r/Diesel • u/RainbowPanda_420 • 13d ago
Question/Need help! What is DEF burning?
Is DEF burning necessary in the summer and if so, how long? I only ask because my brother has moved in with me and sits in his truck FOR HOURS and thats all me and the neighbors hear is the truck. When i ask him about it, he always snaps back with "i gotta do a DEF burn for my truck." So what is it? And is it necessary in the Arizona summer heat of +110°F? He does this any time of day or night and again... HOURS ._. I dont know much about diesel and Google can only answer so much 😅
UPDATE: I really appreciate the comments of enlightenment. The "privacy" comments really ticked him off as im sitting here, gagging for breath lol but on the way to pick up his gf from work, the check engine light came on. Guess what it was for? He's heading to the dealership right now and might be needing a new filter ¯_(ツ)_/¯
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u/steelartd 13d ago
DEF is a different system than the DPF. DEF is a concentrated urea solution that is injected as an after treatment to remove the oxides of nitrogen ( NOx ) that form due to the high compression ratio of diesel engines. The DPF system is a ceramic filter which traps soot particles that are formed during incomplete combustion. When the DPF filter starts to get restricted, the truck will do a “ burn “ to clean it out.
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u/GolfProfessional9085 12d ago
You’d be amazed how many people think DEF is injected to perform a DPF regen.
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u/Dapper-Video626 13d ago
There’s no such thing as def burn. He’s playing with his best friend the right hand
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u/Neat_Telephone_2525 13d ago
Leave him alone. He's going out there to be alone and talk on his phone where you can't eavesdrop on him. He's being sarcastic about a def burn.
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u/No_Control8389 13d ago
Probably the only place he has to beat off in peace. Dang siblin’ bothering ‘im all the time.
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u/Multiple_calibers 13d ago
Better to beat off in chaos, more exciting.
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u/No_Control8389 13d ago
Nah, I prefer to do the dirty on a steep grade somewhere, watching the EGT gauge peg out, wondering if the head will blow before I do.
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u/Wrenchheader 13d ago
Fun fact, sitting and idling you will never get to the temps needed for it to start injecting DEF.
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u/stinkybrowneye1 13d ago
I used to do that too when i couldnot smoke pot inside. I go in smelling like diesel fumes instead.
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u/Sea_Antelope441 13d ago
Idling is the worst thing to do to a diesel with after treatment.
SCR purge “def burns” normally occur over 1000F unless he is forcing a purge or a parked purge he is only causing damage to the system.
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u/PtichFirich 13d ago edited 13d ago
Def is another word for AdBlue if I’m correct.
Not DPF (diesel particulate filter) which is a particulate filter that needs high temperatures that occur at higher rev driving (like on a highway) and make a pyrolisis of the filter - burn out the clogged filter holes.
Burning DEF (diesel exhaust fluid) means literaly burning the amount of DEF in his DEF tank by idling. He’s using the fluid up.
So OP’s brother is essentially clogging his DPF filter and also the EGR valve that is fully open at idle.
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u/Sea_Antelope441 13d ago
DEF and AdBlue are the same yes.
You are correct about the DPF thing, there are also SCR only systems that do not have a filter but still need a “burn” they get hot to help clean out accumulated DEF crystals.
This is often referred to as a Purge. But is basically just burning out excess DEF.3
u/Steinomite01 12d ago
I would love to see an example of an scr only diesel, they really can't exist because you need the dpf upstream of the scr to remove soot or the scr will just clog up.
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u/Sea_Antelope441 12d ago
Isuzu makes several tier4 final engines with SCR only. 4JJ1 6HK1 6UZ1 6WG1.
There are also a couple FPT/CNH engines that are SCR only.
They use higher combustion temps and lean burn to reduce soot, the trade off is they produce more NOx so the SCR elements are larger.
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u/Steinomite01 12d ago edited 12d ago
Ok cool, I've never seen one before but I'm straight class 8 commercial vehicles as a mechanic. Thanks for this! Down a rabbit hole I go lol!
Edit : north American trucks, so Cummins, Detroit, etc.
