No matter what, replace the thermostat and the cap first. If problems persist, then look elsewhere. When very hot and in neutral with the engine at 2000 or so, you should be able to here it come in and out...

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Benzmac:
Donnie Drummonds
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM