Ask TFL: Help, My new 2021 GMC Sierra 1500 Diesel is Having Start-up Problems – Possible Solutions

One cause of the problems may be low quality fuel...

2021 gmc sierra 1500 at4 diesel problem solution bad fuel start-up noise
image: by Nathan.

We recently received this question from Nathan, who just purchased a 2021 GMC Sierra 1500 AT4 with a 3.0L Duramax diesel I6 engine. Nathan writes…

A month ago I purchased a 2021 GMC AT4 with the LM2 Diesel engine. In the beginning, the truck ran flawless, quiet as a gas engine, and sipped fuel like a Prius. One afternoon after work, I went to crank my truck and it wouldn’t start. The engine was just turning over and over without igniting. This finally stopped after the starter timed out, I let it sit a second and pushed the starter again, and cranked right up. My dealer told me they’ve heard of it but no fix had yet to come out. I was advised if it happens again to bring it in. A week or so later I noticed upon taking off at stoplights and low RPM city driving that I had some valve clatter and low droning from my truck. Progressively over time, this has gotten worse and worse to the point I am not sure I can see myself keeping this vehicle. -It’s embarrassing for a $60-70k truck to sound like a clunker. -How can this endure/last when it sounds like metal on metal all the time? My dealer has advised there is no fix and they cannot do anything without codes. I see this is an issue on many of the LM2’s online forums that all have the same issue as mine.

We have not experienced this issue before, but many of your have already responded on our TFLtruck Youtube community page.

TFL viewer, Cordell Robertson provides the following details.

Cordell writes…

Problem #1. What he thinks is metal on metal thankfully is not! It is simply fuel knock, this is caused by the ECU compensating for a bad tank of fuel and misinterpreting it as injector wear. I had the same dreaded sound and vibration on my LM2. The lm2 engine is extraordinarily sensitive to cetane ratings in diesel fuel, it’s actually one of the biggest reasons why GM calls out “top tier” diesel for this engine. If you run a less than stellar tank of fuel the ECU adapts, thinks it’s injector wear and you’re going to get fuel knock. The good news is it’s a very simple reflash of the injector codes which are located on the intercooler in the form of a QR code. However, if he continues to use low-quality fuel the problem will come back and he will continuously have to get the reflash. Highly recommend top-tier diesel so he doesn’t have it keep happening.

This is a video we share on the Facebook page I am a part of.
“Solution @11:30 “(Not my video, no affiliation)
Problem #2. Okay, this one’s a bit complex because the long and the short of it is, GM is still not really sure what’s causing these crank no start problems on these engines. However, I have a couple of engineer and technician friends on GM’s side that keeps me in the loop as much as they can (with publicly available information).

(In reference to the customer service ticket I attached. To be clear the cab does not have to be removed, this particular technician just chose to do it that way). Below I’m going to copy and paste my post that I just posted last night on the “3.0 Duramax owners” Facebook page detailing everything about it. I created this post with help from my engineering contacts that told me what they’re allowed to, and with some technicians that have full access to GM internal tech communications. It was a follow-up post to my original TSB Facebook post 2 weeks prior.

New update to intermittent crank to start
AFTER they have completed the inspection, including rotating the crank four times (PITA on a compression-ignition engine) AND the troubleshooting steps for intermittent crank no start, if no problems are found.
The svc team is to take a picture, and “inspect for metal debris on the surface of the sensor” Including the Camshaft Reluctor. The picture and their findings will get attached to the TAC case (individual engineering ticket). 
Ensure your service departments are creating a TAC case if you have a crank no start, GM needs this information ASAP!!!

******My Original post below******
A possible solution to crank no start.
This is a very labor-intensive job, even just the inspection. I personally suggest trying to capture it on camera to prove that your dealer that it is occurring to convince them to take the 4+ hours to do the inspection.
bulletin May 24, 2021, document ID 5840493
bent trigger wheel on the camshaft.
Normally I would post the internal PDF but I have yet to find that available publicly.

The truck is amazing, but he HAS to run high-quality fuel, or use an additive to boost cetane if he only has less than stellar fuel in his area. From what I’ve been told, GM does not view the injector compensation/fuel knock as an actual problem and more an effect of fuel quality.