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The closed system eliminates practically all use of air in the system and ensures the radiator; heater and engine block
are always in a coolant environment. Coolant is added through the reservoir and not through the radiator filler neck,
eliminating the need to remove the pressure cap routinely. This practicality led to the closed system being used on
most cars today.
Note: Although routine removal of the cap is not recommended, it should be replaced just as often as open cooling
system caps.
THE PRESSURE CAP
The pressure cap is the safety valve of the cooling system. It allows for working temperature above boiling point and it
limits the amount of pressure build-up for safety.
The pressure cap is comprised of the following components:
1. The cap itself, which has two ears to allow for engagement with the filler neck cams. As system pressure is
increased the ears are retained more tightly on the cams. The filler neck cams are designed so that the pressure valve
is pressed on the filler neck with exactly the right amount of pressure. The cams also have a safety stop.
2. A spring disc diaphragm seals against the top of the filler neck and provides friction to hold the cap in place. On
closed systems, an upper sealing gasket seals the atmospheric pressure in.
3. A stainless steel pressure valve spring seals the pressure valve against the lower sealing seat of the filler neck.
Thus permitting pressure to build up as the coolant gets hotter.
4. In the pressure valve itself there is a vacuum relief valve which is either weighted in an open position or closed with
a spring.
THE CLOSED TYPE OR CONSTANT PRESSURE TYPE OF VACUUM RELIEF VALVE
The vacuum relief valve is kept closed by a bronze or stainless steel spring while the engine works and pressure
builds. When the engine is turned off and begins to cool off, a partial vacuum forms which opens the valve to release
excess vacuum pressure in the system.
THE OPEN TYPE OR WEIGHTED TYPE OF VACUUM RELIEF VALVE
The pressure vent type of cap makes use of the vacuum relief valve. It is built in such a way that the vacuum valve
hangs loose and is weighted down with a calibrated weight.
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