Hummer H2 2010 Owner's Manual page 415

2. 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 a hiss is heard, 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.
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 located on the side of the
reservoir.
6-35