The radiator or heat exchanger (or expansion tank) has a pressure control cap with a normally closed valve. The cap is designed to permit a pressure in the cooling system equal to the rating stamped on the top of the cap. A cap with a 9 on the top allows the cooling system to develop 62 kPa (9 lb/in. 2 ) before the valve opens. This system pressure raises the boiling point of the coolant and reduces coolant loss. The maximum allowable coolant temperature, regardless of the pressure cap used, is 99 ° C (210 ° F).
