Chevrolet 1999 Monte Carlo Owner's Manual page 200

How to Add Coolant to the Radiator
NOTICE:
Your engine has a specific radiator fill procedure.
Failure to follow this procedure could cause your
engine to overheat and be severely damaged.
If you hear a hiss, wait for that to stop. A hiss means
there is still some pressure left.
5-18
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
CAUTION:
You can be burned if you spill coolant on hot
engine parts. Coolant contains ethylene glycol
and it will burn if the engine parts are hot
enough. Don't spill coolant on a hot engine.
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the pressure cap, but
now push down as
you turn it. Remove
the pressure cap.