# CEL 004804 fix



## dadasracecar (Sep 12, 2008)

So I got a persistent CEL with this code for several days. The BMWhat app states, " reducing agent no reducing agent pressure buildup possible." Fearing the worst I was looking into deleting all of my SCR system, getting a tune, and rolling coal. I was tired of all of the issues with the DEF system and have spent lots of money to replace and repair that system. As a last ditch effort I decided to see if I could clear the clog just by diluting the DEF. To that end I added a gallon of distilled water to the active tank. The system seems to have unclogged and the CEL went away. Seems everything is functioning properly for over 100 mi and 5 starts. Just thought I'd pass this information along.


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## floydarogers (Oct 11, 2010)

dadasracecar said:


> So I got a persistent CEL with this code for several days. The BMWhat app states, " reducing agent no reducing agent pressure buildup possible." Fearing the worst I was looking into deleting all of my SCR system, getting a tune, and rolling coal. I was tired of all of the issues with the DEF system and have spent lots of money to replace and repair that system. As a last ditch effort I decided to see if I could clear the clog just by diluting the DEF. To that end I added a gallon of distilled water to the active tank. The system seems to have unclogged and the CEL went away. Seems everything is functioning properly for over 100 mi and 5 starts. Just thought I'd pass this information along.


Often, DEF codes go away. The true test is in around 500 miles when the next DPF regeneration is done. That's when the DDE checks the operation of the SCR system.

Ask me how I know.:angel:


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## dadasracecar (Sep 12, 2008)

floydarogers said:


> Often, DEF codes go away. The true test is in around 500 miles when the next DPF regeneration is done. That's when the DDE checks the operation of the SCR system.
> 
> Ask me how I know.:angel:


hmmm. Rats. We'll have to see. I'll hit 500 miles in about a week.


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## floydarogers (Oct 11, 2010)

dadasracecar said:


> hmmm. Rats. We'll have to see. I'll hit 500 miles in about a week.


Good luck.:thumbup:


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## montr (Oct 7, 2006)

dadasracecar said:


> So I got a persistent CEL with this code for several days. The BMWhat app states, " reducing agent no reducing agent pressure buildup possible." Fearing the worst I was looking into deleting all of my SCR system, getting a tune, and rolling coal. I was tired of all of the issues with the DEF system and have spent lots of money to replace and repair that system. As a last ditch effort I decided to see if I could clear the clog just by diluting the DEF. To that end I added a gallon of distilled water to the active tank. The system seems to have unclogged and the CEL went away. Seems everything is functioning properly for over 100 mi and 5 starts. Just thought I'd pass this information along.


Are you planning to flush the water and replace by new DEF?

I am asking this because the system monitor the efficiency of the DEF. Sooner or later, I will detect low efficiency and trigger another SES light.


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## BB_cuda (Nov 8, 2011)

With regard to Floyd's post #2, a better idea might be to drain the diluted DEF, fill with distilled water, drain the distilled water, and then refill with new DEF. Up front, I haven't done this myself but read here enough to know this is good to dissolve crystallized urea. Good luck with the next regen.


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## Doug Huffman (Apr 25, 2015)

More than a kilogram of urea will dissolve in a kilogram-liter of water at room temperature. At 80C four kilograms of urea will dissolve in a kilogram-liter of water.


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## dadasracecar (Sep 12, 2008)

BB_cuda said:


> With regard to Floyd's post #2, a better idea might be to drain the diluted DEF, fill with distilled water, drain the distilled water, and then refill with new DEF. Up front, I haven't done this myself but read here enough to know this is good to dissolve crystallized urea. Good luck with the next regen.


Thought about that but haven't seen a good way to drain the system. Is there a DIY anywhere?


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## dadasracecar (Sep 12, 2008)

montr said:


> Are you planning to flush the water and replace by new DEF?
> 
> I am asking this because the system monitor the efficiency of the DEF. Sooner or later, I will detect low efficiency and trigger another SES light.


I thought about this. I'm not sure how the system would detect the efficiency. Urea is a reducing agent and should lower the concentration of NOx and SOx in the exhaust flow. All of the diesel around here is low sulfur so SOx shouldn't be an issue. NOx in the presence of water will form nitrous acid and possibly hydroxylamine - both of which are different enough so as to fool any sensor. In any case it was only a gallon of water that I was hoping would dissolve excess crystallized urea. The urea concentration will be lower but hopefully not significantly. I'm considering adding a little extra DI water at my next DEF addition to try and prevent the crystallization. Any system that relies on a saturated aqueous solution of anything will be problematic over the long term with buildup. DEF might not be a saturated solution but I bet it's close.


