosyncrasy, like crappy A/C controls (just kidding you guys!). Still, I'd like to get it fixed if it doesn't involve a lot of money.

I'd greatly appreciate your assistance in helping to diagnose the problem. Thanks.

[Edited by BENZ-LGB on 05-14-2001 at 03:21 PM]

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1989 300TE "Alice"
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1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
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2002 C320 Wagon "Molly Fox"

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