It's been a tense few weeks since I lasted posted the thread about my W124 overheating. Since then I had replaced the thermostat, radiator cap, and just recently, the A/C aux fan switch.

That would pretty much cover every major component in the cooling system (water pump, temp sensor, radiator, and expansion tank were replaced within the last 6 months).

SO WHAT gives?????!!!!!!

Well, I suddenly remembered that even though temps remained nominal, even while 104 outside, the coolant would still boil over...the radiator cap would rattle while coolant spilled over...

WAIT A MINUTE...THE RADIATOR CAP RATTLES???

I had taken that cap off dozens of times to refill the tank with water and coolant, but NEVER TURNED THE CAP ALL THE WAY CLOSED!!!!

So this afternoon, I turned it fully clockwise until it seated firmly, and guess what???

NO OVERHEATING...AND NO SPILLED COOLANT!!! By the way it's 101 today, and the A/C was chugging at full tilt!

I told my wife about it excitedly, who just glared at me like I was some idiot...okay, I was some idiot!

Anyway, I dodged the blown head gasket paranoia for at least a little while...maybe I'd better save up for the inevitable, for when it DOES happen down the road...

Thanks to all who helped. I now know more about the cooling system than I ever cared to!

Maybe I can stop starting threads on the tech forum for awhile and go play in the what-cool-parts-I-can-add-to-the-car forum...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car