I am trying to figure this one out. The idle speed is controlled electronically by current to the idle valve from the idle relay. More current, less air, lower idle. There is a coolant sensor switch at the back of the engine that, when it opens, will cause the relay to drop the idle from (I think) 900 to 650 RPM. If the whole system shuts down, the valve stays completely open, which allows enough air to keep the engine at about 1200 RPM. The WUC richens the mixture accoring to temperature but does not affect the idle speed, to my knowledge.

So there are two things that could cause this: some sort of binding of the linkage when cold that holds the throttle open, or some giant source of extra air only when cold.

So I guess I would start by making sure that the mechanical linkage is against the stops when the engine is cold, and that nothing is moving when the engine starts. Also that the cruise control is not affecting the throttle linkage in any way.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe