General Description Of Thermostat Monitoring

The purpose of the coolant thermostat is to effect a quick engine warm up after start. The thermostat is closed after engine start to limit the coolant circulation to the radiator until the thermostat regulating temperature is reached. If the thermostat is stuck open, the coolant circulation will not be limited after start and the engine warm up time will increase. This may cause an increase in emissions.

To monitor the thermostat function, a modelled value for coolant temperature is calculated. This monitoring is used for diagnosing a leaking thermostat or a thermostat stuck in the open position. When the temperature model has reached normal operating temperature the actual coolant temperature is checked to confirm that it has been above the normal thermostat opening temperature for sufficient time. If this is not the case the thermostat is declared stuck open.

Graphs showing the diagnostic operation with typical calibration values are given below.

TCO: coolant temperature (sensor)

TCO_SUB: modelled temperature

Normal Thermostat Operation 

Fig 1: Normal Thermostat Operation - Graph

Courtesy of BMW OF NORTH AMERICA, INC.

If timer is elapsed then thermostat is declared ok.

Thermostat Failure 

Too Slow Coolant Temperature Increase 

Fig 2: Too Slow Coolant Temperature Increase - Graph

Courtesy of BMW OF NORTH AMERICA, INC.

In this case timer is not elapsed: failure is detected. The coolant temperature increase is too slow

Too Low Coolant Temperature 

Fig 3: Too Low Coolant Temperature - Graph

Courtesy of BMW OF NORTH AMERICA, INC.

When TCO_SUB crosses 85.5°C (C_TCO_TH_MIN) then decision is taken.