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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.
 

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Did you fill the DEF tank before you left? I ended up adding 7 gallons on my new truck on a trip over the first 1500 miles but clearly it was only at the minimum levels when I picked it up new. Since then it has settled into expected usage. If it were mine, I would just keep driving it until the warning comes on. Keep a 2.5 gallon DEF container in the truck just in case.
 

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Nutdriver,

Not sure about everyone else but I've done some extensive calculations since I got my truck and figured out that my DEF was being used at a rate of about 777 mi/gal of DEF. Was fun when I went to the dealer after they did my first maintenance and I had to school them on the calculations I did: I asked before they performed the maintenance if they were going to top off my DEF tank since it was at 14% when I brought it in and they said yes, only to discover that a week later it was reading low... I ended up going back to ask them how this was possible and they tried to tell me there might be a leak- I told them I had already checked myself and there was no leak or cracks or anything I could see but that they were welcome to check it but I was pretty sure they didn't fill it. After a lot of back and forth the manager came out and tried to BS me that they had filled up my DEF tank. He was really surprised when I showed him my calculations on this as well and that my numbers showed they must've only put 1 gallon, just enough to indicate the level was "OK", or else something was seriously wrong with the truck and that being the case I wanted them to figure it out and prove it to me. At that point he had no choice but to admit they must've only put 1 gallon, not topped it off. I asked if that was standard practice and he beat around the bush and ultimately said yes, they just put enough to get rid of the percentage warning which I think is ridiculous because if they're supposed to top off fluids at a maintenance interval, this should be included IMO. Anyhow, my calculations have been spot on so far with the 777 mi/gal estimate. I'm sure driving habits can make that differ but that's how mine has been.

On a side note, couldn't help but notice your TDIs in your signature. I had a 2014 VW Jetta Sportwagen TDI. Opted for the buyback and turned her in Nov. 2018 and upgraded to my truck. Got $28,300 from VW for my wagon with over 65k mi. on it. Miss that wagon, was getting 40-55 mpg with it. Enjoy your TDI.
 

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Nutdriver,

Not sure about everyone else but I've done some extensive calculations since I got my truck and figured out that my DEF was being used at a rate of about 777 mi/gal of DEF. Was fun when I went to the dealer after they did my first maintenance and I had to school them on the calculations I did: I asked before they performed the maintenance if they were going to top off my DEF tank since it was at 14% when I brought it in and they said yes, only to discover that a week later it was reading low... I ended up going back to ask them how this was possible and they tried to tell me there might be a leak- I told them I had already checked myself and there was no leak or cracks or anything I could see but that they were welcome to check it but I was pretty sure they didn't fill it. After a lot of back and forth the manager came out and tried to BS me that they had filled up my DEF tank. He was really surprised when I showed him my calculations on this as well and that my numbers showed they must've only put 1 gallon, just enough to indicate the level was "OK", or else something was seriously wrong with the truck and that being the case I wanted them to figure it out and prove it to me. At that point he had no choice but to admit they must've only put 1 gallon, not topped it off. I asked if that was standard practice and he beat around the bush and ultimately said yes, they just put enough to get rid of the percentage warning which I think is ridiculous because if they're supposed to top off fluids at a maintenance interval, this should be included IMO. Anyhow, my calculations have been spot on so far with the 777 mi/gal estimate. I'm sure driving habits can make that differ but that's how mine has been.

On a side note, couldn't help but notice your TDIs in your signature. I had a 2014 VW Jetta Sportwagen TDI. Opted for the buyback and turned her in Nov. 2018 and upgraded to my truck. Got $28,300 from VW for my wagon with over 65k mi. on it. Miss that wagon, was getting 40-55 mpg with it. Enjoy your TDI.
Wonderwoman - I have not tried to be that precise in part because my DEF tank was clearly not full when I picked up my truck new. I am trying to not fill the DEF tank until I get the first 30% warning and noting the miles driven at that point. I am trying to then fill the tank and drive until I get another 30% warning. I would then know roughly how many miles I am getting per refill. My use seems to be within expected levels.

When I had my vehicle serviced for one of the free oil change services, I had asked before the visit and they indicated that they would add 1 gallon to my DEF. I am not sure in hindsight that was done since it was not listed on my service record.

The VW TDI Jetta has been a great daily driver. I have 280,000 miles on it currently. Prior to this 2006 VW, I had a 2000 TDI with 160,000 miles totaled by a deer hit. Both have been reliable cars and the 41-45 mpg has been great. Not unusual to see 60+ mpg when driving with a decent tailwind.
 

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I knew how much the dealer I purchased from had in it when I took it off the lot so I’ve been able to do my calculations the whole time which is how I got that number. I’m sure driving habits probably have a play in it too but in my case I have the same route every day. It’s my daily driver. I definitely get the over 5k miles from a full tank of DEF 🙂

Man... It was so hard for me to part with my wagon. I knew I would probably regret doing the buyback but the money they offered was too insane not to do it. Before the TDI scandal, my intention was to drive it til it fell apart and I figured I could get close to 500k miles from it. I sure miss it but if the truck treats me well, it’ll have been a good move.
 

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Discussion Starter #8
Expected DEF Usage

Did you fill the DEF tank before you left? I ended up adding 7 gallons on my new truck on a trip over the first 1500 miles but clearly it was only at the minimum levels when I picked it up new. Since then it has settled into expected usage. If it were mine, I would just keep driving it until the warning comes on. Keep a 2.5 gallon DEF container in the truck just in case.
Yes, mine was at minimum level when I picked it up. However, since then, I've been recording DEF usage. I do carry DEF with me at all times, since if I put 2.5 gallons in, in 600 miles it's telling me Action Required add DEF. I don't think my expected usage should be 2.5 gallons every 600 miles, it should be more like 2500 miles every 2.5 gallons. My 2016 Chevy Duramax went over 5000 miles on 5 gallons.
 

