What are you referring to or what is the symptom? Core plug? Pressure relief valve?
Core plugs are installed in the engine block after the sand from the casting mold is removed. Sometimes they corrode through and leak coolant. If there's not enough antifreeze in the coolant, it could freeze in cold weather. As the ice expands it often pushed the plugs out before it cracks the block. For this reason, some people call these "freeze plugs".
Radiator caps have a pressure relief valve. The cap holds pressure in the cooling system as the engine warms up. This raises the boiling point of the water in the coolant. Once the design pressure is reached, typically 15 psi, the pressure relief valve opens to let the expanding coolant flow into the reservoir bottle. Later, when the coolant cools, a vacuum valve in the cap opens to allow coolant to be drawn back into the radiator. That prevents vacuum from building in the system that would collapse rubber hoses and the radiator tubes.
There is a pressure relief valve in the air conditioning system in case pressures go too high. The plug will blow out in the receiver drier under the hood rather than risking rupturing the evaporator in the dash board.
Tuesday, April 7th, 2009 AT 10:16 PM