Coolant Reservoir Cap Pressure

Cutaway of radiator cap and reservoir.

Coolant reservoir cap pressure.

But they do a lot more than just that. A cap that is no longer sealing could also cause a leak. Caps are designed to ensure that the prescribed pressure on the cooling system is maintained at all times during operation by allowing air to escape in case of overpressure. The same way a pressure cooker increases the boiling temperature of water.

How does this simple cap do this. If the coolant reservoir cracks or breaks due to old age or overheating it will leak. At operating temperature coolant can evaporate from the open system. If your cap isn t venting this causes excess pressure in the cooling system and can cause leaks or a collapsed radiator hose.

The cap is actually a pressure release valve and on cars it is usually set to 15 psi. Coolant goes into the reservoir tank as it expands. While you re in there check to make sure your coolant overflow tank is working properly. In case of a faulty radiator cap the required pressure fails to build up inside the system leading to overheating.

The radiator cap and expansion tank cap make sure the filling hole of the radiator and expansion tank are sealed off gastight. The constant need to add coolant can also be caused by a leak somewhere else on the engine so a proper diagnosis is recommended. The radiator cap actually increases the boiling point of your coolant by about 45 f 25 c. The radiator cap releases the extra pressure by sending some coolant into the overflow tank.

If you have a bad radiator cap the coolant could get released too quickly and cause the reservoir to overflow.