GMC C series 2002 Owner's Manual page 192

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.
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