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DEF in the fuel tank! &/%!

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I am posting this information in case this happens to you.  I put about a gallon of DEF in the fuel tank.  I was tired and my brain was not paying attention.  Unscrewed the DEF filler, started the DEF pump, stuck in in the fuel tank, started pumping and then I woke up.  I couldn’t believe what I had just done.  Here is what to do to avoid serious problems.


Don’t start the engine!  To move the truck you will have to push the start button though.  Don’t put your foot on the brake or it will start. FYI, hold the start for five seconds. It will turn green.  Put you foot on the brake and shift to neutral.  Here is the rub… when you turn on the ignition the tank fuel pump will prime moving the water through the filter and back to the tank.    Once moved out you foot on the brake and shift back to park or better yet leave it in neutral to avoid moving water through the system more than you have too.  

Lesson #1 - to avoid fuel priming pushing water to the engine you will have to pull the fuel filter.  This will make a big mess while the pump goes through it prime cycle, which I understand to be a timed interval of about 30 seconds. An option would be to disconnect the fuel filter feed hose instead of removing the filter.  

You may think that this is the solution to getting the DEF out but doing this still leaves DEF in the tank.


The fuel tank must be pulled and flushed.  This took about three hours at the diesel shop.  Fill with a few gallons of clean fuel, change the fuel filter, and prime the fuel system five times.  You will probably get a water in fuel service message. Drain the fuel filter using the inner 11/16 nut.  Prime two more times and start the engine.  

Lesson #2- Carry a spare fuel filter and the wrenches to change it if you need to. You may run into bad fuel or repeat my dumb ass mistake. 

I was able to continue my trip the next day without problems.  

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And many bitched about the fill location being in the engine bay from 2011 til 2020...


Sorry, couldn't resist.


Glad you figured out what happened and knew how to react!

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Yep.  Big thing is NOT turning the key on at all so the pump primes in the tank.  


In one of our training videos, they just did a 15 minute segment on this.  DEF sinks right to the bottom of the tank, and that means right at the pickup point for the in tank pump. 


Key on or engine started after putting DEF in the tank means the ENTIRE fuel system has to be REPLACED.  Over 50% of their warranty claims on in tank diesel pumps are accidental DEF in the tank and many have been denied warranty.  


You should see the corrosion that was on those components in the videos...not good.  

Edited by newdude
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Sorry to hear this.  Accidents happen.


It seems that GM should have kept the DEF fill under the hood or on the other side of the vehicle.  I know it is a small % that mix and such but even that stinks and could have been avoided if the DEF location was different. 


Lucky that my DEF is located in the trunk and I only have to fill it maybe once a year. 

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In our chemistry lab we would not allow the mechanical engineers and techs mix DEF anywhere near the lab. It's super corrosive and will ruin ongoing experiments

too.  Modern diesels struggles are great.  Standby if we stay with spark gasoline engines because to meet standards heats gonna get turned up even more 

so that gas particulate filters on exhaust and more cooling of the charge before exhaust will be required. Compression/spark combi makes more sense.


Why new fuels may make more sense for IC engines to go forward. 

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