GMC 1993 Safari Owner's Manual page 219

If the overheat warning continues, there's one more thing you can try. You
can add the proper coolant
mix
directly to the radiator, but be sure the
cooling system
is
cool before you do it.
4
Steam and scalding liquids from a hot cooling system can blow out
I
- 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 pressure 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.
PO425
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