It does a wonderful job doing towing and we love the truck. It has just over 67200 miles on it.

It has 6.6 Duramax diesel with the Allison transmission.

over the last month on three occasions, while not towing, when we made a stop, it has seemed the

transmission did not disengage. 

 I had to almost stand on the brakes to stop but the rear wheels kept slowly turning. The first time,

when I let off the brakes, the truck lunged forward. The other two times I have shifted to neutral

to release the transmission.

 My truck has been parked for a month waiting to get into the GMC service department where we

bought it February, 2022.

I have a 2015 2500HD with the 6.0 and about 95,000mi, and for the past 6 months, I've been having this issue where It'd start pulling timing ( up to 15 degrees of KR) under moderate to heavy throttle, as soon as it hits 3000RPM. There is no audible knock, but 15 degrees of KR sounds very excessive. Higher octane fuel makes the issue better,  and usually after long interstate drives (no heatsoak), it runs great, but generally, it's very bad. Towing a 32FT trailer full of band equipment up a mild hill, it struggles to get out of first gear, and slams into the next gear because the timing is so far retarded before the shift, that it screws with the shift timing.  I have used various fuel system cleaners ( now trying Amsoil Pi, as I know its well proven ) but so far, none have made any difference. I want to check the fuel pressure, but I don't see a test port, and I can't get any OBD app to display fuel pressure.

Here's what confuses me:  Sometimes, under the same conditions, It'll run just fine and pull hard straight through 3000 RPM. The only thing I can find different between the times that it runs like a POS and times that it runs like its brand new, is that when it's running like crap, it shudders at idle- enough to be felt throughout the entire truck.

It pulls the most timing at around 3000, and tapers down as the revs increase.

I got it used, with about 76,000mi, so I'm not clear about its past. Back when it was running horribly I was considering that It might have been tuned by the previous owner, but lately, its been randomly running just fine. 

Its been fine for the past week or so, but I'm still very curious about what could be going on. 

I have TorquePRO recordings of the issue happening, which I may upload later if that would be of any help. I've been hoping for a CEL but of course- no pending or stored codes!

Thank you all! Sorry for the length of the post!

This is on my 2016 GMC 2500HD Sierra Denali 4x4. Diesel, as mentioned. The fuel gauge is one of those graphics generated gauges, not a mechanical one on the dash. 
Anyway, any thoughts on this? It used to go up so smoothly now it seems the sticking is getting worse over the last year or less. 

Is there something I can add to the fuel tank that might help clean the sensor and get that indication back to smooth flowing again? 

Thanks for any help.

About to do my rear rotors and pad. Would like to know the proper torque spec on caliper brakets and slide pins as well.  Thanks! 

They did “burn downs”, tightened a cold air pipe to the engine, and reprogramming the computer.

After driving more that 250 miles to and from the GM dealership and spending numerous days of my time dealing with this issue, I spoke with the dealership owner. He shared the following information:

After speaking with his GM regional representative, he was told that “GM programmed computers on 2017 GMC trucks equipped with the Duramax engine to detect any DEF that didn’t meet the exact same quality specifications as GM DEF”.

More plainly stated, GM is essentially forcing 2017 Duramax owners to buy GM DEF or there is a real chance the check engine light will come on and the vehicle could eventually go into shutdown mode.

This would clearly pose a problem for everyone that buys DEF from anyone other that GM. I have a motor coach that requires DEF. Farmers and other businesses that own 2017 Duramax equipped trucks, and have other vehicles requiring DEF would also be severely impacted. We buy and store DEF from third party sellers, because it is more convenient and much cheaper than buying DEF from GM.

NOTE: A service bulletin has just been released that requires owners to bring their vehicles in to have the computers reprogrammed. This is supposed to reduce the sensitivity related to DEF quality. I had this done to my truck and will do a follow-up post after I add NON GM DEF to my truck.