I have a 1998 BMW 540i with no heat. When I start it and drive for a while the temp gauge will rise almost into the red zone and then it drops back down to normal. After this happens , then I have some heat but not like it should be.
First I checked the coolant level. It was Full. Then I pressure tested the coolant system to make sure it did not have a leak and the customer might have just been filling it. No leak found. Took temperature readings at the upper and lower radiator hose only to find there was a big difference in temperature. This was an indication of a failed thermostat. Replaced thermostat with one from dealer. This thermostat is a complete housing with the temperature sending unit built in. This took care of the problem . Customer happy. Parts and Labor cost $300.00 + tax