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Hey guys,
This might be my first post (I can't remember if I've ever posted before). I read the site all the time to troubleshoot for some of my older LB7 trucks.

Anyways, to the point. I bought two 2016 Chevy 3500 Diesels off the lot in November (well two different dealerships, long story). This truck has about 2500 miles on it and for the past couple months it has randomly started surging or fluctuating RPM's at idle. It will only happens randomly but once it starts it will always do it on that key cycle. Once you shut the truck off, it goes away. So this typically happens when coming to a stop sign or stop light. The RPMs will jump from the normal 800 or so RPM's to 1100 RPM's. Which causes the truck to lunge forward as you're slowing down (unless you press abnormally hard on the brakes). Also, when it starts doing this, the transmission will clunk when it shifts. So there are a few concerns here. Pulling the truck in the garage and running into the wall. Pulling up to a light and running into the back of somebody. And of course the shifting of the transmission which can't be good for the transmission. Also, it does not throw any codes.

We have been given the ridiculous runaround by GM and the dealerships in the area. We recorded the issue and tried to at least talk to a service writer about the problem, and we were told that they couldn't do anything with a video. I understand that he can't fix a truck with a video. But where it is so intermittent and it doesn't throw a code, we thought he might at least be interested to see what our complaint is on a truck that cost $63,000 3 months ago. Nope. So he says bring it back when its doing it and they'll look at it. So a few days go by and it starts doing it again. It's 4:00 at this point, we drop what we're doing and take the truck to the dealer. The service manager says he can't have anybody look at it right now...well that's just convenient now isn't it? So we wait another week till it starts again. We take it to a different dealer to see if they would bother to make time for us. Sure enough they did. We were there 3 hours at the dealership, between waiting, test driving and talking. They acknowledge the problem and we leave it with them to work on it. Truck is there all day, idling and being test driven while they try to diagnose the issue. The tech had supposedly been on the phone with GM tech support (or whatever he called it) off and on all day. They couldn't figure it out. 5 days later he calls and says it was 2 quarts low and transmission fluid and it's fixed. We thought, hey maybe it was that simple, but no. A week and a half later it starts doing it again. It's Saturday this time and they said they wouldn't be able to look at it. So we bring it in Monday, even though the issue was not present on Monday. We figured we would leave it there and let the dealer work on it until it was fixed. 7 days later (today) they say they can't get it to replicate and to come pick it up. WTH!!!!This is one of our work trucks! We bought it for work! We can't stop work in the middle of the day to drive 20-30 miles to the dealer to drop it off! We have talked to GM customer care and I have to be honest, this has been just about the worst experience I've had with a vehicle. The lady we talk to acts like she knows nothing about the case. The third time talking to her she says there has to be a problem with the truck in order for them to fix it! It's like she is unaware that the dealership has had our truck for 12 days trying to find the solution to a REAL problem that the computer is not picking up on. A problem that the tech and service manager personally witnessed and acknowledged was not right! We can't keep bringing the truck in for the same problem and hope that some day they find a solution! In the meantime causing who knows what damage to the transmission! And in the meantime leaving me with a truck that I can't fit with all my tools and work gear. Because at any given moment I'd have to unload it all so I can drop it off with the dealer!

Oh, but they do have a Chevy Cruze they can loan me, if its ever available, which it hasn't been, and not that it would help me tow my gooseneck trailers with skid steers and mini excavators on them!

So, why am I coming to the forums? Well, I'm looking for advice. What would you guys do in this situation? I've told GM they should take this truck back and give me a new one. It is in absolutely perfect condition since I haven't been able to do anything with it yet. Then they can fix it in their own time on their own dime. And I wouldn't be stuck without a truck and losing money every day they have it in the shop! Am I talking to the right people? Do I need to threaten legal action? What the heck do I need to do to get some help?

Thanks all, and sorry for the long rant...a little pissed off right now.
 

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It won't let me edit the post. Sorry its about 750 to 1000 RPM's is the fluctuation. If I could figure out how to upload a video (is that possible?) I would. Not that I'm really looking for diagnosis since it's under warranty. But might help show that I do have a REAL problem.
 

