2008 GMC Sierra 1500 Pickup Owner's Manual - Page 345

2. Then keep turning the pressure cap slowly, and remove it. By this time, the coolant level inside the coolant surge tank could 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. Then replace the pressure cap. Be sure the pressure cap is hand-tight and fully seated. 6. Verify coolant level after the engine is shut off and the coolant is cold. If necessary, repeat coolant fill procedure Steps 1 through 6. Engine Fan Noise Your vehicle has electric cooling fans. You might hear the fans spinning at low speed during most everyday driving. The fans can turn off if no cooling is required. Under heavy vehicle loading, trailer towing, and/or high outside temperatures, or if you are operating the air conditioning system, the fans can change to high speed and you might hear an increase in fan noise. This is normal and indicates that the cooling system is functioning properly. The fans change to low speed when additional cooling is no longer required. 3. Fill the coolant surge tank with the proper mixture, to the FULL COLD mark. 4. With the coolant surge tank pressure cap off, start the engine and let it run until the engine coolant temperature gage indicates approximately 195°F (90°C). 5-33