The Blue Stuff: The Ins And Outs Of Diesel Exhaust Fluid


If you ask an engineer to explain what his job is to a five-year-old, he’ll say that his job is to design stuff that works. Robotics, microprocessors, machinery, aeronautics, and biology all avail themselves of the greatest minds to create working solutions for the problems that humanity faces.

But in this day and age, the task list has shortened quite a bit. Short of science-fiction dream like teleportation or extraterrestrial colonizing, we have largely conquered the problems of yesteryear. Stuff that works is readily available, but stuff that works efficiently is the next step.


In the case of diesels, the mandate from on high (the government, to be exact) is to make these applications burn their fuel more efficiently and leave a less lasting trace upon the atmosphere. For better or worse, greenhouse gases and global warming have caught on with the Feds and made them less accepting of anything that smacks of wasted potential. With diesels in the crosshairs, automakers have adapted and given us a new acronym to remember: DEF, or Diesel Exhaust Fluid.

“New” is a relative term here. By now, manufacturers have all made the change in their diesel platforms, from Nissan to Ram. DEF is as common now as motor oil and isn’t going anywhere, so we wanted to understand how this blue fluid came to be, what it really does, and what bearing it will have on the diesel market in the future.

DEFinitely Good For The Environment

DESCRT copy_edited-1

This diagram shows how DEF is used in the exhaust system to eliminate harmful emissions. Photo: TAC Energy

To understand DEF, we must understand another acronym: SCR, or Selective Catalytic Reduction. SCR refers to a method of converting nitrogen oxides (also referred to as NOx, referring to all of the molecular variations as a whole) into two separate molecules – diatomic nitrogen (N2) and water (H2O) – using a catalyst.

Nitrogen oxides are an offshoot of exhaust gases in diesel, and are considered by the scientific community to primary air pollutants. This puts NOx in league with sulfur oxides (SOx), carbon monoxide (CO), and methane (CH4). According to the World Health Organization, air pollution is linked to some 7,000,000 premature deaths every year, ranging from strokes to lung cancer to coronary heart disease.

Motor vehicles have long been the target of legislation here in the States, starting in the 1970s with the mandated adoption of catalytic converters and Corporate Average Fuel Economy (CAFE) standards. Diesels were no different, and over the years, automakers made every effort to lessen theses applications' undesirable effects on the atmosphere: exhaust gas recirculation (EGR) in 1973, diesel particulate filtration (DPF) in 1985, and SCR in 2007 (despite being used for several years in industrial applications).

Motor vehicles have long been the target of legislation here in the States, starting in the 1970s with the mandated adoption of catalytic converters and Corporate Average Fuel Economy (CAFE) standards. Diesels were no different, and over the years, automakers made every effort to lessen theses applications’ undesirable effects on the atmosphere: exhaust gas recirculation (EGR) in 1973, diesel particulate filtration (DPF) in 1985, and SCR in 2007 (despite being used for several years in industrial applications).

Automakers, always keen on toeing the line when it comes to legislation, took on SCR as early as possible, starting in 2007 in order to meet the federal emissions regulations in 2010. The only thing was that SCR didn’t just happen with tweaks to existing hardware in the motor or exhaust system; it required a catalyst to make it happen, and in this case, that catalyst was DEF.

Breaking down DEF, we find it is made up of two parts: 32.5-percent urea, and 67.5-percent de-ionized water. Urea (CO(NH2)2) is the main ingredient in this whole process, and if you guessed it has something to do with urine, you’d be right: it’s found naturally in mammals like us humans, and is the catalyst responsible for how we metabolize nitrogen in our bodies.

And just as it works in our bodies, so it works in diesel motors. When DEF is mixed with exhaust fumes, the de-ionized water evaporates cleanly and allows the urea to go to work, thermally decomposing to form ammonia (NH3) and isocyanic acid (HNCO), the latter of which hydrolyses (breaks down the compound) to form carbon dioxide (CO2) and ammonia. Ammonia then reduces the nitrogen oxide to diatomic nitrogen (N2), water, and carbon dioxide (which will be absorbed by plant life to make air).


You Saved A Tree… So What?

So here is where we set foot into the area of controversy when it comes to DEF. For all of its supposed good effects on the environment, it has led to some serious and considerable criticism in the diesel world for a few reasons.

