Radiator Cap
A pressurized cap that seals the cooling system, raising the boiling point of the coolant and venting excess pressure to the overflow tank.
The radiator cap is a small but highly engineered valve that seals the vehicle's cooling system. Far more than a simple cover, the radiator cap is designed to maintain a specific pressure within the cooling system (typically 12 to 16 PSI). By keeping the coolant under pressure, the cap raises its boiling point by about 45 degrees Fahrenheit, preventing the engine from boiling over during normal operation.
The cap contains two internal spring-loaded valves. When the coolant heats up and expands, generating pressure that exceeds the cap's rating, the pressure valve opens, allowing excess coolant to flow into the expansion tank. As the engine cools down and pressure drops, the vacuum valve opens, drawing coolant back from the expansion tank into the radiator.
If the radiator cap's rubber seal degrades or the spring weakens, the system will fail to hold pressure, leading to low boiling points, coolant loss, and engine overheating. A failed cap can also cause radiator hoses to collapse under vacuum. Replacing a bad radiator cap is a quick and inexpensive fix for minor cooling issues.