GMC C-Series 2001 Owner's Manual page 195

NOTICE:
Engine damage from running your engine
without coolant isn't covered by your warranty.
If there seems to be no leak, start the engine again. See
if the fan speed increases when idle speed is doubled by
pushing the accelerator pedal down. If it doesn't, your
vehicle needs service. Turn off the engine.
How to Add Coolant to the Coolant
Surge Tank
If you haven't found a problem yet, but the coolant
level isn't at the COLD ADD mark or higher, add a
50/50 mixture of clean, drinkable water and 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 the Caterpillar
Operation & Maintenance Manual for the proper engine
coolant to use.
5-28
R
Diesel Engine
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.
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even a little they