Cadillac 2006 CTS Owner's Manual page 308

CAUTION:
{
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.
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-quarter turn
and then stop.
If you hear a hiss, wait for that to stop.
A hiss means there is still some pressure left.
5-34
2. Then keep turning the pressure cap slowly,
and remove it.