GMC 2003 C4C042 Owner's Manual page 250

4. With the coolant surge tank pressure cap off, start the
engine and let it run until you can feel the upper
radiator hose getting hot. Watch out for the engine
cooling fan.
By this time, the coolant level inside the coolant
surge tank may be lower. If the level is lower, add
more of the proper mixture to the coolant surge
tank until the level reaches slightly above the
FULL mark.
5. Then replace the
pressure cap. Be
sure the pressure
-
cap is hand tight
and fully seated.
5-25