We have noted more M54 engine failures due to overheating than on any of BMW's previous engines. We believe this is due to a combination of factors including the electronic thermostat and high operating temperatures. According to BMW's technical literature, the factory thermostat is computer controlled to operate from 80 ° C to 103 ° C. BMW also reports that a mechanical failsafe does not open until the coolant temperature reaches 110 ° C. That's a 230 ° F temperature band, which may also contribute to engine failures.

Here's some background on map-controlled (electronic) thermostats from DuPont:

"In conventional cooling systems a wax element (this is a mechanical thermostat) keeps the coolant temperature close to about 90 °C. This control method ensures that the motor does not overheat even under extreme conditions, such as very high vehicle speeds, very high ambient temperatures and heavy loads. However, in normal conditions a petrol engine can generally be operated at 110 °C without danger. This high temperature improves tribological conditions in the motor, which in turn reduce friction losses, fuel consumption and exhaust emissions. An electronic map-controlled thermostat can satisfy such variable requirements, because it can adapt the coolant temperature to the motor’s operating conditions within broader limits. With the motor at partial load it maintains the coolant temperature at a consistently high level; at full load, high engine speeds or high outside temperatures it reduces the temperature and thus ensures performance characteristics which match these conditions."

So, when the coolant reaches a certain temperature, a mechanical thermostat opens. The map-controlled thermostat is “variable” based on input from the engine computer. It runs the engines hot (which is why the radiators also fail) for emissions. However, when the electronic thermostat fails, it may continue to run the engines hot without opening up “at full load, high engine speeds or high outside temperatures.” This might be satisfactory if the driver were alerted to the fact that the thermostat had failed. A trouble code is set in the engine computer, which can be read with a diagnostic tool, but nothing appears on the instrument cluster. So, a failed thermostat can take the engine with it.

Our replacement unit will set the same error code, but it does not turn on any lights.

Our M54 Competition Thermostat Plate Kit, a less expensive alternative to our M54 Competition Thermostat Kit, is designed to return the selection of normal engine operating temperature to the owner/builder and improve engine longevity by replacing the failure prone electronically-controlled thermostat with a mechanical unit. (Replaces BMW part no. 11 53 7 509 227)

The cover on the sensor plug harness may require removal to clear the fan once installed