This is a follow up post about an overheating problem with my US spec 1980 450 SL (~240,000 miles with METICULOUS maintenance history). Everything, and I mean EVERYTHING (YES, >*EVERYTHING*< ) is working correctly – you name it & I’ve checked it. Belts, hoses, thermostat, thermal clutch, thermo switch, auxiliary fan, temperature sender, temperature gauge (calibration checked with a digital infrared thermometer), water pump flow capacity, the radiator header tanks removed to check the core, fuel mixture & ignition timing, refilled the cooling system twice to assure the removal of any air bubbles, - EVERYTHING IS FUNCTIONING CORRECTLY – yet the car still runs way too hot in all conditions: highway, city, traffic, etc.

Once it is warmed up & the heat has a chance to build up, it is running between 90 at the lowest, and usually just above 100, sometimes reaching as high as 115. YES, the thermostat is good - it is not only new, I pulled it back out and checked it in a pot of boiling water with a thermometer. It is NOT installed backward (not even possible on this engine).

This is on a car that consistently ran between 82-85 degrees, and would sometimes reach 90+ in traffic on a hot day. But now, now what? WHAT AM I MISSING HERE? Could this be a symptom of a head gasket starting to let go? That’s the only other thing I can think of, but there are no symptoms of that – the compression is good & even across all cylinders, there is no external leakage, no internal leakage, the spark plugs are burning clean, WHAT GIVES ? I am going NUTS!

Please someone help me before they haul me off to the rubber-padded room!

THANKS FOR ANY SUGGESTIONS (other than euthanasia for my baby).