This cap seals the opening on the coolant expansion tank, maintaining system pressure and preventing coolant loss. It sits on the tank neck and allows the cooling circuit to pressurize as the engine warms, which raises the coolant's boiling point. A failing seal or damaged cap causes pressure loss, leading to coolant boil-over and potential engine overheating. The cap is manufactured to the original sealing geometry so it mates correctly with the tank's filler neck and retains the specified pressure rating.