2008 Chevrolet Impala Owner's Manual - Page 281

If the overheat warning continues, there is one more thing you can try. You can add the proper coolant mixture directly to the cooling system through the coolant fill neck on the engine, but be sure the system is cool before you do it. How to Add Coolant to the Cooling System Notice: The engine has a specific cooling system drain and fill procedure. Failure to follow this procedure could cause the engine to overheat and be severely damaged. If the engine's cooling system needs to be drained and re-filled, please see the dealer/retailer. 1. You can remove the pressure cap when the cooling system, including the pressure cap and upper radiator hose is no longer hot. Turn the pressure cap slowly counterclockwise. {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. If you hear a hiss, wait for that to stop. A hiss means that there is still some pressure left. 2. Then keep turning the pressure cap and remove it. 5-35