2005 Buick Century Owner's Manual - Page 240

If the overheated engine warnings continue, there is one more thing that can be done. Add the proper coolant mixture directly to the radiator, but be sure the cooling system is cool before this is done. How to Add Coolant to the Radiator Notice: Your engine has a specific radiator fill procedure. Failure to follow this procedure could cause your engine to overheat and be severely damaged. Remove the radiator pressure cap when the cooling system, including the radiator pressure cap and upper radiator hose, is no longer hot. {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. Turn the pressure cap slowly counterclockwise to its first stop. Do not press down while turning the pressure cap. If a hiss is heard, wait for it to stop. A hiss means there is still some pressure left. 5-30