2009 GMC Envoy Owner's Manual - Page 101

Active Fuel Management™ The vehicle's V8 engine may have Active Fuel Management™. This system allows the engine to operate on either all or half of its cylinders, depending on driving conditions. When less power is required, such as cruising at a constant vehicle speed, the system will operate in the half cylinder mode, allowing the vehicle to achieve better fuel economy. When greater power is required, such as accelerating from a stop, passing, or merging onto a freeway, the system will maintain full-cylinder operation. Automatic Transmission Operation There are several different positions for the gear shift lever. 2-25