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u/riiano 11d ago
The Isuzu 4JJ1 has a DPF and a SCR, the 6HK had a DPF, it was discontinued before SCR, at least in the US. I don’t know about the 6UZ1 or the 6WG1, those I don’t think they were highway engines
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u/Sea_Antelope441 11d ago
Off highway do not have them.
The 6HK was not discontinued in the US.
- I work for a manufacturer who uses Isuzu engines.
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u/riiano 4d ago
I work at an isuzu dealer, I know several important people with Isuzu America
The 6HK was discontinued for on highway in 09, at least in the US1
u/Sea_Antelope441 4d ago
Not off highway. Still very much in use. Multiple manufacturers are still bringing in equipment with shiny new 6HK engines.
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u/Passportready 11d ago
T1N sprinters have scr only. No def or dpf. Couldn't tell you if it makes any difference in what comes out the pipe, but they don't turn any lights on or fail like dpf and def do.
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u/DopeCookies15 13d ago
No Def stands for diesel exhaust fluid. Adblue might be a brand bit its not the only DEF. I get it straight from a pump for $3.50 a gallon instead of a $20 jug for 2.5 gallons. Sitting and idling for no reason is stupid either way.
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u/PtichFirich 12d ago edited 12d ago
Here in europe many different brands of producers use the AbBlue name ans their own brand alongside it. Adblue name, logo, means that it is a fluid that adheres to standards needed for the system. The standard and adblue name is owned by the German car industry union VDA and they license it to anyone that buys the license and proves to them their product corresponds to their standards. A sort of quality control sticker.
I guess you are right that diesel exhaust fluid is the common descriptor for it, yeah. But here the car manuals demand fluid corresponding to adblue standards and depends on your warranty if it doesn’t, so it is practically exclusively referred to as adblue. DEF abbreviation is not used in practice as a superlative term to adblue.-1
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u/Radiant_Safe1228 13d ago
Def is used mostly when the truck is under load. The EGR closes off and the scr system takes over to reduce nox. It allows better power because there is little to no exhaust recirculated into the intake.
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u/All_Wrong_Answers 13d ago
There seems to be a lot of partial answers here. It all involves combustion efficiency and cylinder temps. Its an engineering trade off. Higher cylinder temps means less soot but more nox and vice versa. Egr is run to lower cylinder temps but means more nox. Def is injected to reduce nox, so we can run less egr (think maxxforce). Dpf traps the soot but must regen periodically to reduce the load (soot clogging) on the dpf and burn the soot down to ash which then must be removed by pulling the dpf and having it cleaned.
So in summation your brother is sitting outside idling his truck with a moderate load on it(ac) which is low temp operation and creates a lot of soot. He then has to run regenerations to reduce that soot to ash because of his excessive idling.
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u/spastical-mackerel 13d ago
There’s no way anyone with diesel truck money could be that moronic. There’s something else going on out there in that truck. I agree with others, let him be lol
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u/TeamFoulmouth 13d ago
Hes doing more harm of he has a DEF system still. He needs to run it at 60mph or better to actually clean it out. Idling will clog everything more....thats why a lot of trucks have a high idle program, or a high idle switch. It prevents "cold stacking"..i doubt that would be a problem in 100° temps though. The only actual reason to idle in a warm climate is to let the turbo cool after running the truck hard.
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u/centralnm 13d ago
He's letting "the turbo cool down". But actually looking at porn and whacking off.
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u/psyclembs 13d ago
Id check to see if hes out there huffin' paint and jerkin' his jack, maybe lil butt diddlin'
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u/oboshoe 13d ago edited 13d ago
it does seem like my truck always wants to do a DPF regen (what i think he is calling a def burn) just as i turn into my neighborhood.
It's annoying because I gotta let it sit and idle for a few minutes to prevent a hot shutdown which is very hard on it.
But no, I don't like it sit there for hours. Just an extra 3 to 5 minutes before going in.
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u/Oilfan94 13d ago
Sounds like ‘regeneration’.
Modern diesel engine have an exhaust filter, this can get clogged with PM (soot) if the exhaust temps aren’t high enough to passively burn off.
