Radiator Pressure cap

The radiator pressure cap is more than a cap for your radiator; it keeps your engine cool by sealing and pressurizing the coolant inside it. It is planned to hold the coolant in the radiator under a determined amount of pressure. If it was not kept under pressure, it would start to boil, and soon all the coolant would have boiled away. However, the radiator pressure cap prevents this from happening by exerting enough pressure to keep the coolant from boiling. If your cooling system is under too much pressure, it can “blow its top”! To prevent this, the radiator pressure cap has a pressure relief valve. The valve has a predetermined rating that allows it to take just up to an assured amount of pressure. When you turn the radiator cap on the radiator filler neck of the radiator, you seal the upper and lower sealing surfaces of the radiator filler neck.

The radiator pressure cap actually increases the boiling point of your coolant by about 25 C. How does this simple radiator cap do this? The same way a pressure cooker increases the boiling temperature of water. The radiator cap is actually a pressure release valve, and on trucks it is usually set to 15 psi. The boiling point of water increases when the water is placed under pressure.

The pressure relief valve spring is squashed against the lower seal when you lock the radiator cap. The filler neck has an overflow tube right between the two sealing surfaces. If the pressure in the cooling system exceeds the preset rating of your radiator cap, its pressure relief valve allows the lower seal to be lifted from its seat. Then the overload pressure (coolant, air) can squish through the overflow tube to the ground or the coolant reservoir.

What time the fluid in the automotive cooling system heats up, it expands, causing the force to build up. The radiator cap is the only place where this pressure can run away, so the setting of the spring on the radiator cap determines the highest pressure in the automotive cooling system. When the pressure reaches 14 -15 psi, the pressure pushes the valve open, allowing coolant to run away from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This collection keeps air out of the system. When the radiator cools back down, a vacuum is twisted in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was excluded.