PRIN. OF INTERNAL COMBUSTION ENGINES - OD1619 LESSON 2/TASK 3
Radiator Pressure Cap.
(1) Purpose. Modern automotive vehicles use pressurized cooling systems that
allow a certain amount of pressure to develop within the system as its temperature
goes up. The increase in pressure will raise the boiling point of the coolant
proportionally, helping to prevent boilover.
FIGURE 73. PRESSURE CAP.
(2) Operation (figure 73). The cap contains two springloaded valves that are
normally closed, sealing the system. The larger of these two valves is the
pressure valve, the smaller is the vacuum valve. The pressure valve acts as a
safety valve that will vent any pressure over the rated maximum through the
overflow pipe. The vacuum valve allows air to enter the system as the engine cools
down. This is to prevent atmospheric pressure from collapsing the hoses.
i. Expansion Tank (figure 74 on the following page). Some vehicles use an
expansion tank in their cooling systems. The expansion tank is mounted in series
with the upper radiator hose. It is used to supply extra room for coolant
expansion and generally takes the place of the upper radiator tank. The radiator
pressure cap and the overflow line are also mounted on the expansion tank.