2008 GMC Acadia Owner's Manual - Page 346

8. By this time, the coolant level inside the radiator filler neck may be lower. If the level is lower, add more of the proper DEX-COOL® coolant mixture through the filler neck until the level reaches the base of the filler neck. Replace the pressure cap. At any time during this procedure if coolant begins to flow out of the filler neck, reinstall the pressure cap. Be sure to secure it tightly. 7. Start the engine and let it run until you can feel the upper radiator hose getting hot. Watch out for the engine cooling fan. 5-32