GMC 1997 Safari Owner's Manual page 237

How to Add Coolant to the radiator
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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. Turn the
pressure cap slowly
counterclockwise until it
first stops. (Don't press
down while turning the
pressure cap.)
2. Then keep turning the
pressure cap, but now
push down as you turn
it. Remove 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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