Hummer 2007 H2 Owner's Manual page 424

4. Fill the coolant surge tank with the proper
mixture, to the FULL COLD mark.
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5. With the coolant surge tank pressure cap off,
start the engine and let it run until you can
feel the upper radiator hose getting hot.
Watch out for the engine cooling fan.
By this time, the coolant level inside the
coolant surge tank may be lower. If the level
is lower, add more of the proper mixture
to the coolant surge tank until the level
reaches the FULL COLD mark.
6. Then replace the pressure cap. Be sure the
pressure cap is hand-tight and fully seated.