2009 GMC Sierra misfiring, no codes thrown

I have a 2009 Sierra, quad cab 2WD, 5.3L engine with 145,000 miles. About a week ago i began feeling my engine flutter just a bit (maybe 50 rpm spike) between 40-50mph. It became increasingly worse over the next four days. No check engine light or anything unusual. I was in contact with my mechanic and he looked at it for the past two days. My mechanic found that even at idle my truck had a random misfire on all eight cylinders. When driving the truck, the misfire shifts into only cylinders 2-7. The misfire is incredibly small, 20-30 total per minute at about 1000-1500 rpms. He checked for compression issues, valve issues, transmission issues and everything is reading exactly how it should besides the misfires. At this point my mechanic returned my truck with no charge as he can not diagnose any issues. His recommendation was to take it to a dealer or someone who has potentially seen this before. It’s also worth noting in the past four months I’ve replaced the throttle body, all spark plugs, all wired and one coil pack. It also just received a full synthetic oil change within 1000 miles. I’ve only ever ran e85 gas in it. Please if anyone has anything for me, let me know. Me and my mechanic (who i trust greatly) are lost.

I don’t like to guess because I am wrong enough. Is this a Flex Fuel vehicle ? If it is not then you should not use E85 but E10 fuel . You might even try a tank of E10 and see if does make a difference.

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doesn’t this particular truck have cylinder shutoff . . . ?

Taking it to a shop who has the correct GMC scan tool is what I’d do. Doesn’t have to be a dealership necessarily, but they’d almost certainly have the correct scan tool. It may be they can figure out the problem right away, then if you want you can fix it yourself or take it to your own shop for the repair.

If the compression tests within spec in all cylinders, I’m thinking this is probably one of these

  • ignition module
  • crank position sensor
  • cam position sensor

It’s a flex fuel