Hummer H2 2009 Owner's Manual page 384

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.
2. You can remove the coolant surge tank pressure
cap when the cooling system, including the coolant
surge tank pressure cap and upper radiator hose,
are 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.
3. Then keep turning the pressure cap slowly, and
remove it.
5-30
1. Locate the coolant
surge tank pressure
cap that has this label.
4. Fill the coolant surge tank with the proper mixture,
to the FULL COLD mark.