The idle speed actuator could be it. Start there but know it might not be the problem. We carry these in partsshop. Why use the dealer? They do not know what the part even is.

I have seen many idle speed control units fail at the same time because of too much amperage being pulled from the actuator.

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Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM