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Duramax Fifth Generation: 2011-2016 (LML) Discuss the fifth generation (2011-2016) of the 6.6L Duramax diesel engine & associated components. Engine related discussion ONLY.

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Old 12-19-2019, 10:42 AM   #1 (permalink)
paoutdoorsman
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HELP: 2015 starts and idles but will not take throttle

Need some help on figuring this one out. 2015 GMC Sierra 2500HD Duramax - 125K miles - Single owner - All stock. Historically this has been an intermittent problem that has occurred a half a dozen times or so now over the last 50K miles. It hasn't occurred in over a year (around 20k miles), until last night. It was driven to work in the morning (45 min drive), perfectly fine. Started the truck after work and it fired up and idled fine, but when attempting to drive it just sputters and bucks around when the go pedal is pressed. If you put it park and press the throttle it won't rev smoothly, and won't go to much more than 1300 rpm. It pulses like a smoker pulling tractor building power/boost before dumping the clutch.

In the past, stopping and restarting the truck has resolved the issue. Last night, around 25 restarts and it was still acting the same.

The truck has been randomly setting a P049D, that comes and goes but has not presented any drivability issues. I checked the codes and this was the only code stored.

I suspected possible fuel restriction, but using a Kennedy Diesel test gauge showed only 2" of vacuum.

On a whim, I disconnected both negative battery cables. Cycled the ignition 2 times. Then reconnected the battery cables. The truck started and instantly ran perfectly. It would rev out smoothly and at 3K RPM I still only showed 4" vacuum on the test gauge. Drove the truck home and it ran flawlessly.

6:30 this morning, went out to head to work and it's doing the same thing. Not drivable. Before I try to 'reset' the thing again, what else should a guy check to pinpoint the issue with this animal?

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Old 12-19-2019, 02:40 PM   #2 (permalink)
GL1800
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I had the same symptoms and it turned out to be the transmission Control Module. Changed it out and never looked back.

Jeff

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Old 12-19-2019, 03:37 PM   #3 (permalink)
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I had the same symptoms and it turned out to be the transmission Control Module. Changed it out and never looked back.

Jeff
Interesting. Did yours act up only when in gear, or in park as well? Can I swap in a TCM from another LML to test, or do they have to be programmed?
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Old 12-19-2019, 03:59 PM   #4 (permalink)
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Sounds to me like fuel restriction.
It was 8 degrees in Ohio this morning. It's cold enough to cause gelling problems. Is you fuel treated?
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Old 12-19-2019, 04:58 PM   #5 (permalink)
GL1800
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Originally Posted by paoutdoorsman View Post
Interesting. Did yours act up only when in gear, or in park as well? Can I swap in a TCM from another LML to test, or do they have to be programmed?
I purchased it on Amazon and it was genuine AC Delco. Had to take it to the dealer to get it programmed. Saved a lot by going this way.

Jeff
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Old 12-20-2019, 08:52 AM   #6 (permalink)
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Got this figured out last night. The EGR valve was stuck open at 55%. My Actron scan tool showed the actual vs commanded EGR position and while the truck was being stupid, I could see the ECM was commanding 0% open, but the valve was actually stuck around 55%. A few key cycles later, and a few light hammer taps on the EGR valve, it free'd up and starting moving to commanded position. Instantly the truck runs flawlessly again. New EGR valve ordered.
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Old 12-20-2019, 09:38 AM   #7 (permalink)
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Quote:
Originally Posted by paoutdoorsman View Post
Got this figured out last night. The EGR valve was stuck open at 55%. My Actron scan tool showed the actual vs commanded EGR position and while the truck was being stupid, I could see the ECM was commanding 0% open, but the valve was actually stuck around 55%. A few key cycles later, and a few light hammer taps on the EGR valve, it free'd up and starting moving to commanded position. Instantly the truck runs flawlessly again. New EGR valve ordered.
Glad that you got it sorted out.
Thanks for giving us an update

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Old 12-20-2019, 04:21 PM   #8 (permalink)
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thanks for following up with what the fix was.
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Old 12-20-2019, 05:20 PM   #9 (permalink)
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Gotta love having an assignable cause!
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