Radiator Pressure Cap

The different components of a spring loaded normally closed radiator cap.

The radiator pressure cap prevents coolant loss and increases the cooling system's boiling point. It contains a spring that holds pressure in the system until it reaches a specified pressure. This is because for every pound of spring pressure, the boiling point is increased by 3° F. Water boils at 212° F, increasing the pressure in a closed system increases the boiling point. This means that a cap rated at 13 lbs. will increase the boiling point in a system by 39° F. High-performance caps range from 19 – 32 psi. Manufacturers can design engines with higher operating temperatures.

The radiator cap also allows the engine's coolant to expand and contract without allowing air to enter the cooling system. The upper seal seals and protects the system at all times. After the engine warms and system pressure reaches the caps rated pressure, the pressure spring allows pressurized coolant to flow into the reservoir or coolant overflow tank. This allows for expansion of the heated fluid.

The radiator cap releases pressurized coolant to the overflow tank.

As the cap wears the spring weakens and excessive coolant is allowed to flow into the reservoir tank. This coolant overflow will result in a loss of coolant at the seal between the cap and the radiator as well as the overflow tank. If the system passes a coolant system pressure test with no leaks in the engine or passenger compartments, suspect a faulty radiator cap.

The radiator caps vacuum valve opens to allow coolant to flow from the overflow tank back into the radiator.

The cap also allows the coolant to flow back into the radiator as the engine cools. It contains a vacuum valve. As temperatures drop and the coolant contracts, a vacuum is created in the engine’s cooling system. The vacuum valve opens and allows coolant to flow from the overflow tank back into the radiator. As the pressure inside the system drops, outside air pressure helps the coolant flow in.