Cadillac 2004 XLR - Manual (Page 254)

1. Park the vehicle on a level surface. 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 (left).
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.
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