my 300ce was doing the same thing, it turned out to be the linkage bowden cable was not screwed into the right position, which wouldn't let the linkage press on the micro switch (the switch that makes the car know that the engine should in idle)...dont know if your model even has this cable and switch, but its worth looking into.

__________________
1991 300CE Sportline
1983 300SD TurboDiesel (Gone but not forgotten)