Very Fast DEF Usage - Chevy and GMC Duramax Diesel Forum
2017-19 GM T-K2 Platform Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission, Etc.

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post #1 of 50 (permalink) Old 08-28-2017, 02:15 PM Thread Starter
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Very Fast DEF Usage

I now have 6,000 miles on my 2017 Duramax. Twice now I have had the 30% DEF caution come on and at the same time have had DEF mileage remaining warnings showing only 130 miles and 160 miles. On both occasions, the DEF level went from 30% to reading "Low" after about 50 miles, along with the caution that says something like "DEF Low - engine will soon be speed limited". In my 2015 Duramax, 30% DEF corresponded to approx 1000 miles remaining, now it seems to correspond to about 150 miles remaining. Also, assuming that the DEF tank was full when I got the truck and again full after a service where they said they topped up the DEF tank, both times I got the 30% DEF light after about 1000 miles of towing a 7500 lb travel trailer. I have now used 4 tanks of DEF in 6,000 miles (most of those miles towing the 7500lb travel trailer)

Is this normal? Anyone else seeing this behavior? It sure seems like my truck is burning through DEF at a rapid pace.

P.S. The first time this happened, I had to find DEF in the middle of nowhere, Montana, and filled up with some unknown brand of DEF. Shortly after I got a CEL The code was P0821. This happened again 3000 miles later - same code P0821. The fix, applied after the second incident, according to Chevy dealer was to reprogram the TCM and ECM. No CELs since that fix in around 1000 miles of towing.
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post #2 of 50 (permalink) Old 08-28-2017, 02:44 PM
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You assuming they ever filled the tank is the issue.

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post #3 of 50 (permalink) Old 08-28-2017, 02:49 PM Thread Starter
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You assuming they ever filled the tank is the issue.

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No, even if they didn't fill the tank, having the 30% DEF caution come on at only around 130 miles until "DEF Low" is the real issue. Shouldn't the 30% DEF remaining light come on with around 1000 miles left until DEF is empty. Also, going from 30% DEF to "DEF LOW" in less than 50 miles is also an issue (I think). How many miles of DEF remaining are others seeing when the 30% DEF caution comes on? How many miles between 30% DEF and DEF Low warnings?
Note - both times this happened I was towing 7500 lb travel trailer
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post #4 of 50 (permalink) Old 08-28-2017, 03:26 PM
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I have seen other reports of very heavy DEF usage when towing with the L5P, maybe that is one reason for the increase in DEF capacity in 2017 to 7 gallons. The diesel supplement doesn't make a lot of sense either because it states when the 1,000 mile warning appears approximately 3.8 gallons are needed to fill so unless the warning is VERY conservative the 3.2 gallons in the tank at the 1,000 mile warning isn't expected to last very long. According to my math the manual indicates the 1,000 mile warning comes on with about 45% remaining in the tank (3.2 gallons left out of the original 7).

There is probably a way to make the warnings/level readout more asinine and less useful than current but it would take a room full of consultants to do so. With all of the electronic horsepower currently available a simple and fairly accurate readout of DEF tank fill level and the ability to use the trip computer to display average DEF consumption per 100 miles would make it a lot easier on owners to plan ahead.

I am pretty set on ordering a 2018 Denali to replace my 2006 Sierra SLT but stuff like this makes me sometimes wonder if I am about to make a mistake. The L5P equipped Denali would be my third Dmax powered Sierra and the first two have treated me very well.
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post #5 of 50 (permalink) Old 08-28-2017, 05:05 PM
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i have seen other reports of very heavy def usage when towing with the l5p, maybe that is one reason for the increase in def capacity in 2017 to 7 gallons. The diesel supplement doesn't make a lot of sense either because it states when the 1,000 mile warning appears approximately 3.8 gallons are needed to fill so unless the warning is very conservative the 3.2 gallons in the tank at the 1,000 mile warning isn't expected to last very long. According to my math the manual indicates the 1,000 mile warning comes on with about 45% remaining in the tank (3.2 gallons left out of the original 7).

There is probably a way to make the warnings/level readout more asinine and less useful than current but it would take a room full of consultants to do so. With all of the electronic horsepower currently available a simple and fairly accurate readout of def tank fill level and the ability to use the trip computer to display average def consumption per 100 miles would make it a lot easier on owners to plan ahead.

i am pretty set on ordering a 2018 denali to replace my 2006 sierra slt but stuff like this makes me sometimes wonder if i am about to make a mistake. the l5p equipped denali would be my third dmax powered sierra and the first two have treated me very well.
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post #6 of 50 (permalink) Old 08-28-2017, 06:40 PM
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Dealer probably never filled it.. I know mine never did and I've been putting in 5 gallons every 3 or 4k miles it seems like.. And mine will go from 30% to low in a pretty short time also but I don't really watch that much.. Generally when I first see the message come on when it's just below a thousand mile range I'll just fill it up again..
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post #7 of 50 (permalink) Old 08-28-2017, 10:18 PM
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Very Fast DEF Usage

Monitor the DEF yourself. Do not rely on the dealer. My L5P has used 13 gallons in 8,000 miles. In my opinion is too much. No weird messages. The one time I let it get low I saw a 3xx Miles to go message. There is no denying the DEF gauge sucks.


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Last edited by Werdnamirg; 08-28-2017 at 10:21 PM.
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post #8 of 50 (permalink) Old 08-29-2017, 05:26 AM
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Fill the DEF and fuel tank... with fresh juice right when the DEF warning pops. Write down the mileage. Observe as you "daily drive". Report.

Our L5P with 1800 highway miles, idled in our drive for about 20 minutes then messaged me to drive until message went away. Seems excessive to me that an idle cannot be done... unless this is an engineering punishment for idling too long and should shut it off.

DEF fluid lasted about 1600 miles from dealer delivery. Refill did NOT take 7 gallons...just one jug... which I found interesting... yet the power and speed occurred before I could refill. Next long trip I will carry a jug or perhaps 2..

Last edited by cfell; 08-29-2017 at 05:32 AM.
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post #9 of 50 (permalink) Old 08-29-2017, 07:16 AM
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Our L5P with 1800 highway miles, idled in our drive for about 20 minutes then messaged me to drive until message went away. Seems excessive to me that an idle cannot be done... unless this is an engineering punishment for idling too long and should shut it off.
I believe the message you received was from inconvenient timing; the truck was due for a regen at the time you decided to idle it.

I hope that Edge has their Insight gauge display updated to show soot grams with the L5P before I order my new Denali because it would be very useful to know when the truck is approaching the need to regen so that I can plan my driving accordingly. And being able to force an active regen when soot loading is approaching the point where a regen is automatically triggered would also be valuable so that it could be done while on a normal highway run.
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post #10 of 50 (permalink) Old 08-29-2017, 08:38 AM
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I ONLY tow with my truck and I average about 750 miles a gallon of Def. This is a close guess. The dealership if you look at the original PDI paperwork was ONLY supposed to fill the tank with up to 2.5 gallons. Some people got lucky and there dealership filled it for them. The New L5P uses about 15% more Def than my LMM truck and this is due to more stringent EPA standards.

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