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New 3500 Denali bought new about 2 months ago.
It has 1800 miles on it.
while getting on the freeway at 35-45 mph relatively light acceleration truck stumbles I cant tell if its engine or trans caused. Kind of feels like it runs out of fuel or it doesn't know what gear to be in. Its so fast and so infrequent that that I'm not really paying attention then its back to normal.
It has done it 3 times. about 300 miles 700 miles and today at 1800 miles,
I have towed heavy and love it, no problem other than those 3 times. It was empty and warm when it happened.
Any insight?
Thanks
 

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I have the same issue. I don't have any reason to believe it's fuel related and I'm nearly certain it's the transmission being 'confused' about which gear to go in. The way it happens for me is exactly as you described - typically light throttle input, no load, and I notice it more frequently happens when I'm going up a slight incline.

It's done it for me since I bought the truck and I am approaching 7000 miles now. I plan to address the issue with my dealer at the next service - Despite the fact that my truck is a 2018 bought in December, I think it missed the ECM/TCM reprogram update TSB/Recall that addressed this issue.

That's my hope anyway....
 

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I have the same issue. I don't have any reason to believe it's fuel related and I'm nearly certain it's the transmission being 'confused' about which gear to go in. The way it happens for me is exactly as you described - typically light throttle input, no load, and I notice it more frequently happens when I'm going up a slight incline.

It's done it for me since I bought the truck and I am approaching 7000 miles now. I plan to address the issue with my dealer at the next service - Despite the fact that my truck is a 2018 bought in December, I think it missed the ECM/TCM reprogram update TSB/Recall that addressed this issue.

That's my hope anyway....
You can run the vin thru the 'onstar' phone apps and find out if it needs doing.

It was a recall, so if it was required, and the dealer didn't do it, then they didn't get paid for pdi.
Delivering with an uncompleted recall outstanding is a HUGE no-no.


Is it also more common when the engine and trans aren't up to full operating temperature?
 

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Here I am again thinking I know what you're talking about. What it feels like is the accelerator pedal is not set to the right beginning value for acceleration (pedal %). When you look at the pedal value with a CTS/CTS2, it looks like 0-2% of pedal drives 16-19kpsi of fuel pressure, which causes acceleration. There is little to no wiggle room to engage the pedal without driving acceleration (i.e. can't coast at low speeds). Straddling the 0-2% pedal value makes the truck feels like its on/off'ing.

Is that what your feeling?
 

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You can run the vin thru the 'onstar' phone apps and find out if it needs doing.

It was a recall, so if it was required, and the dealer didn't do it, then they didn't get paid for pdi.
Delivering with an uncompleted recall outstanding is a HUGE no-no.


Is it also more common when the engine and trans aren't up to full operating temperature?
I've let the Onstar trial expire and didn't renew it. The app didn't show it though when I checked and it was still within the trial period.

I'm not sure what else it could be other than that recall. I know my truck was a dealer trade and the dealer I bought it from had a closed service department at the time of delivery. My local dealer is pretty helpful though, so I'm hopeful they're able to sort it out.

There's no real rhyme or reason to the issue. Sometimes it will do it on a flat road, but I notice it mostly when it's a slight incline. I don't drive it aggressively and accelerate moderately, I guess mostly to not have horrible fuel economy. But when it happens, it's usually with very light throttle input and stumbles is the perfect word to describe it.

I also notice that when I am switching between reverse and drive, the drop into the gear can be hard. Not really a clunk, but it's very obvious it went into gear. And if the truck is rolling the slightest bit, it's harsh.

I'm not sure if that's a HD thing, a Duramax thing, or an Allison thing, but gear engagement on my 1500 was smoother in that regard. More subtle, if you will...
 

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Hard shift from between F and R when close to stopped, but not stationary, has been a 'feature' on the Allisons I've owned - unlike the 1500's I've had that could/would do gentle R to F even when 'rolling back' either intentionally or accidentally.



The app should continue to work even after subscription expires (for something like 5 yrs, iirc).
It should still give remote start form phone etc.
Many have posted about this in the past.

Select the status tab and scroll to bottom. then select 'check for recall'.

It should check for the vin of the vehicle the app currently has selected (since you can have multiple vehicles hooked up to the same app)

There is also a section where you can enter a vin for another vehicle for it to check.



If you'd rather, you can also check for recall on the gm websites.


If you'd rather, any service department can check you vin for you.


If you'd rather, you use gm customer service website or phone number to get the vin checked.
 

