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Is Diesel Exhaust Fluid A Joke?

You may have heard someone joke about filling up your blinker fluid in a car. Blinker fluid of course doesn’t exist, and while diesel exhaust fluid may sound like a joke, its not. If you’re looking into purchasing a diesel truck, you might see the need for using a specific fluid and wonder, “What is DEF fluid?” You’ve never had to put this kind of fluid into your gas vehicle before. This is one of the bigger differences between a gas and diesel engine truck, and understanding why it’s important to owning a diesel engine truck. Let’s dive in to what is diesel exhaust fluid.

What's DEF and Why Do You Need It?

What Is Diesel Exhaust Fluid?

Diesel Exhaust Fluid (DEF) is used in diesel engines. In 2008, the Environmental Protection Agency (EPA) issued a mandate requiring the use of diesel particulate filters on all on-road diesel engines.

In chemical terms, it’s a mixture of 67.5% deionized water and 32.5% urea. When the urea is heated, it turns to ammonia. It’s a colorless odorless fluid that usually comes in a blue jug and is called ad blue in Europe. While it generally is non-toxic it can crystalize and make a mess if spilled.

DEF is a mixture sprayed into the exhaust system of diesel trucks. If may seem weird but yes the fluid is pumped into the exhaust just before the catalytic converter to facilitate a reaction that breaks down the generated nitrogen oxides (NOx) into harmless nitrogen and water molecules. Manufacturers use DEF to abide by the EPA’s guidelines.

Using gas pumps to uncover what is def fluid
If you own a diesel truck, you will need to learn the importance of DEF fluid.

What Is the Difference Between a Gas Engine and a Diesel Engine?

Diesel and Gas engines in vehicles are more similar than they are different. They are both four-stroke internal combustion engines that use liquid fuel and share most of the same basic operating principles however there are a few major differences.

Gas and diesel engines both convert fuel into rotational motion via internal combustion pistons. Because diesel fuel is thicker and has more energy density than gasoline, diesel engines operate with about 20% more thermal efficiency than gas engines. This increase translates to more power and torque per fuel used.

A spark controls the gas engine, which ignites the fuel and air mixture and causes the engine to run. On the other hand, a diesel engine is a compression combustion engine that doesn’t require a spark. The air mixture explodes on its own due to the force of the compression.

Why Do Diesel Engines Need Diesel Exhaust Fluid?

After the 2008 mandate, the EPA issued even stricter guidelines in 2010. Diesel engines had to reduce their NOx production, so the DEF additive reduces this emission. Heavy machinery and diesel trucks can only operate within these guidelines by adding DEF fluid.  

This is why you’ll find heavy-duty diesel trucks manufactured after 2010 with a diesel fuel tank and a Diesel Exhaust Fuel tank.

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Refilling diesel truck
Expect to use 2 or 3 gallons of diesel exhaust fluid for every 800 miles driven.

How Often Do You Need to Fill Up the DEF Tank?

It all depends on your mileage as the def consumption matches the fuel consumption. You will use more if you are towing vs operating empty. Each truck is different, but light-duty trucks generally consume 2 or 3 gallons of DEF per 800 miles. The DEF tank is usually 7 to 10 gallons. Most manufacturers have installed an indicator of some kind on diesel trucks to alert the driver of low DEF.

Medium and heavy-duty trucks use more DEF. Generally, for every 50 gallons of diesel fuel, the truck burns 1 gallon of DEF. For example, if a heavy-duty truck drives approximately 120,000 miles a year, the DEF tank will be refilled about 20 times.

Where Can You Get Diesel Exhaust Fluid?

One of the best places to get DEF is at truck stops. You can’t go to any fuel center and expect to find it. Love’s, Flying J, Petro, and Pilot Travel Centers are all good options. The greatest advantage to filling up at these locations is you can fill up with fuel and DEF on the same trip. 

Blue Def DEF002-4PK Diesel Exhaust Fluid, 2.5...
  • One gallon of is used approximately every 300 miles
  • A truck with a 20 gallon DEF tank can go up to 6000 miles between...
  • When used in an SCR system will reduce the levels of NOx...

You can also purchase DEF at retail stores like Walmart, Target, O’Reilly’s Auto Parts, and Advance Auto Parts. But DEF is a substance you must be careful handling if you’re not using a pump at a truck stop.

Different gas pump options including DEF fluid
DEF pumps will be labeled clearly at truck stops for you to use with ease.

How Do You Identify a DEF Filling Pump From a Diesel Fuel Pump?

DEF pumps are usually beside diesel pumps at truck stops. The nozzle is very skinny and much smaller than the diesel nozzle. There is a black flap or covering that holds the DEF nozzle in colder climates. The cord on a DEF nozzle is usually on a reel instead of loosely hanging from the pump like the diesel fuel cord. 

