GMC 2001 Denali Owner's Manual page 267

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.
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