GMC 1996 Savana Van Owner's Manual page 203

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 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.