How to Troubleshoot the Thermostat on a BMW 735I


1. Start the engine and turn the heat and fan blower on full blast. Allow the engine about 10 minutes to heat up. Inspect the windshield for a greasy fog. Inspect the floorboards on the passenger side of the BMW for radiator fluid. If this is the case, the cabin of the vehicle will not warm up and the vents will be blowing cool or lukewarm air. If you notice a greasy fog on the windshield or radiator fluid on the floorboards, the issue lies with the heater core and not the thermostat. The heater core will need to be replaced or repaired. If this is not the case and no fog or fluid is found, the thermostat is the likely problem.
2. Remove the thermostat cover and pop the thermostat out of the intake manifold if the engine has been overheating. Place the cover back onto the intake and allow the engine to run for roughly 10 minutes. A thermostat is designed to 'fail safe.' This means the thermostat will fail in a manner that allows the engine to run too cool and not too hot. If you remove the thermostat from the engine, it will ensure the engine will run cool because radiator fluid will not be restricted due to the missing thermostat. If the engine still overheats, the intake manifold and/or coolant veins inside of the engine have clogged due to sludge buildup. Flushing the coolant system on the BMW should resolve the overheating issue. Since a thermostat costs only a few dollars, it's a good idea to install a new thermostat instead of the old one when you put it back into the intake manifold.
3. Start the engine and allow it to run for 10 minutes. Turn the heat on high and turn the fan blower on high. Inspect the amount of heat blowing out of the vents. If no heat or very little heat is blowing out of the vehicle and there is no fluid on the floorboards or fog, it means the thermostat has 'failed safe' and the thermostat will need to be replaced.