2004 Buick Regal Owner's Manual - Page 243

If the overheated 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. {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. 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. Turn the pressure cap slowly counterclockwise until the first stop. Do not press down while turning the pressure cap. 5-35