I had a similar situation with my 2008 5.3. Ran beautiful on the highway but then, at a stoplight, it would idle rough and throw a cylinder 2 misfire. Hard shift from 1-2 on acceleration then it would smooth out and run good again. Compression test showed bad compression on cylinder 2. Get a leak down test done too. The head was pulled and found to be a bad intake valve seat. The head was rebuilt at it runs perfect again.
aeroplague, take your truck to the dealer. Tell the dealer that OnStar couldn't get to the bottom of it and have them replace the VCIM module. I had 17 phone calls with OnStar, each one over an hour long. They are useless but, if you complain, they will give you complimentary data back. You can monitor your data usage with the myGMC app to see if it is fixed. Make sure you do the things I noted above on Aug 24th.
So, OnStar told me to take the truck to the dealership, since they couldn't solve the problem. The dealership changed the VCIM (Vehicle Communication Interface Module). Also, make sure on your truck, you have Settings>Connections>Wi-Fi Hotspot>Share Hotspot Data>Off On your phone (I have an iPhone), go to Settings>Wi-Fi and 'Forget' your truck's network so that it doesn't try to join. I got OnStar to reinstate my 3GB data plan so I'll keep you informed if the truck is still using data. Unfortunately, the myGMC app is funky, and it just has a spinning 'Updating...' thing. OnStar said to call them back in a couple of days.
I have a 2022 Refreshed Sierra Denali. I received a complimentary 3GB data plan with the truck. Without using Spotify, Pandora, Google Maps, etc., I burned through the 3GB in 1 week and 1500 km. I called OnStar and they gave me another 3GB as goodwill. I have burned through 2.6GB of that over the course of a couple of weeks. My WiFi hotspot is turned off so no phone can connect to my truck. I have never used any of the built in apps directly. Google Maps is on but I have never searched a destination or used the route function. I have spent over an hour, 5 different times with the OnStar people-they don't have a clue. Yesterday, the OnStar rep told be that July 18th, shortly after midnight, my truck used a significant amount of data. Strange thing is I was at my friend's cottage with another vehicle and my truck was sitting in my driveway, at home, obviously off. I can't get any answers from GM so asking for help here. Scott.
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