GMC 2003 Sierra 1500 Pickup Owner's Manual page 417

Adding only plain water to your cooling
system can be dangerous. Plain water, or
some other liquid such as alcohol, can boil
before the proper coolant mixture will. Your
vehicle's coolant warning system is set for
theproper coolant mixture. With plain water or
the wrong mixture, your engine could get too
hot but you wouldn't get the overheat warning.
Your engine could catch fire and you or others
could be burned. Use a 50/50 mixture of clean,
drinkable water and a proper coolant.
Notice: In cold weather, water can freeze and crack
the engine, radiator, heater core and other parts.
Use the recommended coolant and the proper
coolant mixture.
You can be burned if you spill coolant on hot
engine parts. Coolant contains ethylene glycol
and it will burn if the engine parts are hot
enough. Don't spill coolant on a hot engine.
1. Park the vehicle on a
level surface. You can
remove the coolant
surge tank pressure
cap when the cooling
system, including
the coolant surge tank
pressure cap and
upper radiator hose, is
no longer hot.
2. Turn the pressure cap slowly counterclockwise (left)
about one full turn. If you hear a hiss, wait for that to
stop. A hiss means there is still some pressure left.
3. Then keep turning the pressure cap slowly, and
remove it.
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