There seems to be many thoughts on how many pound pressure cap should be used on
engine cooling system. First of all the cooling system is designed to use a pressure cap
to prevent boiling of the coolant. Air in the coolant an result in the loss of coolant from
overflow when the aerated coolant is hot. The heated air expands, increasing the pressure
in the system, causing the pressure cap to open. Similarly, coolant can be displaced through
the overflow if the head gasket leaks compression gases to the coolant system. An incorrect
or malfunctioning cap can result in the loss of coolant and engine running hot. Radiator
Cap Pressure Test. Normal Duty Driving, system temperature 220 DEG F, Correct Cap ( 15 PSI ),
Light Duty Driving, system temperature 210 DEG F, Correct Cap ( 7 PSI )
Dave Atherton