Coolant Cap Pressure

We get asked this question daily and it seems most people misunderstand what the pressure cap does.

Coolant cap pressure. If you have a bad radiator cap the coolant could get released too quickly and cause the reservoir to overflow. 18 pounds is the pressure most radiator caps hold. Also online retailers can provide caps but be sure that you know the correct rating for your vehicle before you buy a radiator cap. How to get a new radiator cap with the proper pressure rating.

The radiator cap allows pressure to build up in the cooling system and will eventually vent that pressure to the overflow bottle as the need arises. Avoid pressure above 18 pounds so you do not damage the cooling system by causing a rupture. But it needs to go somewhere so the cooling system s pressure builds to the maximum pressure limit of the radiator cap s valve. The tank has enough space to contain the extra coolant even if it is a bad day with extremely hot weather.

If there were not a radiator pressure cap installed the system would draw in air. The cap is the only place where this pressure can escape so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the fluid pressure inside the radiator exceeds 15 psi it forces the valve open allowing heat to escape and excess coolant fluid to overflow into the tanks on either side of the radiator. When the pressure reaches 15 psi the pressure pushes the valve open allowing coolant to escape from the cooling system.

The radiator cap acts as a release valve set to open at the maximum pressure point. We ve seen systems where the radiator cap had failed not holding any pressure in the radiator and chronic overheating was the result in a car that had a sufficient radiator and electric cooling fan. Let s take a pan of water and heat it up. Check the pressure on the cap with a radiator cap pressure tester.

The water heats up and boils and 212 degrees. The cap pressure rating has nothing to do with regulating your engine coolant temperature nor will it make your engine run cooler. The cap allows the liquid that was. As the vacuum continues to build up the two way check valve which is exactly what a radiator pressure cap is goes to work.

Your pressure tester should have an adapter to test the radiator cap as well which is a good idea to keep the engine from overheating. The cap does this by a spring loaded valve which serves as a pressure relief valve at a rated pressure. The radiator cap releases the extra pressure by sending some coolant into the overflow tank. You ll notice that there s a plunger on the bottom of the cap.

While you re in there check to make sure your coolant overflow tank is working properly. Usually this is set at a density of 15 pounds per square inch psi. You can find new pressure caps for your radiator in the local auto supply store.