GMC 1994 Yukon Owner's Manual page 220

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.
P
: .
Turn the pressure cap slowly to the left 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.
2. Then keep
turning the
pressure cap, but
now push down
as you turn it.
Remove the
pressure cap.
P
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