DEF (Diesel Exhaust Fluid) is a solution of urea and water. What exactly happens if DEF is inadvertently added to the diesel tank? This YouTube video: https://www.youtube.com/watch?v=aLwcm4PjyIs claims that the urea will crystalize out and cause abrasive damage to the diesel pump and injectors. However this person: https://www.youtube.com/watch?v=Mb9r0tjCWDs appeared to prevent damage by adding two stroke oil and draining liquid from the water separator.

What actually happens if diesel and DEF are shaken together? Does the water mix with the diesel leaving crystals of urea? It seems unlikely.

I would have thought that most water based solutions would settle to the bottom of the diesel tank and be collected by the water separator. How can that cause any damage? Does the large volume added overwhelm the water separator so damage is caused by pumping water? If so would draining the water separator while the engine (or the low pressure pump) is running successfully fix the problem?

Update: This aircraft safety notice states that "DEF crystallizes in jet fuel and clogs fuel filters, which can result in fuel starvation". There is a picture on this site of a clogged filter which also makes the point that the urea solution is corrosive, over time, to the fuel tank. However my question remains how is abrasive damage caused to the fuel pump and injectors?

  • When you "mix" diesel and water you get an emulsion that separates out over time... – Solar Mike Sep 18 '19 at 13:52
  • I could be all kinds of wrong, but isn't DEF mixed into the exhaust? If so, it would never see the injectors, injection pump, or any of the fuel system. – Pᴀᴜʟsᴛᴇʀ2 Sep 18 '19 at 21:25
  • Correct, DEF is normally injected into the exhaust in a controlled fashion <en.wikipedia.org/wiki/Diesel_exhaust_fluid> when it is put in the DEF tank. My question relates to inadvertently adding DEF to the diesel tank. – Ken Sep 18 '19 at 21:50
  • You stated you put diesel into the DEF tank, not the other way around ... therefore my question. If it's something other than that, please edit your question for clarity. As it's written, my comment is completely valid. – Pᴀᴜʟsᴛᴇʀ2 Sep 19 '19 at 1:43
  • My apologies @Pᴀᴜʟsᴛᴇʀ2 and thankyou for the correction. I have edited the question. – Ken Sep 19 '19 at 23:27

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