Cadillac 2009 CTS Owner's Manual page 341

If no coolant is visible in the surge tank, add coolant as
follows:
1. 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-quarter turn and then stop.
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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