Wednesday, December 8, 2021


DEF in a diesel tank? Don’t touch that key!

By Russ and Tiña De Maris
Distraction can lead to disaster. Say you pull your rig into the truck stop fuel island. You’re low on fuel, and your dash warning system says you need DEF (diesel exhaust fluid). It’s been a long day on the road. You pop open the filler lid that hides your fuel cap – and the DEF filler cap.

monsoonmike on

As you stand in a stupor, your reverie is interrupted when you notice you’ve stuck that DEF nozzle – not in the DEF filler – but in the fuel filler! Now you’ve got DEF in a diesel tank! Sorry to say, but you have just seriously complicated your day.

In line for a disaster

If you’ve got a diesel engine truck or motorhome, and you’re a DEF user, you’ve probably cussed the stuff more than once. Sure, diesel exhaust fluid does reduce the amount of nitrogen oxide your engine kicks out. That’s good for air-breathers like us humans. But it does add an additional layer of fussing, and more money at the pump. But if you accidentally put DEF in your diesel engine fuel system, you put yourself in line for a disaster.

DEF is a mixture of water and urea, that “sort of smells like a barnyard” stuff that’s used in a lot of fertilizers. While it’s not good to get water in your diesel tank, a fuel separator system will usually catch water before it gets too far into your fuel system and cause serious damage. Here’s the problem: Fuel separators aren’t designed for, nor are they good for, separating out DEF. The DEF gets past the separator and into the fuel system.

DEF crystalizes. bernhausendiesel on

DEF is extremely corrosive, which is bad for the fuel system itself. And while diesel fuel contains agents to lubricate, DEF works the opposite way – it reduces lubricity. Once in a fuel system, DEF can cause mechanical parts to seize. It will also begin to crystallize, clogging up passageways where fuel needs to move. In short, if you’ve got DEF in your diesel, you’ve got a financial disaster ahead of you.

Don’t touch the starter!

How much of a disaster? Much depends on the action you take the moment you realize that you’ve goofed and put DEF in a diesel tank. Want a small financial disaster? DON’T TOUCH THE STARTER! Instead, call for a tow truck and have your rig towed to a mechanic, pronto. The mechanic will need to take several important steps. Drop the fuel tank and dump the contents. Clean out the fuel tank. Change your fuel filters. Your charges? A red face, and a few hundred dollars. Remember, diesel fuel contaminated with DEF is considered a hazardous waste, and that in itself spells dollar output.

Your second option? Jump in the seat, hit the key, and drive like nothing has happened. Result? The moment the lift pump in your rig begins to push that contaminated fuel, you’ve got problems. It’s highly likely you’ll be looking at replacing your high-pressure fuel pump, your injectors, and a lot of other fuel system goodies. Your charges? Easily SEVERAL THOUSAND dollars. All from having DEF in a diesel tank.

Alternative fixes?

Yes, we’ve heard of folks who, recognizing they’ve goofed, do “alternative” fixes. Drive a little. Crawl under the rig and pull the fuel separator lever and hope that the DEF will miraculously come out. Add various nostrums to the fuel tank to “increase lubricity” like two-cycle engine oil. Repeat all of the above. And maybe some folks have gotten away with it – so far. It may be that the time bomb just hasn’t gone off yet.

Avoiding the problem

How can you prevent the problem from the start? You may be blessed with having a rig where the DEF filler and the fuel ports are far, far away from each other. Some pickup owners cuss when they have to fill up their DEF tanks, because they have to pop the hood and practically crawl in with the engine. That might be enough to help you slow down and think your way out of a problem – before you get into it. And if you need a little reinforcement, watch this video produced by a diesel mechanic that can put the “multi-media fear of god” in you.

Some truck stops have introduced a “DEF mis-fill prevention device” on their bulk DEF pump nozzles. It’s a simple idea: The nozzle is equipped with a magnetically activated system. Until a strong magnetic force comes near the nozzle, you can pull the lever, but no DEF will flow. Equip your DEF port with the appropriate magnetic safety device, the nozzle flows. We’ve heard plenty of griping about these. If you hit a truck stop with the mis-fill preventer and don’t have your rig equipped with the safety device, you’ll have to go in and get one at the fuel desk, sometimes leaving a deposit. Of course, this system won’t stop you from pumping DEF out of a carton or jug into your diesel filler.

Talk to yourself!

If your fuel tank cap and your DEF port are anywhere close to one another, there’s a good chance of a major, expensive goof. So, if that’s the case with your rig, it may be best to slow down and concentrate. Don’t let anyone come chat with you as you fill up. Talk to yourself instead. We know – talking to oneself is generally reserved for folks who’ve had too much ethanol. But, hey. If looking like an idiot can save me potentially thousands of dollars, I’ll talk – and sing – my way through every fill!


Will a DEF head problem ruin your trip?



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10 days ago

Another nice thing about having a gasoline engine. NO DEF required.

Mike Albert
18 days ago

My Mercedes Sprinter has the fuel fill door/cap on the driver’s side chassis/door post. The DEF tank is under the hood on the right side. Very hard to reach the DEF fill cap at some stations. Some of them, I have to move to another island and go the wrong way so the hose can reach. Too hard to lift and hold the container of DEF without spilling, even with a funnel and standing on the front bumper. NOW, the Jeep Wrangler… the Diesel and DEF caps almost touch. Always fill separately.

18 days ago

This exact thing happened to me 3 years ago. I was workamping at Amazon in Kentucky, 60 hour weeks on night shift. On a cold morning, I went outside to fill the Def tank with a jug of Super Tech Def from Walmart. I started to fill the tank when I realized I was pouring it into the diesel spout. I had only poured about 12 oz., but knew it was a bad thing. I researched it online & knew I shouldn’t start the truck. I called the Ford dealer & had the truck towed in. They drained the tank, dropped it, & then flushed the tank, lines, filter housings, etc. Total cost, $1,400. On Fords, the Def spout is just 2″ from the diesel spout.

Mike Carr
18 days ago

I did this a couple years ago, put def in tank of my Ram 1500 Eco-diesel. Fortunately I didn’t touch the key, went to internet and read same info as here. Had it towed, mechanic drained tank, flushed system replaced water filter, about $300 total. No problems other than wounded pride and a good lesson. I was fortunate. Definitely a bigger problem if you start the vehicle.

Jesse Crouse
18 days ago

Check TWICE BEFORE YOU PUMP. An ounce of protection is worth more than a pound of cure. All are OLD sayings but the reason they are old is because they are right.

18 days ago

This will be a lot more than a few hundred dollars, when all said and done probably more in the range of a couple thousand, Yikes

18 days ago
Reply to  Tim

Yes, I noticed the prices in the article are very low, especially in these times. It can cost many thousands

Richard Hughes
18 days ago

I try to keep the DEF filled and do it when I am not fueling. As inconvenient as it is, I use the cartons. Also, my truck has no cap, except the plastic covers on each filler, so I only remove them one at a time. I am considering a lock cap for the fuel.

Thom R
18 days ago
Reply to  Richard Hughes

Same here, I fill DEF from jugs and not at the fuel island where I want to work quickly and get out of the way of semi’s.