2008 Chevrolet Colorado Owner's Manual - Page 301

If the overheat warning continues, there is one more thing you can try. Add the proper mixture directly to the radiator, but be sure the cooling system is cool before you do it. How to Add Coolant to the Radiator {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 radiator pressure cap - even a little - they can come out at high speed. Never turn the cap when the cooling system, including the radiator pressure cap, is hot. Wait for the cooling system and radiator pressure cap to cool if you ever have to turn the pressure cap. 1. Remove the radiator pressure cap when the cooling system, including the upper radiator hose, is no longer hot. Turn the pressure cap slowly counterclockwise about one full turn. If you hear a hiss, wait for that to stop. A hiss means there is still some pressure left. 2. Keep turning the cap to remove it. 3. Fill the radiator with the proper DEX-COOL® coolant mixture, up to the base of the filler neck. See Engine Coolant on page 5-27 for more information about the proper coolant mixture. 5-35