Radiator pressure caps are marked indicating the amount of pressure held in the cooling system by the pressure valve's spring. For domestic vehicles, the pressure the stated in pounds per square inch (psi).Radiator caps for older imported vehicles may marked "0.9", which indicates that the pressure rating the cap is 0.9 times normal atmospheric pressure. Sine atmospheric pressure is 14.7 psi, a 0.9 cap has a pressu