First of all, it creates an extra layer of maintenance. By and large, people hate chores and maintenance, and since DEF is an expendable resource in a diesel vehicle, it will add to the cost of ownership and become a time investment without immediate or discernible payoff, making it psychologically unrewarding.

Moreover, in some applications, the filler neck for urea is very close to the filler neck for diesel fuel. The more ignorant or lazy diesel driver may not be able to tell which is which when parked at the pump (as some fuel stations offer DEF at the pump), and as Gary Maschner of Strictly Diesel tells us, this is a common issue. “We’ve had a number of Ram and Ford trucks with damaged fuel systems because they put DEF in the wrong filler,” he said. “On the Duramax truck, the DEF cap is very close to the washer fluid cap, so you can guess what happens to those.”

This is a shot of a 2012 F-250's DEF tank. The tank was cracked near the base due to unknown causes.

Gary Maschner of Strictly Diesel in Phoenix, Arizona, has seen time and again what DEF can do to a truck. “I’ve seen customers pour DEF into the fuel filler neck and have it mess up everything,” he said. “It’s common enough that Ford sells a parts kit with injectors, pump, sensors, and regulators, basically everything forward of the fuel tank. We’ve had good luck with insurance companies covering these repairs, and it’s a good thing too, since the jobs cost between $10,000 to $11,000.”

On top of all of this, DEF is startlingly delicate. Its properties are such that it must be stored between 40 to 80 degrees Fahrenheit, and will last about a year before it turns to pot. However, if the DEF freezes (which it does at 12 degrees Fahrenheit), it’s still safe to use once it thaws. Exposure to air will cause crystallization, so using airtight-sealed containers is the only feasible way to store DEF. On that note, thanks to DEF’s corrosive properties, it should not be used with brass or copper; plastic or stainless steel are the best bets.

DEF is especially susceptible to contamination, as one teaspoon of table salt will ruin 5,000 gallons of the stuff. Considering all of the materials you might have in your garage, from paint thinner to silicone grease, it’s incredibly important that DEF never comes into contact with items used with other substances, and vice versa. So, if you use a funnel to pour DEF into the vehicle’s storage tank, be sure that funnel is used for DEF and DEF only.



Usage of DEF in a typical diesel truck varies from manufacturer to manufacturer, but the average usage rate is about five gallons for every 1,600 miles of travel. As an example, a 2015 Ford F-350 gets about 15 mpg average, and has a 35-gallon fuel tank and a 5-gallon DEF tank. Assuming he drives 1,000 miles a month, this means the average F-350 owner refills his DEF tank about once every two months; more often than an oil change, but thankfully cheaper.

You Saved A Tree And Gained Some Power, That’s What!


It might seem like DEF causes more problems than it fixes, from corrosion to chore-adding to cost considerations. But DEF has some plus sides, too.

For one, it has allowed trucks to surpass fuel mileage and power potentials compared with year’s past. Since SCR works on the exhaust side of the internal combustion process, it doesn’t have a direct effect on how much air or fuel can be consumed inside the motor, which increases the potential for horsepower and torque in trucks. And with regard to fuel mileage, increases of up to five percent have been observed.

Gary added, “DEF has also been linked to fewer regenerations in diesel trucks. This is a good thing because it reduces fuel consumption to clean out the DPF, meaning better fuel efficiency overall. When these trucks go into regenerations, there’s also the risk of fuel getting into the crankcase and contaminating the oil in there, so with fewer regenerations, there’s less wear on the engine.”

Also, cost is a factor, but Gary affirmed it was not a big deal. “The trucks that do run DEF are better off and more reliable than the models that do use it,” he said. “Reducing emissions and wear on the motor is less with running DEF.”


Though Peak BlueDEF has cornered the market in DEF, other offerings come from Ford Motorcraft and AC Delco, and even Mercedes and Chrysler have their own versions.

Here To Stay

For better or worse, DEF is not going anywhere. The future of diesel trucks depends on the abilities of engineers to make trucks cleaner and more reliable, and DEF seems to be the cure du jour for placating emissions standards.

What do you think will happen to diesel’ efficiency as time goes on? Let us know in the comments below.

About the author

David Chick

David Chick comes to us ready for adventure. With passions that span clean and fast Corvettes all the way to down and dirty off-road vehicles (just ask him about his dream Jurassic Park Explorer), David's eclectic tastes lend well to his multiple automotive writing passions.
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