GMC 2002 Sierra - Manual (Page 343)

5-16
If no coolant is visible in the surge tank, add coolant
as follows:
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 coolant
surge tank pressure cap
-- even a little -- they
can come out at high speed. Never turn the
cap when the cooling system, including the
coolant surge tank pressure cap, is hot. Wait for
the cooling system and coolant surge tank
pressure cap to cool if you ever have to turn
the pressure cap.