How does a radiator cap work?

The car’s radiator acts as a heat exchanger, transferring excess heat from the engine’s coolant fluid into the air. The radiator is composed of tubes that carry the coolant fluid, a protective cap that’s actually a pressure valve and a tank on each side to catch the coolant fluid overflow. In addition, the tubes carrying the coolant fluid usually contain a turbulator, which agitates the fluid inside. This way, the coolant fluid is mixed together, cooling all the fluid evenly, and not just cooling the fluid that touches the sides of the tubes. By creating turbulence inside the tubes, the fluid can be used more effectively.

When coolant fluid overheats, it expands, causing the fluid to become highly pressurized. When it enters the radiator, the pressure increases even more because it’s in an enclosed space. The radiator cap acts as a release valve set to open at the maximum pressure point. Usually this is set at a density of 15 pounds per square inch (psi). 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. Once the radiator cools down, the coolant fluid in the overflow tanks gets sucked back into the pump, continuing its route through the cooling system.

Cars with automatic transmissions cool transmission fluid in the same way with a separate heat-exchange circuit built into the radiator. This two-step process of cooling the transmission fluid is equivalent to a radiator within a radiator. As the heated transmission fluid enters the transmission cooler, the oil’s heat is exchanged with the coolant fluid in the radiator, making the transmission fluid cooler while heating the coolant fluid instead. Then the coolant fluid’s heat is transferred to air in the radiator itself.