A few days ago, my 1992 745 non turbo over heated on my way in to work. It was one of those very very cold mornings, about 0 degrees F, minus a couple or so. The car never put out any heat, but the gauge just sky rocketed right up to the red zone. Sounds like classic symptoms of a bad thermostat, but new coolant, pressure cap on the expansion tank and 92C degree thermostat last fall. No visible leaks and coolant visible in the tank.

I babied her home and took the truck to work. Took off the thermo next day, (quite a bit warmer now, near freezing), and tested it in boiling water, and could clearly see that it opened and closed as the water heated or cooled. (Used ice cubes) Put it back together, now it runs fine. What gives? Am somewhat leery about why this happened, could there be any other cause? Stuck heater control valve perhaps? Would this cause the same symptoms of overheating without any heat? Maybe the radiator is plugged up and it being cold the slushy antifreeze could not circulate?? Plastic tank on the rad, maybe time to replace...but cant pony up the $225 for a new til I am sure of a cure. Will replace the heater valve in the meantime, that one is cheap and easy. If I do end up going the rad replacement route, what is the best brand replacement? All metal, or plastic/ aluminum or brass???

I would appreciate any input that anyone might be able to offer

Thanks in advance

Steve Martinsen in Northwest CT

PS starts and runs fine otherwise and with only 150k miles ready for more.