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Excessive DEF consumption, 2019 Denali Duramax

33K views 81 replies 27 participants last post by  redwngr 
#1 ·
I have just over 13,000 miles on my 2019 GMC Sierra Denali Duramax now, all has been OK until this trip. My fuel mileage had been increasing, etc. as the miles went on. This trip, my DEF usage has gone to 7 gallons every 1500+ miles, fuel mileage has dropped to 9 mpg (it had been over 12 towing), I don't get the usual DEF notifications. No 30% or 1000 mile notification, just an Action Required Now-engine will shut down in 500 miles. The power seems to be OK. I have taken it to a shop on this trip, they checked the system for leaks and said they make some ECM updates--it didn't change anything. I have a GM reference number now so I'll be taking it in when I get home. Just wondering if anyone has any info I can help the dealer with. Thanks.
 
#42 ·
I'm still on my test after the dealer replaced the DEF injector, I have 3500 miles on the tank now and DEF level is still OK. The most I had logged before was 2500 miles not towing. Since this virus pandemic, it's been hard to get miles on but I may be heading to Texas in a couple weeks so that will be a good test.
 
#47 ·
Welcome to the club. Just dropped mine off this AM. Told them not to have me come back for it till something has been done. I’ll post what I find out. My expectations are low.


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#48 ·
They found nothing as I expected aside from a bad NOx sensor. I actually spoke with the mechanic who said he looked at everything he could and found nothing. Said the next step will be to press GM when it does it again.


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#50 ·
Sorry. Texas public school education..... writing skills are lacking. Anyway, after 800 miles I got the Def Empty message stating 500 miles till a limit of 65MPH. At that time, 2 gallons at the pump is all it would take. Sure seems like a sensor, but the dealer computer reads info from the same source, and at the shop it was apparently accurate. Mechanic was as baffled as I was.

Previous to that, earlier in the trip I had a message that Def was low(not empty)or something to that effect threatening impending speed reductions. On that occasion I added 5 gallons. It was boxed def so maybe could’ve taken more. That was legit low, but where were the early warnings?

Another time during the trip I got a message def at 30% 222 miles to go. Why is empty 500 to a 65MPH limit and 30% 222 miles. That eventually went away and changed to OK. I think my truck was smoking some of the good stuff in Colorado while I wasn’t looking.

As to the mileage, you’re right. That fifth wheel has a lot of frontal surface area. That’s a lot of resistance to fight.




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#52 ·
Sorry. Texas public school education..... writing skills are lacking. Anyway, after 800 miles I got the Def Empty message stating 500 miles till a limit of 65MPH. At that time, 2 gallons at the pump is all it would take. Sure seems like a sensor, but the dealer computer reads info from the same source, and at the shop it was apparently accurate. Mechanic was as baffled as I was.

Previous to that, earlier in the trip I had a message that Def was low(not empty)or something to that effect threatening impending speed reductions. On that occasion I added 5 gallons. It was boxed def so maybe could’ve taken more. That was legit low, but where were the early warnings?

Another time during the trip I got a message def at 30% 222 miles to go. Why is empty 500 to a 65MPH limit and 30% 222 miles. That eventually went away and changed to OK. I think my truck was smoking some of the good stuff in Colorado while I wasn’t looking.

As to the mileage, you’re right. That fifth wheel has a lot of frontal surface area. That’s a lot of resistance to fight.........
Hey... Nothing wrong with your writing skills. Thanks for the additional insights. Still a theory but this seems to point to an erratic DEF sensor. Maybe those that are more familiar with this system will comment. Sounds like your actual DEF consumption if only 2 gallons for 800 miles getting 8 mpg would be close to normal.
 
#54 ·
From what I understand, the ecm calculates def level more by consumption than by the level sensor. I believe the level sensor is just for the initial check during an ignition cycle. During vehicle operation, I don't think the ecm or reductant controller are watching it that closely. All of the system data can be seen with the GM scan tool. Techs need to know what to look for and what to do with that info first though.
 
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#55 ·
I was wondering the same. Since all these messages came up while towing I was thinking it must be looking at a trend or straight consumption to calculate this. In fact, on my LML my Edge tuner which I use as a monitor had a DEF level indication. On this engine it’s not an option. Only urea pressure. Maybe ties into what’s mentioned above. Also, that would explain why, when the tech was scanning everything it was fine. It’s not under load. Nothing is being consumed. So then, if the initial levels are correct how would they correct this?


