GMC SIERRA 2002 Manual page 345

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) about
one full turn. If you hear a hiss, wait
for that to stop. A hiss means there is still some
pressure left.
2. Then keep turning the pressure cap slowly, and
remove it.
5-18
Your vehicle may be
equipped with one of
the two caps shown.
3. Then fill the coolant surge tank with the proper
mixture, to the FILL COLD mark.