Radiator Pressure Cap
pressure cap (fig. 6-6)
is used on nearly all of the modern engines. The radiator cap locks onto the radiator tank
filler neck Rubber or metal seals make the cap-to-neck joint airtight. The functions of
the pressure cap are as follows:
the top of the radiator tiller neck to prevent leakage.
system to raise boiling point of coolant.
excess pressure to protect against system damage.
a closed system, it allows coolant flow into and from the coolant reservoir.
cap pressure valve consists of a spring-loaded disc that contacts the filler neck. The
spring pushes the valve into the neck to form a seal. Under 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.
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 hoses.
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.
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.