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#57 ·
I am at my whits end and now researching Fords and Ram's. I can't afford to keep pulling this trailer cross country at 6mpg. I've had this truck to two dealerships and they can't find anything wrong with it. If I could find a low mileage LML I would go back cause I got 10-12 pulling with it and rarely if ever had to put DEF fluid in. I have to carry extra DEF with this L5P. My RV is 8K empty 10K max and this ridiculous.
 
#58 ·
After going to 3 dealers, I went back to my selling dealer (they are 3 hours away) and within 3 1/2 hours they found the DEF injector putting over twice the DEF into the system. They replaced the injector, reprogramed the system and I'm still on test. But I'm now at just over 3500 miles on the DEF tank, hoping it gets up to 6000 or 7000 miles like it's supposed to. I never got over 2500 miles before, 1500 miles towing. And my fuel mileage has gone up 5 miles/gallon. I'm getting 22mpg not towing, 14 towing now. 26000 miles on the truck now. 2019 Denali.
 
#60 ·
OK, I'm back from my trip to Texas after the DEF injector nozzle was replaced. My DEF usage did improve--it went up to just under 3800 miles on the 7 gallon tank, not towing. The truck is back at the dealer now but again, the dealer is telling me that GM is saying there isn't a usage chart so the usage is typical and to release the truck back to me. The dealer did say they tested the NOx sensor and the rest of the system and they found no problem. Also, the first notice I get is Action required, exhaust fluid low, speed limited soon. The dealer is now looking into that while they're waiting for parts for the engine block heater recall parts to come in. I don't know where to go from here, I really don't want to be adding DEF every 2000 miles when we start traveling with our RV. Does anyone have any authorized GM publications that might help me with this problem? Thanks.
 
#61 ·
Again, the GMC dealer is saying GM won't authorize any repairs because there is no specification for DEF usage. The dealer tells me the usage is way above what it should be but they can't get GM to authorize work to be done. On the forums, I see that most DEF usage issues have been resolved with replacement of the Nox sensors. And since the DEF injector nozzle has been replaced, would it benefit me to pay to have the Nox1 and Nox 2 sensors replaced? I never get any CEL's and the only DEF notification i get is the Action Required, low DEF, add DEF now. This last test I did, I got 3700+ miles on the 7 gallon DEF tank, not towing. We're going to start traveling more and I really don't want to have to be adding DEF every 2000 miles.
Thanks for any recommendations.
 
#62 ·
I get just about 900 miles per gallon of DEF in my 2020 Denali. My truck has almost 35k on it now. Will tell you that 2 weeks ago when I got my fuel filter changed there was yet another software update for the DEF system that was required.
 
#64 ·
I had a Nox sensor replaced and all seemed good. Not towing, I was at 3500 miles and only added 2.5 gallons at the pump to top it off. This week, with 1487 miles since that DEF top off I went straight from OK to Def Empty 500 miles till speed limited. I topped it off at that point. Only took 4.7 gallons. This was about 1250 miles of towing my fifth wheel and the rest a city/highway mix. Figured I used up about 140 gallons of fuel give or take, so that’s about 3% per gallon of fuel on DEF. A little high, but not too concerning.

What’s concerning is once again, the message. It’s coming prematurely and skipping the early warnings. This happened while towing. I dropped off the trailer and shut to truck off to get fuel. When I powered it back up, DEF was OK again. This was before I added the 4.7.

Not the end of the world, but frustrating as there’s no codes. Nothing visible to the dealer to fix. I do have an appt next week with them, but thinking I’m waisting my time. Thoughts?


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#65 ·
I had a Nox sensor replaced and all seemed good. Not towing, I was at 3500 miles and only added 2.5 gallons at the pump to top it off. This week, with 1487 miles since that DEF top off I went straight from OK to Def Empty 500 miles till speed limited. I topped it off at that point. Only took 4.7 gallons. This was about 1250 miles of towing my fifth wheel and the rest a city/highway mix. Figured I used up about 140 gallons of fuel give or take, so that’s about 3% per gallon of fuel on DEF. A little high, but not too concerning.

What’s concerning is once again, the message. It’s coming prematurely and skipping the early warnings. This happened while towing. I dropped off the trailer and shut to truck off to get fuel. When I powered it back up, DEF was OK again. This was before I added the 4.7.

Not the end of the world, but frustrating as there’s no codes. Nothing visible to the dealer to fix. I do have an appt next week with them, but thinking I’m waisting my time. Thoughts?


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No you are not.. Find another Dealer.. This is by no means normal at all!
 
#67 ·
I have stayed pretty disciplined and waited to fill the def until the first warning goes off. After the warning I have typically still been driving for another 100 to 200 mi and add 5 gallons. Nearly all of this use is pulling a 5,000 lb trailer.. I've had two mileage durations of 2,000 mi each, and a third at about 2200 mi between fills. I'm using def at a rate of 2.5% compared to fuel.
 
