2005 Cadillac STS Owner's Manual - Page 322

If no coolant is visible in the surge tank, add coolant as follows: 1. You can remove the coolant surge tank pressure cap when the cooling system, including the coolant surge tank pressure cap and upper radiator hose, is no longer hot. Turn the pressure cap slowly counterclockwise (left) about one-quarter turn and then stop. If you hear a hiss, wait for that to stop. A hiss means there is still some pressure left. 3. Fill the coolant surge tank with the proper mixture, to slightly above the FULL COLD/FROID line on the side of the coolant surge tank. 4. With the coolant surge tank pressure cap off, start the engine and let it run until you can feel the upper radiator hose getting hot. The upper radiator hose is the largest of the hoses which comes out of the radiator, on the passenger's side of the vehicle. Watch out for the engine cooling fans. By this time, the coolant level inside the coolant surge tank may be lower. If the level is lower, add more of the proper mixture to the coolant surge tank until the level reaches the FULL COLD/FROID line on the side of the coolant surge tank. 2. Then keep turning the pressure cap slowly, and remove it. 5-32