2005 Pontiac GTO Owner's Manual - Page 218

To add coolant to the overflow reservoir, do the following: 1. Remove the coolant overflow reservoir cap when the cooling system is no longer hot. If there is still no coolant visible on the dipstick, there is one more thing you can try. You can add the proper coolant mixture directly to the radiator, but be sure the system is cool before you do it. {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. 2. Fill the coolant overflow reservoir with the proper mixture, until it reaches the bottom arrow on the dipstick. 3. Replace the dipstick, then press down on the coolant reservoir cap while turning it clockwise until it stops, to lock it into position. 5-32