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Similar Problem

I also have a 2019 Denali 3500HD, and have also been seeing very high DEF usage. It used over 12 gallons of DEF in the last 2400 miles. 2200 of the miles were towing a boat (~10,000 lbs). I also am not getting any warnings. It goes from "OK" to speed will be reduced to 65 in x miles. I had to put 2.5 gallons in every 400 miles not he return trip home. Also no leaks in the system and no drop in power or performance. Please update this post if you get it fixed.
 

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OK, here's my calculations: '18 High Country Dually, 98% of mileage of just over 15K pulling heavy approx 19.5K pounds.
I wait until I get the 30% DEF warning and at that point I can dump in 5 Gal of DEF. Just did the cal and I'm burning
a Gallon of DEF every 471 miles, that's a lot more than my '11 LML did pulling the same weight.
 

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GM has issues with the DEF gauge readout. I know mine is a 2020 BUT it happens on the earlier 18 and 19's as well. Mine jumps around even when it is almost full shows half or on a recent trip gave me the warning of xxxxx miles until empty. It took 1.4 gallons to fill it to the top when showing almost empty. After filling went to half then 2 days later went to full.

Point is it is not accurate and I carry DEF all of the time. Gave GM the notification letter (Dealer got a copy as well) per the instructions, call to them verified and have a case number now. Pursuing this Dealer under the Vernon Articles (Texas LL). The software was flashed on the 1st attempt did nothing and waiting for the strike to be over and the repair backlog at my dealer which is one of the biggest ones GM has because of parts non availability. This will be the 2nd attempt out of 3 and the Dealer is working with GM to determine what the next repair attempt will entail.

Do not want to be in the middle of nowhere and the truck decide it is out of DEF and goes into limp mode. Will just let them replace it if they cannot fix it.
 

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I too maintain a spreadsheet. On my '19 with 7,800 miles I'm getting 770 miles/gallon. About 70% of miles in town, 30% hiway and towing
 

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OK, here's my calculations: '18 High Country Dually, 98% of mileage of just over 15K pulling heavy approx 19.5K pounds.

I wait until I get the 30% DEF warning and at that point I can dump in 5 Gal of DEF. Just did the cal and I'm burning

a Gallon of DEF every 471 miles, that's a lot more than my '11 LML did pulling the same weight.


That would probably be about right. How many gallons of diesel did you burn over the same time. Def usage is usually 1-2 % of diesel usage.

One guy on another forum said the def could be diluted to about 66% of normal def and it would still work fine. That would be 2 gallons of def and a gallon of distilled water. But I wouldn’t do this in a real cold climate.
 

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One guy on another forum said the def could be diluted to about 66% of normal def and it would still work fine.
I always find these posts somewhat humorous as with no data and just some "person" on the internet says ok do "this" some people actually do it. When something goes wrong and for whatever reason they (GM warranty issue only) check the DEF fluid just like they do the fuel and it is not to specification -- you are out of luck.
 

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Based on 20k miles of driving with all fillups (Diesel and DEF) recorded, both towing and non-towing, my 2018 L5P burns DEF at approximately a 2.2% rate. At that rate, I burn about a Gallon of DEF for every 45 gallons of fuel.
 

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One guy on another forum said the def could be diluted to about 66% of normal def and it would still work fine. That would be 2 gallons of def and a gallon of distilled water. But I wouldn’t do this in a real cold climate.
I always find these posts somewhat humorous as with no data and just some "person" on the internet says ok do "this" some people actually do it. When something goes wrong and for whatever reason they (GM warranty issue only) check the DEF fluid just like they do the fuel and it is not to specification -- you are out of luck.
Saw that post too.

The part I found amusing/curious it that DEF is already 'diluted'....(2/3 is water, 1/3 is liquid urea).



FWIW, one of the reasons for the existing ratio is because that ratio freezes a low temp (about 12*F) and at the ratio used it will not 'salt out' during the freeze thaw cycle.

https://www.cumminsfiltration.com/sites/default/files/MB10033.pdf
 

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I haven't calculated how much Fuel I've used but I've had my 2019 since August 6, 2019 I just rolled over 4900 miles and I've put 4 x 2.5 (10 gallons) of DEF in it so far. They swore it was full when I left the lot but based on some of your comments it probably wasn't. Of that 4900 miles I did 1300 miles pulling my 38' 10K lb travel trailer and got a whopping 7 miles per gallon doing it. I hear wait for the break in but I traded my 2018 w/48K miles on it for this 2019 (long story) and it never got better than 7-8mpg pulling. Anyway, I know this is about DEF but seems based on some of your calculations for 10 gallons of DEF I should have gone 7000 miles. I'd say I'm using a lot. I used a ton on that 1300 mile trip as I had to stop about 800 miles in and add 5 gallons. Thank God I found a 24 hour Wal-Mart in the middle of no where cause I was stupid and started the trip without a spare bottle. It went from you have 999 miles to go to you are about to have problems in about 30 miles. It seems to be using quite a bit. I'll be keeping an eye on this and this post to see what you guys come up with.
 

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I have searched for your excessive regen recall and do not get a single hit. What year vehicle do you have? Can you provide any more info??

Thanks
 
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