I've got a 1992 740 wagon. Recently, the heat stopped. There is air coming through the vents and the fan works fine. Also, the temperature gauge has been going into the red zone for awhile. It's at that point that I'll suddenly get some heat through the vents. Once it's fallen out of the red zone, the heat stops. I replaced the thermostat but still have this problem. Any suggestions?

 

You may have some major cooling system problem such as bad head gasket which makes you lose coolant etc. Such erratic heat would be due to uneven flow through the heater core. Check the whole thing thoroughly.