There is a problem with the active fuel management systems on ALL 2007-2008 Sierras that have the 5.3L or 6.0. The 4.8L model does not have active fuel management. For the old timers out there, it is the same issue as the Cadillac V8-6-4 engine. Make no mistake, If your vehicle has active fuel management, you have a problem! GM is aware and do not have a fix for this issue.