GMC 2010 T-Series Owner's Manual page 176

How to Add Coolant to the Surge Tank
WARNING:
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.
Notice: This vehicle has a specific coolant fill
procedure. Failure to follow this procedure could
cause the engine to overheat and be severely
damaged.
If no coolant is visible in the surge tank, add coolant as
follows:
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. 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.
2. Then keep turning the pressure cap slowly, and
remove it.
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3. Fill the coolant surge tank with the proper mixture,
to the COLD FULL mark.