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## Doug Huffman (Apr 25, 2015)

dadasracecar said:


> [ ... ] DEF might not be a saturated solution but I bet it's close.


Nope. 0.325 urea


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## dadasracecar (Sep 12, 2008)

Doug Huffman said:


> Nope. 0.325 urea


Is that 0.325 molar, wt%, v/v%? What's the saturation concentration? The only reason I said it was close is that every time I spill even a little bit, that spot almost immediately dries leaving a white solid - urea.


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## Doug Huffman (Apr 25, 2015)

Of course, the water evaporates away leaving the dissolved solid urea crystals. But they'll rinse away.

The saturation concentration varies with temperature, from slightly greater than 1 w/w at 20°C to 4 w/w at 80°C.

If you understand molarity and density concentration fractions then you should be able to ask your own questions and find your own answers, if ...

For those that don't, slightly more than a pound of urea can be completely dissolved in a pint (=lbm) of water at room temperature.


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## dadasracecar (Sep 12, 2008)

Doug Huffman said:


> Of course, the water evaporates away leaving the dissolved solid urea crystals. But they'll rinse away.
> 
> The saturation concentration varies with temperature, from slightly greater than 1 w/w at 20°C to 4 w/w at 80°C.
> 
> ...


Yeah thanks. So 5.4 M. That's pretty darn concentrated which explains the buildup problems associated. And yeah, I know about concentration. Just wasn't well versed in the particulars of DEF.


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## dadasracecar (Sep 12, 2008)

Doug Huffman said:


> Of course, the water evaporates away leaving the dissolved solid urea crystals. But they'll rinse away.
> 
> The saturation concentration varies with temperature, from slightly greater than 1 w/w at 20°C to 4 w/w at 80°C.
> 
> ...


Yeah thanks. So 5.4 M. That's pretty darn concentrated which explains the buildup problems associated. And yeah, I know about concentration. Just wasn't well versed in the particulars of DEF.


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## Doug Huffman (Apr 25, 2015)

Well, no. At room temperature 32.5% w/w is about 30% concentrated and a long way from 100% concentrated. What build up problems are there?

For those trying to keep up with his logic, a mole of urea is about 60 grams and a mole of water is 18 grams. That's the mass weight of 6 X 10^23 molecules, the unit of stuff.

A liter of water mass weighs 1000 grams and is 1000/18 moles of water.


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## dadasracecar (Sep 12, 2008)

Doug Huffman said:


> Well, no. At room temperature 32.5% w/w is about 30% concentrated and a long way from 100% concentrated. What build up problems are there?
> 
> For those trying to keep up with his logic, a mole of urea is about 60 grams and a mole of water is 18 grams. That's the mass weight of 6 X 10^23 molecules, the unit of stuff.
> 
> A liter of water mass weighs 1000 grams and is 1000/18 moles of water.


Read the original post. The CEL actually says buildup possible. Every time I open the filler ports on my DEF tanks I see crystalline urea crusting up. Some times the crystals extend partially over the mouth. I'm not going to argue a perspective of concentration with you. To me 5.4 M solution is very concentrated. Make a 5.4 M saltwater solution and taste it. Make a 5.4 M solution of sugar and water. 300ish grams of solid dissolved in 1L of water is concentrated. A white residue forms almost instantly when any DEF spills.


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## Doug Huffman (Apr 25, 2015)

dadasracecar said:


> The BMWhat app states, " reducing agent no reducing agent pressure buildup possible."


I read it as Reducing Agent: No reducing agent pressure buildup possible. Were there commas in your original indication or not?


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## dadasracecar (Sep 12, 2008)

Doug Huffman said:


> I read it as Reducing Agent: No reducing agent pressure buildup possible. Were there commas in your original indication or not?


THere were no commas anywhere. This was from the BMWhat app. I read it as "Reducing Agent: No reducing agent pressure, buildup possible"


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## Doug Huffman (Apr 25, 2015)

Bishop Ockham's Razor works here for me. _Numquam ponenda est pluralitas sine necessitate._ Plurality must never be posited without necessity.


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## dadasracecar (Sep 12, 2008)

So I've gotten several CELs for Reducing Agent Low Efficiency because I put water in the tank. lol. But the pressure buildup CEL has only returned once. I'm thinking a gallon of DI water once in a while helps prevent buildups and clogging.


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