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Are the rpm's actually fluctuating, or does it just go up in rpm and stay there? Could it maybe be trying to go into a regen, but something isn't quite right? I'd think that would throw a code though as picky as the emissions system is.
 

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They are fluctuating. So as you're at idle, it'll rev up quickly, then drop slowly back down to normal idle, then it'll sit there at normal idle for a couple seconds and rev back up. It'll continue doing that until you shut the truck off and start it again. Then it goes away. No codes, nothing. I kinda thought it was something to do with the high idle or the regen as you called it.
 

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My 2014 GMC 6.6 is doing a similar thing as you described. The engine temp will be fully warmed up and the outside temp is 65-70 and after driving for 20 minutes I will notice the RPM's are at 800 when they are normally around 600 at idle. When I come to a stop it seems that I need to apply more pressure on the brakes to stop and like you described you feel like you are going to hit something. When I'm sitting in traffic it also feels like the truck launches at the car in front of me when I let off the brake to idle forward. I have a close friend that works for GM as a parts manager and he pulled the records up on my truck and it shows that before my truck even left the plant it had a Idle Control Speed Module replaced and my friend said that was common
. I wanted to look here on the forum before buying one to see if anyone had the same issue or if this was normal. I first suspected it was something to do with the DEF system operating but this is all new to me. Just a note: I have not made any mods to my truck that would be related to this. It sounds like your situation has been a real pain which I'm very familiar with being that I had to tell a service tech at Jeep how to repair my 2014 Grand Cherokee after it was in the shop for three months and he replied with " I tried what you said and it worked". I have no faith in these dealerships anymore. I hope we can get some answers here. Good luck!
 

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Hey guys,
This might be my first post (I can't remember if I've ever posted before). I read the site all the time to troubleshoot for some of my older LB7 trucks.

Anyways, to the point. I bought two 2016 Chevy 3500 Diesels off the lot in November (well two different dealerships, long story). This truck has about 2500 miles on it and for the past couple months it has randomly started surging or fluctuating RPM's at idle. It will only happens randomly but once it starts it will always do it on that key cycle. Once you shut the truck off, it goes away. So this typically happens when coming to a stop sign or stop light. The RPMs will jump from the normal 800 or so RPM's to 1100 RPM's. Which causes the truck to lunge forward as you're slowing down (unless you press abnormally hard on the brakes). Also, when it starts doing this, the transmission will clunk when it shifts. So there are a few concerns here. Pulling the truck in the garage and running into the wall. Pulling up to a light and running into the back of somebody. And of course the shifting of the transmission which can't be good for the transmission. Also, it does not throw any codes.

We have been given the ridiculous runaround by GM and the dealerships in the area. We recorded the issue and tried to at least talk to a service writer about the problem, and we were told that they couldn't do anything with a video. I understand that he can't fix a truck with a video. But where it is so intermittent and it doesn't throw a code, we thought he might at least be interested to see what our complaint is on a truck that cost $63,000 3 months ago. Nope. So he says bring it back when its doing it and they'll look at it. So a few days go by and it starts doing it again. It's 4:00 at this point, we drop what we're doing and take the truck to the dealer. The service manager says he can't have anybody look at it right now...well that's just convenient now isn't it? So we wait another week till it starts again. We take it to a different dealer to see if they would bother to make time for us. Sure enough they did. We were there 3 hours at the dealership, between waiting, test driving and talking. They acknowledge the problem and we leave it with them to work on it. Truck is there all day, idling and being test driven while they try to diagnose the issue. The tech had supposedly been on the phone with GM tech support (or whatever he called it) off and on all day. They couldn't figure it out. 5 days later he calls and says it was 2 quarts low and transmission fluid and it's fixed. We thought, hey maybe it was that simple, but no. A week and a half later it starts doing it again. It's Saturday this time and they said they wouldn't be able to look at it. So we bring it in Monday, even though the issue was not present on Monday. We figured we would leave it there and let the dealer work on it until it was fixed. 7 days later (today) they say they can't get it to replicate and to come pick it up. WTH!!!!This is one of our work trucks! We bought it for work! We can't stop work in the middle of the day to drive 20-30 miles to the dealer to drop it off! We have talked to GM customer care and I have to be honest, this has been just about the worst experience I've had with a vehicle. The lady we talk to acts like she knows nothing about the case. The third time talking to her she says there has to be a problem with the truck in order for them to fix it! It's like she is unaware that the dealership has had our truck for 12 days trying to find the solution to a REAL problem that the computer is not picking up on. A problem that the tech and service manager personally witnessed and acknowledged was not right! We can't keep bringing the truck in for the same problem and hope that some day they find a solution! In the meantime causing who knows what damage to the transmission! And in the meantime leaving me with a truck that I can't fit with all my tools and work gear. Because at any given moment I'd have to unload it all so I can drop it off with the dealer!

