Chevrolet 2008 Avalanche Owner's Manual page 394

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. Do not spill coolant on a hot engine.
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.
5-34
1. You can remove the
coolant surge tank
pressure cap when
the cooling system,
including the coolant
surge tank pressure
cap and upper radiator
hose, is no longer hot.
2. Then keep turning the pressure cap slowly, and
remove it.
3. Fill the coolant surge tank with the proper mixture,
to the FULL COLD mark.