Coolant Reservoir Cap Pressure

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

Coolant reservoir cap pressure.

How does this simple cap do this. The radiator cap releases the extra pressure by sending some coolant into the overflow tank. 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. A reservoir is under pressure in normal circumstances in order to increase the coolant s boiling point.

The constant need to add coolant can also be caused by a leak somewhere else on the engine so a proper diagnosis is recommended. While you re in there check to make sure your coolant overflow tank is working properly. A cap that is no longer sealing could also cause a leak. If your cap isn t venting this causes excess pressure in the cooling system and can cause leaks or a collapsed radiator hose.

Another symptom of a potential problem with the coolant reservoir is coolant leaks. The radiator cap actually increases the boiling point of your coolant by about 45 f 25 c. If the coolant reservoir cracks or breaks due to old age or overheating it will leak. But they do a lot more than just that.

Cutaway of radiator cap and reservoir. At operating temperature coolant can evaporate from the open system. The radiator cap and expansion tank cap make sure the filling hole of the radiator and expansion tank are sealed off gastight. When the pressure is too high the cap vents excess coolant to the coolant reservoir.

In case of a faulty radiator cap the required pressure fails to build up inside the system leading to overheating.