2020 6.6L Duramax DEF Usage

cNate · June 22, 2020

I know our tanks hold 5 gallons of DEF fluid. I seem to burn through a gallon about ever 300-400 miles when towing. Makes sense that it would burn faster while towing. Dealership said I should be getting closer to 1k miles per gallon on DEF. Even when I’m not towing, I’ve not seen anywhere near that number. Anyone else been tracking how many miles per gallon they burn? Towing/not towing?

I don’t mind the cost of the DEF, just want to make sure I don’t get caught out in the dessert or mountains on crawl mode. I get pretty remote at times. I carry a couple boxes in the truck for long hauls. It’s also way cheaper if you fill up at truck stops versus buying Blue DEF online or at stores.

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Waaazooo · June 23, 2020

I know our tanks hold 5 gallons of DEF fluid. I seem to burn through a gallon about ever 300-400 miles when towing. Makes sense that it would burn faster while towing. Dealership said I should be getting closer to 1k miles per gallon on DEF. Even when I’m not towing, I’ve not seen anywhere near that number. Anyone else been tracking how many miles per gallon they burn? Towing/not towing?

I don’t mind the cost of the DEF, just want to make sure I don’t get caught out in the dessert or mountains on crawl mode. I get pretty remote at times. I carry a couple boxes in the truck for long hauls. It’s also way cheaper if you fill up at truck stops versus buying Blue DEF online or at stores.

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In a perfect world where you only working the engine at “x” load.. would that happen. You definitely use more def under higher engine load than at idle. I don’t know the math to figure it out. I don’t fill it up all the way. Usually around 100 miles to empty I put in 5 gallons and it lasts me around 3800-4000 miles...ish.

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ssowie · June 24, 2020

I believe there is a tsb or update that I saw on my last service paperwork about a def update that they went ahead and did. Maybe check with your dealer.

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redwngr · June 30, 2020

On 6/21/2020 at 11:01 PM, cNate said:

I know our tanks hold 5 gallons of DEF fluid. I seem to burn through a gallon about ever 300-400 miles when towing. Makes sense that it would burn faster while towing. Dealership said I should be getting closer to 1k miles per gallon on DEF. Even when I’m not towing, I’ve not seen anywhere near that number. Anyone else been tracking how many miles per gallon they burn? Towing/not towing?

I don’t mind the cost of the DEF, just want to make sure I don’t get caught out in the dessert or mountains on crawl mode. I get pretty remote at times. I carry a couple boxes in the truck for long hauls. It’s also way cheaper if you fill up at truck stops versus buying Blue DEF online or at stores.

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My LML got between 1100 and 1200 miles per gallon def not towing (which was most of the time)

My L5P seems to get a little less, but still around 1100 (hard to be precise, since a fill lasts so long, especially with the larger 7.0 gal tank)

Don't forget that when it gets to 'empty', you still have 500 miles before it limits to 65 mph, and then another 75 miles to 55 speed limit.

Even not towing, you probably need fuel within that range -- and the fuel locations likely also have def. (either boxes or bulk)

cNate · July 1, 2020

Thanks for the reply, that kind of mileage is in-line with what I expected not towing. I have a 2020 with the L5P mated to the Allison 10 speed. I thought it was 7 gallons too but the manual says 5 gallons for my truck and when it’s down to one bar, I can only add back 4.5 gallons. Have you been able to put more than 5 in yours?

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wtbrowne · July 1, 2020

Page 89 in the Diesel Supplement Manual. Def capacity is 7 gallons. The page you're showing says that 4.5 gallons is the approximate minimum amount of def that can be added at one bar showing. I don't see any mention of a 5 gal capacity.

Edited July 1, 2020 by wtbrowne
Added text

greatmizzou · July 1, 2020

What a nightmare of a thing to do to a truck. Is this all from the mall crawlers running coal all the time?

DEF was not the main reason but one of the little reasons I didnt want to upgrade to Diesel. If I didnt need the power I certainly didnt need the hassle

cNate · July 1, 2020

Page 89 in the Diesel Supplement Manual. Def capacity is 7 gallons. The page you're showing says that 4.5 gallons is the approximate minimum amount of def that can be added at one bar showing. I don't see any mention of a 5 gal capacity.

Thank you. I was factoring each bar was a half gallon and added up all the bars. Appreciate you pointing it out in the manual.

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Tampa_HD · July 1, 2020

My understanding is 1 bar on the gauge is 2.5 gallons. At 2 gallons it begins limp mode, not sure what happens if you go to 0 gallons DEF.

