Theory Of Operation

The Coolant Thermostat is a flow regulating valve that separates the engine cooling circuit from the radiator cooling circuit governed by coolant temperature. The coolant thermostat's purpose is to control the coolant flow to regulate the engine coolant at a predetermined temperature. A properly functioning thermostat allows the coolant temperature in the engine part of the cooling system circuit to warm up quickly to the desired temperature. Once the desired coolant temperature is reached, the thermostat controls the coolant flow from the engine to the outer, radiator circuit to maintain a constant coolant temperature. The thermostat is made to only open fully at the desired coolant temperature. A low engine coolant temperature results in high hydrocarbon and CO engine-out emissions due to relatively low temperatures of the combustion chamber walls. Due to this emissions increase it is necessary to detect a malfunctioning thermostat. The diagnostic is conducted during the warm-up phase of the engine as well as during steady state engine operations to monitor any potential coolant temperature drop below a defined temperature limit.

This diagnostic detects Thermostat stuck open failures. This can result in coolant warm-up delay or coolant temperature drop during engine operation. The warm-up behavior test monitors the measured engine coolant temperature for warm-up after engine start. The measured temperature is compared to a model of engine coolant temperature based on normal warm up characteristics. The steady-state test monitors engine coolant temperature drop during engine operation. The described rationality checks will detect coolant thermostat (coolant temperature below thermostat regulating temperature) failures.