Thermostat: Description and Operation

The engine cooling thermostats are wax pellet driven, reverse poppet choke type. They are designed to provide the fastest warm up possible by preventing leakage through them and to guarantee a minimum engine operating temperature of 88 to 93° C (192 to 199° F). They also automatically reach wide open so they do not restrict flow to the radiator as temperature of the coolant rises in hot weather to around 104° C (220° F). Above this temperature the coolant temperature is controlled by the radiator, fan, and ambient temperature, not the thermostat.
The thermostat is operated by a wax filled container (pellet) which is sealed. When heated coolant reaches a predetermined temperature, the wax expands enough to overcome the closing spring and water pump pressure, which forces the valve to open.
Coolant leakage into the wax pellet will cause a thermostat to fail open. Do not attempt to free-up a stuck open thermostat.