Pontiac 1998 Trans Sport Owner's Manual page 312

How to Add Coolant to the Radiator
1. You can remove the radiator pressure cap when
the cooling system, including the radiator pressure
cap and upper radiator hose, is no longer hot,
T u r n
the pressure cap slowly counterclockwise
until it first stops. (Don't press down while turning
the pressure cap.)
If you hear a hiss, wait for that to stop. A hiss means
there is still some pressure left.
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