Just bought a 2010 Sierra with a 5.3 litre 6 speed trans. Couple thousand kms on it so far. Watching the instant economy on the display I see it switch to 4 cyl mode only when I go down a significant hill or take my foot off the gas. All the literature says it will switch under light load conditions. I drive through valley where I live along side a river going downstream, no load, a couple extra people in cab and as I slow down slightly it will switch but as soon as I try to maintain 90 to 100 km (55 to 60 mph), it immediately switches to 8 cyl.

I called service at my dealer and his first question was "Well, does it do 4 cyl at 10 mph?" I was a little disgusted. He didn't seem to know really how the system worked and is now checking into it. Come on! Really?

I think it's not working properly. Any help on this from anyone would be appreciated

Thanks -- Kelly