Oh, but they do have a Chevy Cruze they can loan me, if its ever available, which it hasn't been, and not that it would help me tow my gooseneck trailers with skid steers and mini excavators on them!

So, why am I coming to the forums? Well, I'm looking for advice. What would you guys do in this situation? I've told GM they should take this truck back and give me a new one. It is in absolutely perfect condition since I haven't been able to do anything with it yet. Then they can fix it in their own time on their own dime. And I wouldn't be stuck without a truck and losing money every day they have it in the shop! Am I talking to the right people? Do I need to threaten legal action? What the heck do I need to do to get some help?

Thanks all, and sorry for the long rant...a little pissed off right now.

I have a 2016 LTZ and have been experiencing the same problem when the truck ideling high it will slam into gear and make a horrible clunking noise. I took it to the dealer also and they said I had to bring it to them when it was doing it. Have you had any updates on what the problem was or if it went back to the dealer?

Thanks
 

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Hi guys. Just chiming in here as well. Bone stock, mothered, garage queen of a 2015.5 Denali truck and idle according to the dealer is supposed to be 675, mine is jumping to 950 and holding through multiple key cycles. Dealer said it is a setting discussed in the owners manual and changeable in "Settings" on the dash and it's purpose is to create fewer emissions while cold. Well, that sounds great but......I already knew I had the fast idle option set at "Off" and I confirmed it during one of these episodes that the truck was plenty warmed up and operating very contently at 210 degrees. Something smells in Denmark.
 
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IMO you've got a problem in a wiring harness or on a circuit board (a very tiny bleed over of voltage or short out caused by temperature or vibration) and they may never figure it out. I had a Mercury outboard that had a problem like that and I eventually sold the boat because of it.
 

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2015 gmc

My 2014 GMC 6.6 is doing a similar thing as you described. The engine temp will be fully warmed up and the outside temp is 65-70 and after driving for 20 minutes I will notice the RPM's are at 800 when they are normally around 600 at idle. When I come to a stop it seems that I need to apply more pressure on the brakes to stop and like you described you feel like you are going to hit something. When I'm sitting in traffic it also feels like the truck launches at the car in front of me when I let off the brake to idle forward. I have a close friend that works for GM as a parts manager and he pulled the records up on my truck and it shows that before my truck even left the plant it had a Idle Control Speed Module replaced and my friend said that was common
. I wanted to look here on the forum before buying one to see if anyone had the same issue or if this was normal. I first suspected it was something to do with the DEF system operating but this is all new to me. Just a note: I have not made any mods to my truck that would be related to this. It sounds like your situation has been a real pain which I'm very familiar with being that I had to tell a service tech at Jeep how to repair my 2014 Grand Cherokee after it was in the shop for three months and he replied with " I tried what you said and it worked". I have no faith in these dealerships anymore. I hope we can get some answers here. Good luck!
Has anyone figured out what was causing this? I am having the same issue and it is bugging the shit out of me!
 

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I have the same issue also, 2016 denali Duramax idle at 950 rpm most the of time and does Not throw any codes either. finally bite the bullet brought it in dealer hopefully the computer at the dealership figure out the issue. Unfortunately could not do shit, so after 2 days sitting at the dealer they finally found the engine harness was rubbing against the egr/ac condenser causing a shortage. they replace 2-3 wire were burned down to copper. problem solved and cost me $525 plus tax. all they did was using electrical tape and zip ties the harness out of the way.
 

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The wire rub has been an issue since the 2011 models. I found the issue on my ‘12 and fixed it before it caused any problems. These symptoms are possibly because of the rub issue.
@smokester, has the dealer reflashed the ecm and tcm?
 
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