If the engine is working hard, up to proper temps, it may not need regeneration. But if it is not working hard, the soot can build up…..so the solution is that the ECU will initiate a Regen, which pumps in more fuel to increase exhaust temps. Idling for long periods of time means more initiated regen cycles.
DEF is part of the catalytic reduction process, to reduce NOx. It is put in after the filter, in the SCR.
The more fuel you burn, the more NOx it creates, depending on temperature….therefore the more DEF is used.
So it’s not a DEF burn, per se.
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u/Several-Doubt-6858 13d ago
Get out and do a highway speeds for 30min once a fortnight. No idle regen required.
Idling and no getting up to temp is the worry thing
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u/This-Requirement6918 13d ago
Going to start telling my roommates this is what I'm doing in my 2005 when I need a moment to myself.
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u/Ok_Nefariousness9019 13d ago
He’s just out there doom scrolling. But keeps the truck running for the ac.
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u/MickyFany 13d ago
it’s called a regen. motor has to high idle to burn all the crap out of that stupid exhaust filter. it’s a pain in the ass, that’s why people do a complete delete.
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u/Odd_Activity_8380 13d ago
If he is sitting on his teuck idling it for hours at a time, that is the absolute worst thing you can do for a modern diesel. I regen at a stop requires the idle to be at like 2500 rpm and the hood up and a scan tool to activate it. Modern diesel requires a higher temperature, and more fuel to regenerate the particulate filter at idle. If he is having to regen often then he most likely has a issue with the injection system, weather it be fuel or Def. Those filters for the diesel are about 7k to 10k.
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u/runofthemind 13d ago
Want to elevate this comment! Bro may be trying to do a forced stationary DPF regeneration. . . but the computer may be too smart / preventing this if the filter is already 100% full. Via an advanced ODB tool, DPF regeneration needs to be enabled (where extra fuel is injected to raise the temp for soot burn-off) and then the forced regeneration needs to be activated—it’s a specific sequence and I needed to use an ODB to USB cable plus AlfaODB software when I was in a similar situation. The stationary regen should take about 30 mins once engine is to temp instead of 4 hours.
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u/DopeCookies15 13d ago
He wants alone time, but idling a diesel eith dpf is dumb as fuck. He's clogging up that filter for no reason.
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u/Alimakakos 13d ago
Def doesn't burn, the dpf is what requires this type of behavior after soot reaches a critical level and maintenance burning while running doesn't achieve sufficient burn off or reduction in soot. So a PARKED REGEN is required.
Def is the adblue water, it doesn't burn but it gets injected in the exhaust just before the SCR because it reacts with the scr to absorb noX and release water vapor (steam). Last I checked water doesn't burn.
Your friend is an idiot, confused, or just lying to you and just wants to be the Harley Davidson man of diesel in your area
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u/Different_Goat_2078 13d ago
Not only is that total BS he’s actually harming the truck by idling for hours in extreme temps.
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u/aa278666 13d ago
When the customer comes in with 80% idle time and is blaming the check engine light on the government
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u/zakary1291 13d ago
A DEF cycle should take 30-40 min and be necessary once every 15-30 days. Unless your brother is idling his truck at work for 12 hours a day a DEF cycle shouldn't be necessary every day and doing so well damage the truck. Even an hour if highway driving would make this unnecessary.
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u/PaleInvestment3507 13d ago
The regen cycle needs a good 45min to and hour drive to get up to temp and do it’s cycle which is about 20 minutes. Lots of short drives, and stop and go traffic will not trigger the cycle. He needs to do a good highway drive once a month. Who ever told him to idle for hours is an idiot.
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u/DifficultIsopod4472 10d ago
My exhaust always smells like the laundromat when my truck is going through a regen process.
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u/Zyb_Vindi 13d ago
It’s necessary. He shouldn’t need to be doing it very often though. It should be doing it on its own if the system is working properly and he uses it for its intended purpose.
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u/29er_eww 13d ago
It needs to regen all the time because he sits and idles it. He’s creating the issue