GMC SIERRA CK2 1999 Owner's Manual page 261

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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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