Thermostat or both bad (archive)

The engine temp should be normal at highway speeds. If the temp needle barely registers at high speed, I'd suspect the thermostat. A bad heater valve wouldn't cause that symptom. Even if the coolant is warm enough to run the heater, it's not at the correct temperature for the engine.

It's possible that both the thermostat and the heater valve are bad.

Questions: If you've been driving at high speed, and are idling, how long does the temp needle take to get up to a correct level? How long before you get good heat in the cabin?

If you get good heat within 30 sec. or so, I would guess the heater valve is part of your problem, because the coolant is sufficiently warm, but the valve isn't diverting it to the heater core.

86 535i. Cold day today. Heat would only come on when car idled in traffic. At highway speed heat would almost cut off, then return at stop light and low speed. Coolant level OK. Temp gauge barely registered at highway speed(rather than normal mid-range), but would increase when car idled. Have checked archives...disagreement over whether it's the thermostat or the heater valve. This past summer did replace thermostat, and lower temp. switches for aux. fan. What do you think? Thermostat broken or heater valve needs rebuild? Or....? Thank you.

Follow Ups:

[ Follow Ups ] [ 5-series (E28) Message Board ] [ Msg. Board FAQ ]

Questions, comments, or problems, please visit the Roadfly help desk.