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I Accidentally Poured DEF into my Diesel Fuel Tank

We know it sounds ridiculous, but it happens. Although we’re big Ford & Cummins advocates here at Texas Diesel, they both could have placed their DEF fill in a different place to at least alleviate confusion or distractedness while filling your def tank. Here are all of your common questions answered in a nutshell about accidentally putting DEF fluid into your fuel tank.

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Here is why you don’t want to run your truck with DEF fluid in the tank, beyond the obvious:

  • DEF fluid is comprised of 67.5% water and 32.5% Urea. If we go back to our science classes in high school, you’ll remember that if you mix oil and water, the water/urea is going to settle down and oil is going to sit on top. This is exactly why you don’t want to run your truck unless absolutely necessary. All of the DEF fluid is going to sit on the bottom of your tank which is where your sending unit grabs fluid from. In all cases, we never recommend running your truck with any sort of liquid, other than diesel, in your fuel tank.

  • If you put DEF into your diesel fuel tank and you haven’t started or keyed on (IE. Low side Fuel pump isn’t pumping) your truck you’re in luck, you’re still in the safe zone. Regardless of what any dealership or repair shop tells you your tank can be dropped, cleaned, and replaced with no issue. We advise replacing the fuel sending unit to be thorough with the repair. We have yet to need a fuel system replacement after these symptoms presented themselves in this manner. If you absolutely have to start the truck to move it onto a trailer or into a better location you can attempt to unplug the power wires going into your fuel delivery pump and very quickly move the vehicle. In this case, it is very unlikely that you’ll have any contamination issues. We recommend not running the truck longer than 20-30 seconds though. If your truck won’t start without the low-side fuel pump plugged in, then we advise being very quick about moving the vehicle! Should you need to start the truck with DEF in the tank then we advise having your sending unit and all hard lines from the tank to the engine cleaned. We also advise having your feed and return lines from the top side fuel filter to the CP4 pump removed and allowed to dry to see if any crystallization is present. Make sure to not clean these lines as we are looking for the DEF to crystalize. If no crystallization is present, more than likely you’re in the clear! Disclaimer: WE NEVER ADVISE CUSTOMERS TO START THEIR VEHICLE AFTER ANY SORT OF CONTAMINATION HAS ENTERED THE FUEL SYSTEM. DO THIS AT YOUR OWN RISK.

  • I started and ran my truck for a good period of time (more than 1 minute of driving or idling) with DEF fluid in the tank. In this case, it is very likely you’re going to have DEF contamination in the fuel system. Although your truck may still be running it won’t last long. At some point, the DEF fluid will begin to crystalize and create clumps in your lines, injectors, CP4 pump, Fuel filters ETC. In this case, you can try replacing just what Is broken but it isn’t going to fix your problem in the long run. By not replacing the entire fuel system at the same time you’re going to cause yourself more downtime, money, and headache. At Texas Diesel, we refuse to replace individual fuel system components which have been contaminated by DEF, water, or other debris. The only way to fix this problem effectively is by replacing the entire system at once.

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