e34 535i sudden overheating

[JonE]

make sure the small "pisser" line from the coolant reservior tank isn't clogged, it should stream a bit of coolant, you'll see it if you have the coolant tank cap off.

[gale]

Mine just did the exact same thing last Friday after returning from lunch, had it warmed up to operating temp & then parked it for 45 minutes or so. The needle was on a rocket sled to the red zone. I'm hoping it's just the thermostat but fear it could be the headgasket. It did this about 4 or 5 summers ago and a new thermostat fixed it so thinking after only 30k miles, it's too soon to be the thermostat again -- don't know for sure, they fail sometimes. It cooled down right away to the 5/8 tick mark on the drive back after turning the heater full blast. I bled it after it cooled down and it was dead-nuts 12:00 o'clock on the drive home in 95F heat with the a/c on.

I'm doing the shotgun approach and ordered new t-stat, fan clutch, water pump, aux. fan switch. Cap is good. Radiator is fresh, never had trouble bleeding it. I don't have any record of when the waterpump was changed if ever, and can stop the fan at will with my fingers without nicking them. I've been running the blue stuff 50/50 distilled all along.

If it happens again "then off with its head", been waiting for an excuse to extrude-hone the intake & bore my throttle body & put a hotter cam in it anyway.