fins. Heat is carried away from the tubes and fins by the
seat prevents leakage between the cap and the filler
movement of the air through the core.
Connections must be provided to carry the water
The pressure valve acts as a safety valve to relieve
from the engine water jacket to the radiator, and from
extra pressure within the system. The cooling system
the radiator back to the engine. Vibration and
may be designed to operate at various pressures, 4 to 17
movement between the radiator and the engine could
psi, depending on the manufacturer's specifications.
cause breakage of the metal pipe. For this reason,
The pressure valve in the cap is preset by the
flexible hose is used for radiator connections.
manufacturer. When replacing a pressure cap, make
Sometimes, pieces of pipe are placed in sections of
sure you use a cap with the proper pressure setting,
long hoses. This reduces the danger of hose collapse
which is usually marked on the top surface of the cap.
and makes a stronger connection.
The vacuum valve opens only when the pressure
within the cooling system drops below the outside air
Radiator Pressure Cap
pressure as the engine cools down. This automatic
action of the vacuum valve prevents collapse of the
The radiator pressure cap (fig. 5-4) is used on
hose and the radiator.
nearly all modern engines. The pressure cap closes off
the overflow pipe and prevents loss of coolant during
normal operation. It also allows a certain amount of
pressure to develop within the cooling system. The
Always remove the radiator cap slowly and
pressure raises the boiling point of the coolant
carefully. Removing the cap from a hot,
approximately 3 degrees for each psi (pound per square
pressurized radiator can cause serious burns from
inch) and permits the engine to operate at higher
escaping steam and coolant.
temperatures without loss of coolant from boiling.
The pressure cap contains two spring-loaded
valves. The larger valve is called the pressure valve
and the smaller one is called the vacuum valve. A
The water pump is the heart of the cooling system.
shoulder in the radiator filler neck provides a seat for
the bottom of the cap assembly, and a gasket on this
Most engines use a centrifugal water pump, like the
Figure 5-4.--Radiator pressure cap.