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Here I am again thinking I know what you're talking about. What it feels like is the accelerator pedal is not set to the right beginning value for acceleration (pedal %). When you look at the pedal value with a CTS/CTS2, it looks like 0-2% of pedal drives 16-19kpsi of fuel pressure, which causes acceleration. There is little to no wiggle room to engage the pedal without driving acceleration (i.e. can't coast at low speeds). Straddling the 0-2% pedal value makes the truck feels like its on/off'ing.

Is that what your feeling?
I wouldn't that it feels like it's on/off'ing really. I feel like the engine speed/transmission speed get to a point right on the edge of a shift point. If the road were down hill or perfectly flat, the upshift would occur and there would be no issue. But it feels like when it happens on an incline, the transmission gets 'confused' as to which gear it should use to climb the incline and gets stuck in the upshift. It doesn't happen between every gear - it's either 3 to 4 or 4 to 5, I'm not 100% certain which one it is. All other upshifts are smooth and without notice.

Hard shift from between F and R when close to stopped, but not stationary, has been a 'feature' on the Allisons I've owned - unlike the 1500's I've had that could/would do gentle R to F even when 'rolling back' either intentionally or accidentally.



The app should continue to work even after subscription expires (for something like 5 yrs, iirc).
It should still give remote start form phone etc.
Many have posted about this in the past.

Select the status tab and scroll to bottom. then select 'check for recall'.

It should check for the vin of the vehicle the app currently has selected (since you can have multiple vehicles hooked up to the same app)

There is also a section where you can enter a vin for another vehicle for it to check.



If you'd rather, you can also check for recall on the gm websites.


If you'd rather, any service department can check you vin for you.


If you'd rather, you use gm customer service website or phone number to get the vin checked.
Well, I confirmed again on the owners center on the gmc.com site - my VIN doesn't have any open recalls.

Perhaps I can convince my dealer to perform the recall service again (the programming) and see if it corrects the issue. Or maybe there will be historical codes that indicate what the issue might be.

Hopefully I'll be in for a service this week and will have good news to report afterwards. The problem is, it's a sporadic issue and as consistent as the variables are when it occurs, it's difficult to reproduce on-demand.
 

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Mine does the same thing but only when first driven for the day...it stutters at 51 mph. It is warmed up prior to driving.
 

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I had my truck in for its first service and raised the issue of the transmission stumble that I have experienced regularly, though it's difficult to force/reproduce on-demand.

Of course, they did not find anything wrong and the transmission is functioning as designed. They even went so far as to ask if I've owned/driven and Allison transmission before.

While I haven't, and I understand it may be less refined as it's built for heavy duty use, I am 100% positive that the stumble I have isn't a design feature! I don't think the issue is mechanical, but rather electronic within the TCM.

Is it possible to reset the TCM/flash it back to new to start the learning process over again? I've asked the service consultant, but he didn't know the answer to that either.
 

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sounds like a bad injector.. my buddy here at work had that happen on his.
 

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I had my truck in for its first service and raised the issue of the transmission stumble that I have experienced regularly, though it's difficult to force/reproduce on-demand.

Of course, they did not find anything wrong and the transmission is functioning as designed. They even went so far as to ask if I've owned/driven and Allison transmission before.

While I haven't, and I understand it may be less refined as it's built for heavy duty use, I am 100% positive that the stumble I have isn't a design feature! I don't think the issue is mechanical, but rather electronic within the TCM.

Is it possible to reset the TCM/flash it back to new to start the learning process over again? I've asked the service consultant, but he didn't know the answer to that either.
I think you posted this happens always at light throttle.

Does it also happen at the same speeds.

Like 33 or 34, 42 or 43 and 53?

If you can find these points and drive 1 or 2 mph below them will it just stay in the lower gear?

If you run just above them and it shifts, can you then slow down a little below those points and it holds the higher gear?

The speeds might be higher when the transmission and engine are cold.
Do the trials when it is warmed up.
 

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I think you posted this happens always at light throttle.

Does it also happen at the same speeds.

Like 33 or 34, 42 or 43 and 53?

If you can find these points and drive 1 or 2 mph below them will it just stay in the lower gear?

If you run just above them and it shifts, can you then slow down a little below those points and it holds the higher gear?

The speeds might be higher when the transmission and engine are cold.
Do the trials when it is warmed up.
What you are describing sounds really, really accurate. The one sticking point I have had with this issue and the ECM/TCM recall is that I have noticed this happened between gear 3 & 4, 4 & 5, and sometimes 5 & 6. When the engine and transmission are cold, the gears do hold a little longer.

