Coolant Reservoir Cap Pressure

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.

Coolant reservoir cap pressure.

The radiator cap releases the extra pressure by sending some coolant into the overflow tank. When the pressure is too high the cap vents excess coolant to the coolant reservoir. A cap that is no longer sealing could also cause a leak. The same way a pressure cooker increases the boiling temperature of water.

At operating temperature coolant can evaporate from the open system. The constant need to add coolant can also be caused by a leak somewhere else on the engine so a proper diagnosis is recommended. But they do a lot more than just that. In case of a faulty radiator cap the required pressure fails to build up inside the system leading to overheating.

How does this simple cap do this. If you have a bad radiator cap the coolant could get released too quickly and cause the reservoir to overflow. The radiator cap actually increases the boiling point of your coolant by about 45 f 25 c. The radiator cap and expansion tank cap make sure the filling hole of the radiator and expansion tank are sealed off gastight.

Another symptom of a potential problem with the coolant reservoir is coolant leaks. Coolant goes into the reservoir tank as it expands. The cap is actually a pressure release valve and on cars it is usually set to 15 psi. Cutaway of radiator cap and reservoir.

While you re in there check to make sure your coolant overflow tank is working properly.