Coolant Flow

Coolant Flow

Thermostat closed
From the coolant pump - via a passage into the engine block - up through the cylinder heads - through the thermostat housing's lower outlet - through the hose to the heat exchanger - through the return hose from the heat exchanger - back to the pump.

With the thermostat normally open
From the coolant pump - via a passage into the engine block - up through the cylinder heads - through the thermostat housing's upper outlet (past the thermostat) - through the radiator - back to the pump.
A small quantity of coolant passes through the thermostat's lower outlet to the heat exchanger and back to the coolant pump.

With the thermostat fully open
The thermostat closes the lower passage through the thermostat housing thus forcing the full flow of coolant through the radiator.