Buick 2008 Allure Owner's Manual page 296

When the coolant level in the coolant recovery tank is at
the COLD FILL line, start the vehicle.
If the overheat warning continues, there is one more
thing you can try. You can add the proper coolant
mixture directly to the cooling system through the
coolant fill neck under the pressure cap, but be sure the
system is cool before you do it.
CAUTION:
{
Steam and scalding liquids from a hot cooling
system can blow out and burn you badly. They
are under pressure, and if you turn the radiator
pressure cap — even a little — they can come
out at high speed. Never turn the cap when the
cooling system, including the radiator pressure
cap, is hot. Wait for the cooling system and
radiator pressure cap to cool if you ever have
to turn the pressure cap.
5-36
How to Add Coolant to the Cooling
System (3.6L V6 Engine)
Notice: The engine has a specific cooling system
drain and fill procedure. Failure to follow this
procedure could cause the engine to overheat and
be severely damaged. If the engine's cooling system
needs to be drained and re-filled, please see the
dealer/retailer.
Turn the pressure cap slowly counterclockwise.
If you hear a hiss, wait for that to stop. A hiss
means that there is still some pressure left.
2. Then keep turning the pressure cap and remove it.
1. You can remove the
pressure cap when
the cooling system,
including the pressure
cap and upper radiator
hose is no longer hot.