The thermostat controls the coolant flow through the inlet side of the engine. As the thermostat opens, cold coolant from the radiator passes through the thermostat. This causes the thermostat to close, until the thermostat again reaches normal operating temperature.

The slow bleeding of cold coolant into a hot engine helps to prevent thermal shock that results from a sudden rush of the cold coolant. The thermostat remains open after the entire cooling system has reached normal operating temperature.

Inspect the engine coolant thermostat opening temperature.