What You DEF Need to Know About Diesel Exhaust Fluid

What makes diesel engines so intriguing? Now there’s a loaded question for ya. Take your pick: their sheer intricacy, ridiculous capabilities in horsepower and torque, response to power-adders, and the list goes on. Sure, there are things that diesels share with their gasoline-platform cousins. Though, for every single data point that is similar, there are three more that separate the designs almost entirely. Case in point: Diesel Exhaust Fluid (or DEF for short). For beginners, DEF may seem like an alien concept but make no mistake—it’s an integral part of diesel ownership.

So, what exactly is Diesel Exhaust Fluid and why do I need it?

Diesel Exhaust Fluid is used in passenger trucks as a way of reducing the harmful emissions of spent diesel fuel, namely nitrogen oxide (NOx) emissions. This is important because (as the EPA points out) these highly-reactive gases, which are formed when fuel is burned at high temps, are pretty nasty—poisonous, in fact. Often appearing as a brown-colored gas, “NOx is a strong oxidizing agent and plays a major role in the atmospheric reactions with volatile organic compounds that produce ozone (smog) on hot summer days.”

Basically, it’s not something you want to spend time breathing in, and it’s pretty hard on the environment too.

That’s why, in 2010, it became standard that all diesel trucks be equipped with systems dedicated to putting DEF to use. Composed of 32.5% urea and 67.5% de-ionized water, Diesel Exhaust Fluid is sprayed into a vehicle’s exhaust system to help breakdown the NOx emissions, turning them into harmless nitrogen and water. If you’re thinking, “Urea. That sounds oddly like ‘urine,’” you’re actually on the right brainwave. Urea is an organic compound produced when a body metabolizes protein. (And yes, we expel it as urine.) But despite DEF earning the nickname “pig urine,” Diesel Exhaust Fluid is actually made of commercial-grade urea—synthetic ammonia and carbon.

So, to simplify things: the urea and water in DEF heat up, producing ammonia. This ammonia then breaks down and neutralizes the NOx gases. Less toxic elements come out of the tail pipe, and everyone breathes a little easier.

Changing Diesel Exhaust Fluid

Yeah, we know. Diesel Exhaust Fluid is one more thing to add to your list of routine maintenance. Not to mention, it also requires a little more attention than some other diesel fluids.

First, the regulations that put DEF into place are pretty strict, meaning automakers have had to take them very seriously. And that trickles down to you, because your diesel truck is programmed to reduce engine power if you run out of DEF. While you’ll receive a warning when levels are low, the average diesel truck burns about one gallon of DEF every 200-300 miles. This just so happens to be about how long a tank of gas in your truck should last. While you can always top off at a station, keeping an extra gallon or two of DEF on hand is a smart move. (PEAK is a proud supplier of DEF for trucks and sells a gallon jug for roughly $5.)

Filling up is easy. Just look for the DEF tank, which is usually marked with a blue filling cap and the words (you guessed it) “Diesel Exhaust Fluid.” The free service site, discoverdef.com reports that while some people have managed to confuse their diesel fluid and DEF tanks, it’s rare. “The standard nozzle diameter for DEF is 0.75 inches (19mm), compared to 0.87 inches (22 mm) for diesel, preventing the diesel nozzle from ever being inserted into the DEF tank,” the site points out. But if, by some stroke of terrible luck or stupidity, you do confuse the two, get to a dealer ASAP and don’t drive the vehicle. Even small amounts of diesel fuel can wreak havoc on the system.

Storing Diesel Exhaust Fluid

If you decide to keep some spare fluid around, keep in mind that DEF is a water-based solution, so that means it will freeze in the wintertime. According to Cummins Filtration, “A 32.5% solution of DEF will begin to crystallize and freeze at 12 °F (-11 °C). At 32.5%, both the urea and water will freeze at the same rate, ensuring that as it thaws, the fluid does not become diluted, or over concentrated.”

Good news, since it can also freeze in your tank. Luckily, the Selective Catalytic Reduction (SCR) system that allows DEF to break down NOx emissions is also designed to heat the tank and lines, so everything can thaw properly—although, it will take some time. In short, be mindful of where you store your spare Diesel Exhaust Fluid and where you park your truck.

Additionally, that DEF has a shelf life. Usually, Diesel Exhaust Fluid will last you about two years. But if it’s stored in an exceedingly warm space (over 86°F/30°C) or in direct sunlight, expect that number to drop. Compared to the other fluids in your vehicle, DEF is the least hazardous, but it should still be handled carefully. So, don’t store it in containers made of carbon steel, aluminum, copper, or zinc, as it can corrode the metal. It’s also worth noting that some trucks will have a filter in the DEF system, and you’ll want to be mindful of its condition to ensure the system can work properly.

One More Thing for the Road…

Some people claim DEF systems have improved their vehicle’s fuel efficiency and power. Others see it as a burden and an unnecessarily complicated add-on to an otherwise blissfully-simple engine. (Get the joke?Because there’s really nothing simple about diesel ownership.) Whatever side you fall on, remember that deleting a truck’s DEF system means that vehicle will no longer be street legal. Ultimately, the DEF system really doesn’t do anything to harm power output, and removing it will require everything to be tuned afterwards. Perhaps some weight savings may be of consideration, but with all the power that diesel is pumping out, is it really worth the trouble?