Cadillac 2011 STS Owner's Manual page 376

Adding only plain water or some other liquid to the
cooling system can be dangerous. Plain water
and other liquids, can boil before the proper
coolant mixture will. The vehicle's coolant warning
system is set for the proper coolant mixture. With
plain water or the wrong mixture, the engine could
get too hot but you would not get the overheat
warning. The engine could catch fire and you or
others could be burned. Use a 50/50 mixture of
clean, drinkable water and DEX-COOL coolant.
Use a 50/50 mixture of clean, drinkable water and
®
DEX-COOL coolant. If using this mixture, nothing else
needs to be added. This mixture:
Gives freezing protection down to 34°F ( 37°C),
outside temperature.
Gives boiling protection up to 265°F (129°C),
engine temperature.
6-26
Protects against rust and corrosion.
Will not damage aluminum parts.
Helps keep the proper engine temperature.