According to the Haynes repair manual for Chevy/GMC trucks 1988 thru 2000 the cap should be a 15 psi radiator cap.
Not knowing what vehicle, 15 psi is probably average.
Look on the radiator cap - there should be a psi rating stamped on there.
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.
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 2002 Mercury Sable : 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
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 )