First thing is to make sure your throttle linkage is set correctly, and that the microswitch that engages the idle circuit is indeed being actuated when the accelerator is let up.

If that is working properly, I would start checking for vacuum leaks. There are several places for those on an M103.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition