I'll bet you've got some air leaks somewhere else that is causing the idle to be high. Try spraying a little carb cleaner or starting fluid around the injector seals and towards the ring seals between the upper and lower manifolds to see if you can detect a leak that way. My '79 also has that idle valve, and it is a pretty positive device.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C