Cadillac 2007 XLR - Manual (Page 395)

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.
If you hear a hiss, wait for that to stop. A hiss
means there is still some pressure left.
2. Then keep turning the cap and remove it.
3. Fill the coolant surge tank with the proper
mixture until the level inside the surge tank
stabilizes at the FULL COLD mark on the front
of the surge tank.
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