Hi Guys, new member here. My buddy has a 2006 Chevy 3500 with the 8.1 allison combo. He bought this truck used last summer. He has been getting awful mileage with it. We kind of wrote it off as nature of the beast. However, getting 8-9mpg unloaded cruising down the hwy is just not right. So, he hooked up my NGK powerdex handheld wideband, and took it for a ride. Pretty much anything other than idle, the A/F ratio drops. Even at light throttle pulling from a stoplight, it drops to the 13's. It really doesnt maintain the 14.7 like my 1997 454 Vortec does at all times except when in PE mode.

He replaced the 02 sensors thinking that could be it, but no change. The is no SES light on the dash. Any other ideas on where to look? Like a bad coolant sensor, MAF sensor, etc? I was thinking maybe its not going into closed loop or something. Thanks!