Hello Scotty I've been having a problem with my 2011 GMC Sierra Denali. My alcohol content goes up every time I refuel using E87. I reset it with my tool scanner and it goes down to 3% and starts like new, but after I have to refuel again my alcohol content goes up to 25% and start rough on cold start until I reset it again. It happens every time I refuel. It doesn't throw any codes and dealer can't figure it out. It is a Flex Fuel and I have tried E85 and still same problems? Any Idea what it could be? I would highly appreciate your help. Thank You So Much.

I would flush out the tank first, you may have water or something else in there. If not that, let the dealer figure it out, it's their stupid design after all, VERY complex and prone to breaking down on gm products