Poor planning to make it so you have to add 4.5 gallons to fill it up, but you have to buy it in 2.5 gallon units.

The small things that annoy me

redwngr · July 2, 2020

15 hours ago, Tampa_HD said:

My understanding is 1 bar on the gauge is 2.5 gallons. At 2 gallons it begins limp mode, not sure what happens if you go to 0 gallons DEF.

Poor planning to make it so you have to add 4.5 gallons to fill it up, but you have to buy it in 2.5 gallon units.

The small things that annoy me

It does not 'limp' at 2 gallons.

It does not even limp at 'empty'.

When it does Speed limit is still full power, but the max speed is reduced from the normal limit of approx 100 mph.

'The term Limp', as normally refers to a 'low power mode'

At approx 1000 miles to empty it gives a (one time) message indicating approx range remaining (like fuel range, it is an estimate base on 'recent' usage rate)

From about 300 miles range, it will give a message indicating approx 'miles to empty' at each start cycle.

When it is empty, escalating warnings/actions begin.

The first is a warning - '500 miles til 65 speed limit '.

For that 500 miles, it still runs normally, but there will be a coundown screen on the dic that cannot be acknowledged so it goes away.

I've intentionally taken my '18 L5P this far and ran about 60 miles looking at the countdown......

(never tried it on the LML)

At 500 miles, speed limit to 65 begins and warning of '75(iirc) miles til 55 mph speed limit'.

Then 5 mph speed limit.

For most, the 500 miles is going to get the truck to it's next gets fuel stop.

Most fuel stops also have DEF.

Until 2020MY the def bargraph didn't exist.

DEF DIC page would indicate 'OK' until the tank was down to approx 30% remaining. Then the page dispayed the approx %.

Those trucks also gave the 'miles to empty' warnings, and when empty gave the 'miles to speed limit' warnings.

2011 LML did not have any sort of def gauge. It's not that tough to manage. (and LML had a small (5.3 gal) def tank.

DEF level sensing wasn't super accurate on LML either. Mine wouldn't hold 5 gallons even at the 300 mile warning level (but lots of others reported being able to)

At the time, 2.5gal boxes were the most convenient for me -- so rather then dealing with a part box I changed to partial fill and just adding one.

1200 miles per gallon, so one box about every 3000 miles.

Point of the LML stuff is that part fill is always an option.

Yes, there are trucks out there with def systems in need of repair.

And we are learning that the DEF display doesn't operate like a typical variable gauge.

Tampa_HD · July 2, 2020

@redwngr

Wow, that's all really good information. Glad to hear she isn't going to just leave me stuck without much warning. I had a 2016 Duramax and made it to the first level warning and got scared so I stopped in the middle of no where and spent an hour looking for DEF. I guess I could have made it the 200 miles home.

I look forward to being able to fill it off the pump and not get the crystal residue on things under the hood when a small drop spills.

Thanks again for the information!

redwngr · July 3, 2020

11 hours ago, Tampa_HD said:

@redwngr

Wow, that's all really good information. Glad to hear she isn't going to just leave me stuck without much warning. I had a 2016 Duramax and made it to the first level warning and got scared so I stopped in the middle of no where and spent an hour looking for DEF. I guess I could have made it the 200 miles home.

I look forward to being able to fill it off the pump and not get the crystal residue on things under the hood when a small drop spills.

Thanks again for the information!

If you spill a little, grab a hose and wash it down.

It mixes easily with water.

cNate · July 4, 2020

Appreciate all the great info on the thread. There’s another thread on this with a similar title and people are mentioning a DEF system recall, a program update and one person had to have their exhaust repaired which was causing over usage. Search DEF on 2020 forum for more info.

Nate

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cNate · July 8, 2020

Just brought the truck in for oil change, DEF top off, tire rotation, and replace the fuel filter (at 29% after 12k miles). Lots of towing so not getting anywhere near the 18-20k it’s expected to last.

I asked them to check DEF system for recall or SERVICE BULLETIN and they found a software reprogram. They think it’ll help with the sensor reading the levels accurately now. I was having a problem where I fill up and it shows it was still half empty for a few hundred miles. Sometimes it was showing full immediately. Software update likely won’t help with mileage. Techs thought 300-400 miles per gallon of DEF while towing was on par with what they’ve seen. Will report back after I put another 1k miles or so on the truck to let you know if it helps with sensor or burn rate. See bulletin number below.

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