#70 ·
Carl. He seemed like he was doing all he could. Last time, he took me back into the shop to talk to the tech which I appreciated as I didn’t get how a Nox sensor could fix my issue. The tech explained how he ran all the computer tests and all was good. This time I’m telling them not to call me till something has been replaced or reprogrammed.


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#71 ·
Carl. He seemed like he was doing all he could. Last time, he took me back into the shop to talk to the tech which I appreciated as I didn’t get how a Nox sensor could fix my issue. The tech explained how he ran all the computer tests and all was good. This time I’m telling them not to call me till something has been replaced or reprogrammed.


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Have not met Carl. When you go in next time talk to Joe Laubhan he is the Service Director. Office is at the end of where the service drive in area is glass around it on the south end. BE NICE!!!!
 
#72 ·
Thanks for the advice. The people I’ve dealt with up there have been great so far. This my second truck from them. They might take a different approach anyway given this is a repeat item. We’ll see. I’ll post up after that appt on the 21st.


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#73 ·
Well, they kept my car for about 1/2 the day and just did a reflash of the ECM. I don’t think it’ll fix it, but it’s one more thing to exhaust. I won’t know till next year as all my towing will be short trips of a few hundred miles. Empty it’ll take 5 months to work through the def. if it does it again, I’m going to have to stop being nice.

Too bad you can’t delete an L5P very easily these days.

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#74 ·
Well I know this is the Duramax Thread but I wound up trading my 2019 Denali HD for a 2019 Ford F250 Platinum. Pulled my trailer from Arizona to Moab, Utah and then over to Rocky Mountain National Park down to Durango and back to Arizona. It was 2400 trip all told. I averaged a little over 11 mpg on the trip including pulling in the Mountains and didn't have to put DEF in the whole trip. This truck has an actual DEF guage and I used about 3/4 of a tank which is approximately 5 gallons and I put 5 Gallons in when I got home. That sure beats the 5 gallons of DEF per 800 miles and 6mpg of Diesel I was getting out of my Denali. Never thought I'd go back to Ford after my 6.0L debacle but so far I'm pretty pleased. No weird shifting issues and my trailer brakes actually work now. Both my 2018 and 2019 had issues with the trailer brake controller. The Denali did hold down hill grades a lot better with the 10K trailer than the Ford does as I go to Colorado often and know I didn't ever have to touch my brakes going over the Berthoud Pass by Winter Park in the Denali but did several times in the Ford. Good luck to you guys I hope GM gets your issues worked out. I was tired of dealing with them and their BS.
 
#75 ·
Congrats on the new truck, they do look good!

Thats to bad the dealers were unable to get your truck sorted out. I absolutely love my 18, it was a bit sketchy at first as I had the chuggle and odd shifting at the beginning. After a bunch of towing and changing the trans fluid out to Transynd it has been great. Funny you had a 15 that you loved, my 15 was a nightmare. I swear the dealer was sabotaging that truck as it would get fixed and have to go right back for another problem.

Anyways, enjoy the new truck.
 
#78 ·
I have a 2019 3500 Duramax Denali . The dealership has no idea how much Def it should use. I was told by service manager that I should fill up twice e a year. I get bout 3000 miles to a 5 gallon tank. Anyone out there no a good solution to deleting the entire Def system ?
 
#79 ·
IIRC, DEF usage should be somewhere around 1% to 2% of your fuel consumption. So if you burned 100 gallons of fuel, you can plan on 2 gallons or so of DEF being used.

Deleting emissions equipment is a federal crime, and due to recent litigation by the EPA where forum posts were used in the legal proceedings, we cannot allow discussion in the open forum that pertains to deleting or otherwise altering the emissions control systems on these trucks. This is being done to protect our members and the forum itself.
 
#82 ·
My 2018 L5P rarely got worked, and it gave a little better than 1100 miles per gallon of def.
It would give over 7000 miles per tank doing the daily driver job.
Logged several fills over 6000, and put in 5.4x gallons to fill it to giving hand calcs between 1110 and 1140 mpg.
(6183/5.44, 6101/5.47 so 1135 and 1115 mpg)

I use bulk def locations so that it can be filled to the top each time for hand calcs like is commonly done to for fuel mileage.

I rarely fill before the 1000 mile to empty warning.
I have taken the L5P all the way to empty and watched the start of the 500 miles to speed limit countdown, just to see if/how it worked.

The countdown to speed limit can't be cleared from the DIC, and does countdown mile-to-mile when driving. Refilled it after about 60 miles. It took 6.35 gallons to fill it to the top.
 
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