A pellet-type thermostat in the coolant outlet passage controls the flow of engine coolant to provide fast engine warm-up and regulate coolant temperatures. A wax pellet element in the thermostat expands when heated and contracts when cooled. The pellet is connected through a piston to a valve. When the pellet is heated, pressure is exerted against a rubber diaphragm to force the valve open. As the pellet cools, the contraction allows a spring to close the valve. This allows the valve to remain closed while the coolant is cold, preventing circulation of coolant through the radiator. At this point, coolant circulates throughout the engine to warm the engine quickly and evenly.
As the engine warms, the pellet expands and the thermostat valve opens. The coolant then flows through the radiator where heat is passed through the radiator walls. This opening and closing of the thermostat allows the cooling system to keep the engine in the correct operating temperature range.