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.
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 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
On a 1998 Ford Expedition : 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.
On a 1998 Ford Taurus : ( the thread on cap on your engine coolant reservoir is the 16 PSI pressure cap/ radiator cap )
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 Expedition : The thread on cap on your engine coolant reservoir is the 16 PSI pressure cap / radiator cap