Thermostat: Testing and Inspection

The thermostat controls coolant flow into the engine (inlet side). As the thermostat opens, cold coolant from the radiator passes through the thermostat causing it to close until the thermostat again reaches normal operating temperature. This slow bleeding o cold coolant into a hot engine helps prevent thermal shock due to the sudden rush of cold coolant. The thermostat will remain open after the entire cooling system has reached normal operating temperature. Inspect engine coolant thermostat opening temperature.