A pellet type thermostat, in the coolant outlet passage, controls the flow of the engine coolant, in order to provide fast engine warm-up. The thermostat also regulates the coolant temperature. A wax pellet element in the thermostat expands when heated. The wax pellet contracts when cooled. The pellet is connected through a piston to a valve. When the pellet is heated, pressure exerts against a rubber diaphragm to force the valve open. As the pellet cools, the contraction allows a spring to close the valve. The valve remains closed while the coolant is cold, preventing circulation of coolant through the radiator. The coolant circulates throughout the engine with the thermostat closed. The engine warms up quickly and evenly. As the engine warms, the pellet expands and the thermostat valve begins to open. Coolant now flows through the radiator. The thermostat operation permits enough coolant to enter the radiator in order to keep the engine within the normal operating temperature limits. The thermostat curently used on the S/T Trucks has a nominal rating of 91°C (195°F).