GMC 2001 Yukon XL Owner's Manual page 280

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
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.
R
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COOL
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even a little they
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