2003 Buick Regal Owner's Manual - Page 238

When the coolant in the coolant recovery tank is at the COLD mark, start your vehicle. If the overheat warning continues, there's one more thing you can try. You canadd the proper coolant mixture directly to the radiator, but be sure the cooling system is cool before you do it. 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 capto 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 it first stops. Don't press down while turning the pressure cap. 5-33