Monitoring description

The coolant thermostat monitoring is done to detect a slow warm-up due to heat losses through thermostat and radiator. It is based on the comparison of the measured ECT sensor signal and the calculated ECT model (TCO_SUB).

The ECT model calculation is depending on the speed of the water pump, engine load and ambient temperature.

A malfunctioning coolant thermostat is detected, if the calculated ECT model has exceeded the threshold 1 (P0128) and the measured ECT sensor signal remains below threshold 2 (P0128).

Before a malfunctioning coolant thermostat is entered into failure memory, the conditions concerning low load, deceleration duration and IAT during the monitoring are checked. If the monitoring conditions are met, the coolant thermostat is entered into failure memory. Otherwise the coolant thermostat monitoring is inhibited for this driving cycle.

Example of monitoring Method: 

Fig 1: Engine Cooling System Monitoring Graph

Courtesy of BMW OF NORTH AMERICA, INC.

A comparison between the measured engine coolant temperature (ECT) and the fault detection criteria temperature is done after a specific time interval. The interval itself is based on the engine coolant temperature model.

As soon as the model temperature exceeds the thermostat regulation (opening) temperature and all other monitoring conditions are fulfilled at the same time, a valid diagnosis occurs.

At that time, if the measured engine coolant temperature is higher than Fault detection criteria (thermostat regulation temperature -11°K), the thermostat is concluded as normal thermostat.

On the contrary, if the measured coolant temperature is lower than Fault detection criteria (thermostat regulation temperature -11°K), the thermostat is concluded as opened stuck thermostat.

The thermostat regulation (opening) temperature is determined by the hardware. It is always 97°C. The fault detection criteria at the thermostat is therefore 86°C.

The ECT-sensor is not directly at the thermostat. This results in a temperature difference between ECT and Thermostat. Therefore we use calculated models: