Chevrolet Impala 2007 Owner's Manual page 330

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 on the engine,
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.
330
How to Add Coolant to the Cooling
System
Notice: Your engine has a specific cooling
system drain and fill procedure. Failure
to follow this procedure could cause your
engine to overheat and be severely damaged.
If your engine's cooling system needs to
be drained and re-filled, please see your
dealer/retailer.
If you hear a hiss, wait for that to stop. A hiss
means that there is still some pressure left.
1. You can remove the
pressure cap when
the cooling system,
including the
pressure cap and
upper radiator hose
is no longer hot.
Turn the pressure
cap slowly
counterclockwise.