Cadillac 1999 DeVille Owner's Manual page 252

yellowblue
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.
5-14