The temperature of the engine coolant is automatically controlled by a blocking-type thermostat located in a
housing attached to the water outlet end of the cylinder head. A single thermostat is used in the in-line engines.
At coolant temperatures below approximately 170 (77 ), the thermostat valve remains closed and blocks
the flow of coolant through the radiator. During this period, the coolant circulates through the cylinder block and
head and then back to the suction-side of the pump via the bypass tube. As the coolant temperature rises, the
thermostat valve begins to open, restricting the bypass system and permits the coolant to circulate through the
radiator. However, with the valve fully opened in the in-line engine, a very small portion of the coolant will
continue to circulate through the by-pass tube, while the major portion will pass through the radiator. A properly
operating thermostat is essential for efficient operation of the engine. If the engine operating temperature
deviates from the normal range of 160-185 (71-85 ), remove and check the thermostat.
1. Drain the cooling system to the necessary level by opening the drain valves.
2. Remove the hose connections between the thermostat housing water-outlet elbow and the radiator or
3. Loosen the bolts and remove the water-outlet elbow from the thermostat housing on the in-line engine
(Fig. 1). Take out the thermostat.
If the action of the thermostat has become impaired due to accumulated rust
Figure 1. Thermostat Housing Details and Relative Location of Parts