GMC 2009 Yukon XL Owner's Manual page 429

If no coolant is visible in the surge tank, add coolant as
follows:
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. Keep turning the pressure cap slowly and remove it.
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.
3. Fill the coolant surge tank with the proper mixture
to the FULL COLD mark.
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