The PSI (Pounds per Square Inch) rating is what pressure the radiator cap is designed to hold up to but not exceed in the cooling system.
18 psi
There should be 15-17 psi in the cooling system when warmed up under normal operation. Any more than that the radiator cap should release.
The cap is faulty, replace it.
With a radiator cap pressure tester. The tool usually comes along with a cooling system pressure test kit.
Radiator caps are pressure relief valves. They are rated at "pounds per square inch" and are designed to release some pressure from the radiator if it gets too high. If an engine overheats it can boil the coolant and this will cause the radiator cap to let out steam. Radiator caps DO lose their ability to regulate pressure and can release too soon. If the engine has not overheated, have the radiator cap tested and replaced as necessary.
Look on the radiator cap - there should be a psi rating stamped on there.
16 #
16 psi
On a 2003 Mercury Sable : The thread on cap on the engine coolant reservoir is the 16 PSI pressure cap / radiator cap
On a 1998 Ford Expedition : The thread on cap on your engine coolant reservoir is the 16 PSI pressure cap / radiator cap
On a 2001 Ford Expedition : The thread on cap on your engine coolant reservoir is the ( 16 PSI ) pressue cap / radiator cap
On a 2002 Mercury Sable : The thread on cap on your engine coolant reservoir is the 16 PSI pressure cap / radiator cap
It should be printed on the radiator cap. Most are around 16 psi.It should be printed on the radiator cap. Most are around 16 psi.
The thread on cap on your engine coolant reservoir is your pressure cap / radiator cap ( 16 PSI ) on a 1996 Ford Taurus ( all engines )
On a 1998 Ford Taurus : ( the thread on cap on your engine coolant reservoir is the 16 PSI pressure cap/ radiator cap )
On a 1995 Ford Crown Victoria : The thread on cap on your engine coolant reservoir is the 16 PSI pressure cap / radiator cap
On a 1998 Ford Expedition : The thread on cap on your engine coolant reservoir is the 16 PSI pressure cap / radiator cap