Chuck,

I know this is a reply to an old post, but I'm going through the archives looking for a solution to a surging idle speed on my 500SL and came across this thread. First, the idle speed for the Euro 500SL is 475rpm...according to my mechanic.

Problem I'm having is I can be sitting at idle (475-500rpm) and the engine will surge anywhere from 100-300rpm. The idle never gets up to 1500rpm, which seems to be typical with the idle speed control unit. The idle speed control valve and unit were both replaced a couple years ago, so I'm hoping this isn't the problem so soon.

It doesn't appear to be an OVP problem either. The OVP was replaced about three years ago as well.

I'm leaning (hoping) toward the ignition control module, which I happen to have a spare in the trunk. However, I'm open for suggestions if anyone has something to offer.

Thanks,

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000