This radiator coolant tank cap seals the filler neck of the cooling system's expansion tank. It maintains the system's operating pressure, raising the boiling point of the coolant to prevent cavitation at the water pump. A worn or leaking cap allows coolant loss and air ingress, which leads to localized overheating and reduced cooling efficiency. The cap's pressure-relief valve opens at the specified rating to protect the radiator and hoses from overpressure. The manufacturer constructs the cap from materials rated for sustained exposure to hot coolant and steam. On reinstallation, ensure the sealing gasket sits cleanly in the neck and the cap is turned fully to its locked position.