Vibration in truck/Steering wheel under acceleration only
I have a 2019 3500HD L5P Dually.
I have been chasing down a shake in the truck since I bought it. Had tires balanced/Road force balanced by the dealers so that is taken out of the equation for me at this point. This definitely made the shakes in the truck quite a bit better but there is still a lingering issue that the dealer is saying "that's just what the truck does".
When I'm accelerating from 65-72, especially with a load, the whole truck will start to shake. When it's cold it's minor and just a nuisance. Once the truck warms up after 15-20 miles of highway it continues to get's worse until it get's to just a stable shake through the truck while maintaining in the 65-72 speed range. It seems to be tied to engine revs to a certain degree as I can feel it shaking at bit when accelerating at lower RPMS from 45 to 55 as well, but it's not as pronounced as 65-72. When towing it gets especially bad going up a hill/accelerating up a hill.
Once I let go of the fuel the shake completely disappears. This goes doubly so towing and going down a hill after a climb. If I accelerate faster than 65/72 and let off the throttle, there is no shake in this speed range, it smooths out completely as it decelerates. I don't need to put it in neutral/take it out of gear either.
I duplicated it for the tech and service manager and they acknowledged the truck is doing it and explained it as being "economy mode" to try and save fuel or some weird explanation so it causes some shaking, but it's a constant shake, so I'm not convinced. It went through 3 techs at the dealer apparently and they all think there is nothing wrong. They said they checked the mounts for cracking or any issues and didn't see anything that would cause this kind of engine feedback, they rechecked tire balancing and they did some checks to the transmission.
I'm kind of at a loss here as to what the issue is that could cause this vibration from the engine and give this info to get GM to actually fix it. I owned an LML before this that definitely didn't have this issue so I know it's not something the "truck just does".
Any insight is appreciated.