Lesson 5/Learning Event 2
FIGURE 76. PRESSURE RADIATOR CAP.
The pressure radiator cap has two spring-loaded valves that are normally in the closed position. The largest of
the two valves is the pressure valve, and the smaller is a vacuum valve. When the cap is installed in the
radiator filler neck, it seals at the bottom as well as at the top of the filler neck.
When the engine is cold, there is no pressure in the cooling system. As the water in the cooling system warms
up, it expands. Since the cooling system is sealed by the cap, the water expansion causes pressure to build up.
When the pressure builds up enough to overcome the spring tension holding the pressure valve closed, it lifts
the valve. The excess pressure escapes by the pressure valve and out through the radiator overflow pipe. The
amount of pressure the cooling system is able to build up depends on the strength of the pressure valve spring.
In some vehicles as much as 15 PSI is developed in the cooling system.