Well, while I am out of the country, my wife reported strange behavior in our '90 240 sedan. The other day she was driving into town and the temp gauge went up nearly to the red and the heat didn't work. Just as she was about to turn and head home (bless her!), all of the sudden, the heat started working and the temp went down.

Now, since I can't diagnose, all I did was sit and ponder the problem. On the surface, it sounds typically like symptoms of a froze up cooling system. However, the temps were only around or slightly below freezing. And, while I haven't flushed the radiator in the past year, I'm sure the coolant is fresh enough to the point that it didn't freeze up.

After a frozen radiator, my second thought would be a stuck thermostat. But sitting here thinking of how the heater works, it takes hot water from the engine block, feeds it through the heater core and then sends it back to the water pump. Therefore, if the thermostat didn't open, I would still expect to get heat out of the system.

Does this logic hold true?