GMC 2004 Canyon Owner's Manual page 297

When the coolant in the coolant recovery tank is at the
FULL COLD mark, start your vehicle.
If the overheat warning continues, there's one more
thing you can try. You can add the proper mixture
directly to the radiator, but be sure the cooling system
is cool before you do it.
CAUTION:
{
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.
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