Figure 6-6.Radiator pressure cap.
pressure, the boiling point of water increases. Normally
water boils at 212°F. However, for every pound of
pressure increase, the boiling point goes up 3°F.
Typical radiator cap pressure is 12 to 16 psi. This
raises the boiling point of the engine coolant to about
250°F to 260°F. Many surfaces inside the water jackets
can be above 212°F.
If the engine overheats and the pressure exceeds the
cap rating, the pressure valve opens. Excess pressure
forces coolant out of the overflow tube and into the
reservoir or onto the ground. This prevents high
pressure from rupturing the radiator, gaskets, seals, or
The radiator cap vacuum valve opens to allow
reverse flow back into the radiator when the coolant
temperature drops after engine operation. It is a smaller
valve located in the center, bottom of the cap.
The cooling and contraction of the coolant and air in
the system could decrease coolant volume and pressure.
Outside atmospheric pressure could then crush inward
on the hoses and radiator. Without a cap vacuum or vent
valve, the radiator hose and radiator could collapse.
Always remove the radiator cap slowly
and carefully. Removing the radiator cap from
a hot pressurized system can cause serious
burns from escaping steam and coolant.
The water pump is an impeller or centrifugal pump
that forces coolant through the engine block, cylinder
head, intake manifold, hoses, and radiator (fig. 6-7). It
is driven by a fan belt running off the crankshaft pulley.
The major parts of a typical water pump include the
Figure 6-7.Water pump.