Chevrolet 2003 Blazer Owner's Manual page 295

Steam and scalding liquids from a hot cooling
system can blow out 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, i s hot. Wait for the cooling system and
radiator pressure cap to cool if you ever have
to turn the pressure cap.
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 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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