Chevrolet 1999 Astro Owner's Manual page 226

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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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