Coolant Reservoir Cap
A pressure-rated cap on the coolant reservoir or radiator that regulates pressure in the cooling system.
A coolant reservoir cap (or radiator cap) is a critical safety device designed to regulate pressure inside the engine cooling system. By sealing the system, the cap allows the coolant pressure to rise as the engine warms up, typically to between 12 and 16 PSI. According to physical laws, raising the pressure increases the boiling point of the coolant, preventing the engine from boiling over during heavy use.
The cap contains a spring-loaded pressure relief valve and a vacuum valve. If the cooling system pressure exceeds the cap's rated limit, the spring compresses, opening the valve to route excess hot coolant into the overflow recovery tank. As the engine cools down and pressure drops, the vacuum valve opens to draw the coolant back into the radiator, preventing the hoses from collapsing.
A failing reservoir cap can cause serious problems: if it fails to hold pressure, the coolant will boil at a lower temperature, leading to engine overheating and coolant loss. If it fails to release pressure, the excessive pressure can blow out radiator tanks, water pumps, or coolant hoses. Testing the cap with a pressure tester is a standard diagnostic step.