The throttle has to close fully, and the throttle switch work consistnently. or the idle controller will not get a ground signal, and won't "know" that it should start controlling idle speed.

I fought with this on my 380SL, which the PO had allowed to sit for three years.

I started with a constant high idle. Found no voltage at the idle valve. Replaced the idle relay and got an intermittent high idle.

Cleaned the idle control valve, and it became less frequent.

Lubricated all linkage pivots, cleaned around the throttle stop, and cleaned the throttle body and throttle valve, (you will be amazed at what's there) and it became less frequent.

Replaced the throttle position switch, and cleaned the grounds a) at the right lower corner of the dash (you will need to check the location for the 126) and b) on the engine hoist point. Problem has not recurred.

So you have an interesting choice. It's almost impossible to tell if it's a throttle closing problem or the relay, if it's intermittent. So you can start chasing either depending on whether you want to spend money or tinker.

__________________
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