The diesel pump at truck stops is normally green or black, while the DEF pump is usually blue (but don’t always trust the colors). Make sure to always read the label before pumping.

What Are the Cons of a DEF Truck?

The biggest con of a DEF truck is the hassle of keeping an eye on the DEF tank and refilling it as needed. In the early generations of DEF systems, there were a lot of failures as well. DEF head failures are common and sometimes the systems can cause reliability issues for the vehicle. In general DEF systems have become much more reliable in newer generation systems and the biggest drawback is just having to purchase another fluid for your vehicle.

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Man pouring DEF fluid into car after discovering what is DEF fluid
Running out of diesel exhaust fluid will cause your engine to stop running.

What Happens to the Engine If the DEF Tank Is Empty?

Most times your vehicle will continue to operate without DEF but at a very limited power level. You may not even be able to drive the speed limit. If you let it go too long the engine may shut down completely and not restart. Its also possible to end up damaging the exhaust system if you let the DEF run out. 

Always pay attention to the gauge or sensor that monitors the DEF level.

Is All DEF Fluid the Same?

There are 12 major manufacturers of DEF. All of these manufacturers should be following rigid guidelines set by the EPA. This means there should be no difference between brands. All Diesel Exhaust Fluid should contain 67.5% distilled or deionized water and 32.5% high-grade pharmaceutical urea. 

If you’re looking at a cheaper off-brand, you just want to make sure it is EPA-approved and check if it’s gone through testing. This will be on the bottle if you’re purchasing DEF from a retail store.

WHAT Happens When You RUN OUT of DEF?? *Diesel Exhaust Fluid*

Take Care of Your Diesel Engine by Understanding Diesel Exhaust Fluid

For your diesel truck to have a long life, proper maintenance is key. Replacing the air filters, changing the oil, and adding DEF are all part of taking care of a diesel truck. If you take care of your diesel engine, it could last twice as long as its gas engine counterparts.

So what is Diesel Exhaust Fluid? A critical component to your diesel engine truck!

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About Tom Morton

Tom, a Pacific Northwest native, is our technical genius. Born in Washington and raised in Alaska before settling in Michigan. He's the man who keeps our operation running, both figuratively and literally.

With a background in Electrical Engineering, Tom specializes in RV solar systems and lithium batteries. He made history as the first documented individual to use a Tesla battery module as an RV battery. Tom has personally assisted countless RVers with system installations and has educated thousands more through his videos and articles.

Cinematography is another of Tom's passions, showcased in his work on the Go North series. You can see his camera skills on display in The RVers TV show on Discovery Channel and PBS where he also stars as a co-host.

Tom's mechanical expertise extends beyond RVs to boats, planes, and all things mechanical. He's renowned for taking on maintenance and repair projects single-handedly and is often spotted underneath RVs, making him the technical backbone of our endeavors.

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Ray Davis

Monday 9th of January 2023

Informative article. Perhaps in a future article or an addendum to this one, you might present information on the importance of the "freshness," of DEF and the corresponding label ID code breakdown, as well as a strong suggestion to rinse any surfaces that the Fluid may be accidentally spilled on. Ray

Gilles Mercier

Monday 9th of January 2023

Good morning Tom and Caitlin. Would like to add an important point about diesel engine equiped with DEFsystem. Bottes DPF (diesel particul filter) and SCR ( selective particul filter) are really sensitive and expensive parts. They want to run with clean fuel and Adblue and that's it. Never add aditive, gaz or other magic stuff in your fuel or urea. You Will destroye the chemical propriety coating the interior of DPF / SCR and soon, you Will have to deal with engine F/C for low DEF system efficiency or DPF regeneration problem. Another point to mention is; burned oil and glycol are ennemys of DPF and SCR. So as soon you see an abnormal oil consommation, or have to add coolant in your expansion container, you have to rush à diagnostic of those issue, before concequancial damage to your DPF /SCR. Often, they Will have to be cleaned after repairs are donc. Have fun in 2023. Gilles

Mortons on the Move

Monday 16th of January 2023

Thanks for the comment! These are great points we did not cover. For sure any emission from an engine besides normal burn will screw with any emissions system!

Kevin Linton

Monday 9th of January 2023

Hey Tom Really love reading your blog and since I have a diesel truck loved this article. I’m one of those guys that’s not happy with the fact I have to be mandated to put def in but I do. But I’d love for you to research and write an article about just how much we’re doing to the environment by manufacturing def along with the process of making all those plastic jugs and cardboard boxes the ultimately wind up in a landfill somewhere or on the side of the road since the jug can’t be reused. In my opinion we’re not saving the environment by using def we’re hurting it more. Thanks and see you in the road.

Mortons on the Move

Monday 23rd of January 2023

Good points, I really feel it's a double-edged sword, you're always putting it somewhere... However, NOX emissions are a big human health hazard, and particularly in dense urban areas I do believe it's a benefit to our lungs.