Sounds like you're referring to the radiator cap which has a pressure relief valve that will maintain up to 15 pounds of pressure on the cooling system. For every pound of pressure, the boiling point of the water in the coolant is raised three degrees. Since it's common for the coolant to get well over 212 degrees, it's possible the water is boiling out of the system and leaving as steam.
Saturday, July 9th, 2011 AT 12:25 AM