Lesson 5/Learning Event 1
out of the cylinder to open the thermostat valve. Tension to hold the valve closed is supplied by a spring. The
pellet-type thermostat is not sensitive to pressure, so it works well in a pressurized cooling system.
Thermostats that are used in military vehicles generally start to open at about 180F of coolant temperature.
This may vary a little from one engine model to another, depending on the engine design. However, the
boiling point of the coolant must always be considered when determining just how hot the engine should run.
If the coolant is allowed to boil, it will turn to steam and lose its cooling qualities. Therefore, the coolant
temperature must always be kept safely below its boiling point (water boils at 212F at sea level).
When military vehicles are used in extremely cold climates, they are generally equipped with a radiator cover to
help the thermostat raise the engine temperature. The radiator cover may be a piece of canvas or it may be
metal shutters. Either type of cover can be closed varying amounts to reduce the amount of cooling air that
flows through the radiator core.
Pressure Radiator Cap
It has been found that increasing the amount of pressure on water will raise its boiling point. Therefore, most
liquid-cooled engines are designed to operate with a pressure in the cooling system to provide a greater
difference in the coolant boiling point and operating temperature. Each pound of pressure on the water in the
cooling system will raise the coolant's boiling point about 3. The pressure is regulated by a pressure-type cap
on the radiator.