Chevrolet 2007 TrailBlazer Owner's Manua page 423

If the overheat warning continues, there is one
more thing you can try. You can add the proper
coolant mixture directly to the radiator, but be sure
the cooling system is cool before you do it.
CAUTION:
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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, is 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
about one full turn. 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.
Remove the pressure cap.
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