GMC 2001 C3 Owner's Manual page 257

How to Add Coolant to the Coolant
Surge Tank
If you haven't found a problem yet, but the coolant level
isn't at or above the FILL COLD mark, add a 50/50
mixture of clean, drinkable water and DEX
coolant at the coolant surge tank, but be sure the cooling
system, including the coolant surge tank pressure cap, is
cool before you do it. See "Engine Coolant" in the Index
for more information.
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
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
R
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COOL
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even a little they can
CAUTION:
Adding only plain water to your cooling system can
be dangerous. Plain water, or some other liquid
like alcohol, can boil before the proper coolant
mixture will. Your vehicle's coolant warning system
is set for the proper coolant mixture. With plain
water or the wrong mixture, your engine could
CAUTION: (Continued)