Here's one I REALLY can't explain. My 83 300D has been running hot all winter and spring, about 100C on the gauge. This morning on the way to work it was about 50~55 F and the car went right up to 100C as usual. This afternoon coming home it was about 80F. The damn temp guage never went over 90C all the way home, Hi-Speed, dragass in traffic, rock steady at 90C. Strange thing is this is how it ran for me last summer when I first got it. Last summer was very hot and the car never went over 90C. This winter it always ran hot. I'm quite confused and can only think it's got to do with the radiator fan. The aux fan has been hard-wired on the past couple of weeks and basically made no apprecable difference. What gives with this thing? I'm swapping out the original metal fan and clutch with a plastic one out of a 300SD and putting in a new water pump too. Strange car but fun.

I had the same problem. When it's really hot the temperature was fine (always between 85-98) but when the weather got mild (50-60F) the car ran hot (98-100*C) unless I drive the car really hard then the temperature would drop again until I drive normally where the water temp would go to 100C again.

Turned out that my T-stat was being weird. I replaced that and all everything was fine.

with the new T-Stat the car actually did the same thing again once after I didn't drive it for a few weeks (in the middle of winter)

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2008 BMW 335i Coupe

I did just change the thermostat about 3 months ago. I have yet another new one that I'm going to put in when I swap out the fan and water pump. I'm going to flush out the system too but when I drained the coolant last time it was very clean and as much as I could see of inside the radiator looked good and clean too. 100C doesn't worry me a whole lot but it will sometimes start creeping a little higher and then I worry some. I do drive the car very hard, 80 miles a day, 80 miles per hour, pedal to the metal, no abuse, just wringing out as much acceleration as I can. Oil every 3K miles and 30 MPG.