GMC 1999 Sierra 1500 Pickup Owner's Manual page 256

If the overheat warning continues, there's one more thing you can try. You
can add the proper coolant mix directly to the radiator, but be sure the
radiator is cool before you do it.
'
Steam and scalding liquids from a hot cooling system can blow out
I-
and burn you badly. They are under pressure, and if you turn the
radiator pressure cap-even a little-they can come out at high
speed. Never turn the cap when the cooling system, including the
radiator pressure cap, is hot. Wait for the cooling system and radiator
pressure cap to cool if you ever have to turn the pressure cap.
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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. 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.
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