Most of the time, the upshift occurs without issue and the higher gear will hold, even if I slow down under the original shift point. What I feel like happens is that when it does occur, the transmission feels conditions are met for an upshift, but midshift, it determines that the current gear is also a good choice, and hence the stumble. Every time though, the higher gear is chosen.

Now that it is getting to be warmer weather, I haven't noticed occurring as frequently. I won't know if that's for sure the case until next winter rolls around and I can continue to observe. That is, unless the issue is fixed prior to that.

I appreciate all the help on this. Overall, the truck is so much fun to drive and I'm so happy I made the jump to the Duramax/Allison/2500HD.

ETA: The speeds you gave seem pretty consistent with the shift points, despite the stumble or not. Of course this is unloaded/unladen everyday driving conditions.
 

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Sounds very familiar to my complete Injector failure at 5700 miles on my 2018 3500 L5P that happened yesterday. Take it to get checked out before you get stranded.
 

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Mine didn't give any warning and I wasn't due in for any scheduled maintenance for another month to know if it was sending codes. The one time I let it get under 100 miles before adding def it sent two emails. I figure anything remotely important would come as an email as well. Engine light didn't come on until 20 miles after the misfiring began. I was a mile from the dealership so I limped it in at low rpms where the shuddering was minimal.
 

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Mine didn't give any warning and I wasn't due in for any scheduled maintenance for another month to know if it was sending codes. The one time I let it get under 100 miles before adding def it sent two emails. I figure anything remotely important would come as an email as well. Engine light didn't come on until 20 miles after the misfiring began. I was a mile from the dealership so I limped it in at low rpms where the shuddering was minimal.
Only been on the site 3 time to tell us all of 8 injectors failed at 3500 miles??? If I were GM I would ask you what crappy fuel you were using but being a bit more realistic your just trolling.

And maybe every little problem people feel then they also will have all of their 8 injectors fail too!!!! Wow that is a lot of injectors!!!!
 

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I started a thread on it in hopes someone else was familiar. No idea if it was all 8, but he said they were ordering a complete set of 9 (1 for the emissions system) and replacing all of them before running any more tests. See his email below.

I have a SHELL company gas card. All fill ups have been at one of two SHELL stations so far, 90% at the one by my house where I have filled up for 14 years. Never had a fuel issue before with major brand pump diesel.

Here is the correspondence between Demontrond GMC in Houston, TX service advisor and myself:

Hoang Bui
May 14 (3 days ago)

to me

Good evening!



So it looks like we just got the diagnosis back on your truck a few minutes ago and we’re checking on availability of parts for now. You’ve got fuel injectors out in that engine, causing multiple misfires. For now that’s all we’re looking at for a fix but that is yet to be determined. We’ll put the injectors in and double check for you.



I will have to give you a loaner truck to drive while I get this done for you, if you could swing by. I’ll need your driver’s license, credit/debit card, and a valid insurance to issue you a loaner truck.



Let me know if you have any questions or need anything from us in the meantime! I do have a phone number 281-987-0697 but I’m unsure if that’s your business number or cell phone. I hadn’t called to update yet as I’m still figuring out how long the parts will be.



Thanks!

Hoang Bui

281-877-3045

[email protected]



From: Tommy Rice [mailto:[email protected]]
Sent: Monday, May 14, 2018 3:31 PM
To: Hoang Bui <[email protected]>
Subject: White Denali 3500 Diesel[/COLOR]
 

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Discussion Starter #18
Took to dealer

I'm the original poster.
Follow up is I took truck to the dealer, they said they have had reports of the same issue I'm having. They said all modules had the latest updates. The Technician said the only thing he could was to put transmission in relearn mode. He also said to drive it a little hard. put foot into it some. these trucks don't like to be babied too much.
i'll give it a try.
 

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Just joined the forum. Greetings all. I have a 2018 3500 duramax and its doing probably the same thing at intermittent times. It seems like at about 40-50 mph range and accelerating there is a stagger and would agree with some others it seems to be a transmission issue (going in and out of kick down mode??).. told dealer and as is intermittently issue he didn't think there would be codes to pull but am taking it in for 6 month service and will verify updates he checked on were done as they were SUPPOSED to have been already complied with. Will post update after visit.
 

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I had this issue last June during a 4,000 mile vacation around the western US. Three times it "stumbled" under acceleration getting onto the highway/freeway. Mine acted as though someone had shut the fuel off for a moment and then turned it back on. This occurred on three separate days after fueling at three different stations, always major name brand. Took it to the local dealer upon returning home and they "could not duplicate" the problem. No further issues since then for mine.

Hope you can get yours figured out.

Paul
 
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