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 pressure raises the
boiling point of the coolant approximately 3 degrees for
each pound and permits the engine to operate at higher
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 shoulder
in the radiator filler neck provides a seat for the bottom
of the cap assembly and a gasket on this seat prevents
leakage between the cap and the filler neck.
The pressure valve acts as a safety valve to relieve
extra pressure within the system. The cooling system
may be designed to operate at various pressures between
4 and 17 psi, depending on the manufacturers
specifications. The pressure valve in the cap is preset by
the manufacturer. When replacing a pressure cap, make
sure you use a cap with the proper pressure setting that
is usually marked on the top surface of the cap.
The vacuum valve opens only when the pressure
within the cooling system drops below the outside air
pressure as the engine cools down. This automatic
action of the vacuum valve prevents collapse of the
hoses and the radiator.
Because it has a sweet taste, animals and
children sometimes ingest spilled coolant. The
lead content that antifreeze absorbs while in use
makes it a hazardous waste and it cannot be
disposed of by being dumped on the ground. It
must be containerized and turned in for
The water pump is the heart of the cooling system.
Most engines use a centrifugal water pump (fig. 1-46)
that provides a large volume capacity and is nonpositive
in displacement. This type of pump has an impeller with
blades that force the coolant outward as the impeller
rotates. The shaft on which the impeller is mounted is
usually driven by a fan belt and revolves in a bushing or
in ball bearings inside the housing. For different cooling
systems, pumps vary considerably in construction of
seals, bearings, mounting, and drive.
ALWAYS REMOVE THE RADIATOR
CAP SLOWLY AND CAREFULLY. Remov-
ing the cap from a hot, pressurized radiator can
cause serious burns from escaping steam and
COOLANT AND ANTIFREEZE. Since water
is easily obtained, is cheap, and has the ability to transfer
heat readily, it has served as a basic coolant for many
years. Some properties of water, such as the boiling
point, freezing point, and natural corrosive action on
metals, limit its usefulness as a coolant. This is counter-
acted by the use of an antifreeze. Manufactured under
many different trade names, the most commonly used
type of antifreeze is ethylene glycol. Ethylene glycol is
a chemical compound composed of a mixture of
ethylene and glycerine derivatives. Maximum freezing
protection is achieved by mixing 60% ethylene glycol
with 40% water. This mixture protects the cooling
system to a temperature as low as minus 62°F. Ethylene
glycol has a very high boiling point, does not evaporate
easily, is noncorrosive, and is practically nonflammable.
Figure 1-46.-Water pump.