What Happens If You Put Diesel Exhaust Fluid (DEF) In A Diesel Tank?

Diesel fuel has undoubtedly taken a lump or two over the last decade-and-a-half. But even Volkswagen's infamous "Dieselgate" scandal couldn't completely derail diesel engines as a viable, more efficient alternative to those run on traditional gasoline. A few years before those VW shenanigans, the desire for cleaner diesel even led to the invention of Diesel Exhaust Fluid, an additive designed to help reduce the emission of harmful NOx (nitrogen oxides) in the exhaust created from burning diesel fuel.

The additive is, however, not added to diesel fuel before you pump it into your vehicle's gas tank, and is instead poured or pumped in via a separate port. As said port is often located next to the one meant for fuel, it can be easy to become confused and add DEF directly into the diesel tank. The consequences of such a mix-up can be quite dire, and the contaminated fuel can even lead to engine failure.

In the best case scenario, you'll catch the mistake before you start your engine, as the real damage caused by DEF in your diesel tank comes once a vehicle's engine turns over and tainted fuel works its way through the fuel system. Catching the mistake early may well save you a bundle in repair costs, though repair work may still be required. If you don't catch the error, serious engine damage can occur, as the presence of DEF can result in the clogging of your vehicle's fuel system and damage to its fuel injectors. 

If you do crank your engine after adding Diesel Exhaust Fluid to your fuel tank, you likely won't be on the road for very long, as the presence of DEF in your fuel system will almost certainly result in the engine stalling soon after it's engaged. As we noted, the damage done in even a short burst of driving with DEF in the fuel system could be catastrophic. So, if you happen to catch the mistake at the pump, do not turn the car on, and instead, seek the help of an automotive professional immediately to correct the issue.

If you're lucky, the filling station you're at has a garage attached, and the repair work can be done there. Otherwise, you'll need to engage a tow truck, which is obviously also true if you don't catch the mistake and your engine fails on the road. As for the repair work required, if the problem is caught early, you may only need a mechanic to drain and flush the fuel system. If not, some vital engine components may have been damaged beyond repair and will need to be replaced. In that case, repair costs will be considerably higher.

The good news is that it should be hard to mix up the DEF tank for the diesel fuel tank, as the port for the former tends to boast a different color cap. So, please pay attention to what you're doing the next time you pull up to the pump to fill up on either.