2003 GMC Sierra 1500 Pickup Owner's Manual - Page 418

4. Then fill the coolant surge tank with the proper mixture, to the FULL COLD mark. 5. 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. Watch out for the engine cooling fan. 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 mark 5-43