Buick Enclave Owner's Manual page 277

1. Detach the fasteners and lift off
the panel that covers the
radiator cap.
2. Remove the radiator pressure
cap when the cooling system,
including the upper radiator
hose, is no longer hot. Turn the
pressure cap slowly
counterclockwise about one
full turn.
If a hiss is heard, wait for that to
stop. A hiss means there is still
some pressure left in the
system.
3. Keep turning the pressure cap
slowly and remove it.
4. Fill the radiator to the base of
the filler neck with the proper
DEX-COOL coolant mixture.
5. Fill the coolant recovery tank to
the FULL COLD mark.
6. Reinstall the cap on the coolant
recovery tank but leave the
radiator pressure cap off.
7. Start the engine and let it run
until the upper radiator hose
feels warm. Any time during this
procedure, watch out for the
engine cooling fans.