Remember when unleaded gas wasn’t regular? Look at what most of us have seen in our lifetime: catalytic converters, unleaded gasoline and ultra-low sulphur diesel fuel, just to name a few. Yes sir, changes come, and there isn’t much an old timer can do about it.
With the transition to Tier IV diesel motors, we have seen changes to our trucks and tractors. We have been introduced to diesel exhaust fluid (DEF) and the need to provide our new diesel-powered vehicles with DEF on a routine basis.
DEF is an aqueous urea solution made with 32.5 percent urea and 67.5 percent deionized water. It is standardized as AUS 32 (aqueous urea solution) in ISO 22241. DEF is used as a consumable in selective catalytic reduction (SCR) in order to lower nitrogen oxide (NOx) concentration in the diesel exhaust emissions from diesel engines.
Because the fluid is consumed during the system’s normal operation, it requires the vehicle’s owner to not only monitor the fluids level but also refill it when it’s been used up.
SCR technology uses ammonia to break down NOx emissions produced during diesel combustion into nitrogen and water. SCR has become the technology of choice for a majority of truck and engine manufacturers to meet the stringent 2010 emissions standards set by the EPA for heavy-duty trucks.
The biggest benefit of SCR for the vehicle owner is in the fuel savings the technology provides. Because SCR deals with NOx outside the engine, manufacturers are once again able to tune their engines to run more efficiently and produce more power. The increase in engine efficiency also leads to a reduction in particulate matter, resulting in less-frequent regeneration of the DEF and adding to the increased fuel economy.
It is recommended we store exhaust fluid between 40 and 80ºF, and it has an effective shelf life of one year when stored at 80ºF. Prolonged storage above 86ºF will cause hydrolysis to occur. The most important quality of DEF is its purity.
You may be thinking, “I have urea and water in quantity here on the farm … I will just mix my own.” The one-word answer to that is: don’t. Impurities can wreak havoc on a SCR system; a teaspoon of salt will contaminate 5,000 gallons of DEF, so the quality standard (the ISO 22241 listed above) is very important.
Contamination concerns are one consideration for you in considering how much DEF you need to keep on hand and how large of a container to keep around. DEF is available in containers ranging from 1- to 275-gallon totes.
If you feel you need a larger volume around your equipment, make sure your delivery system maintains the cleanest conditions possible. The most common storage vessel is a 2.5-gallon jug with a disposable filler pipe included in the box with the jug. Use them once and throw them away.
One of the biggest complaints about the use of SCR is: Eventually you are going to run out of exhaust fluid. Refilling your vehicle’s DEF tank is simple. DEF is available at most auto parts stores and truck stops across the country, making it incredibly easy to locate, and new brands and retail outlets are popping up constantly.
Since DEF is nontoxic, virtually anybody can refill his or her vehicle simply by purchasing the proper amount of fluid and locating the fill port, usually next to the fuel filler.
The worst thing that can happen to an SCR system is being filled with contaminated or incorrect fluid. This can potentially cause thousands of dollars in damage to the emissions system and leave you stranded. What can also leave you with a thumb sticking out by the side of the road is running the DEF tank dry. The EPA requires vehicle manufacturers have measures in place to ensure equipped vehicles cannot run without exhaust fluid.
Vehicle manufacturers all handle this in slightly different ways. Some employ the use of a gauge, while some have a simple warning light. Generally speaking, however, when the DEF tank level drops below 10 percent, a warning of some kind will be displayed on the dash, indicating it’s time to fill up.
The warnings will get progressively more frequent, brighter or louder as the level continues to decrease. If the vehicle is allowed to run out of fluid, one of two things will happen: Either engine power will be cut and speed limited to essentially a “limp mode,” or the vehicle will not start until the fluid is replenished.
“Can DEF go bad?” is a topic of discussion on many internet chat sites and owner threads. The answer is: Yes, DEF can go bad but, if stored properly and protected from temperature extremes (especially on the high end … over 90ºF), DEF should last a minimum of a year.
I would also add it is wise to purchase DEF from an outlet that moves DEF regularly and doesn’t have “old stock” on the shelf. DEF can freeze at temperatures below 12ºF; however, it will work after it thaws with no side effects detrimental to the motor. Freezing is a problem in the vehicle if the tank is overfilled. The tank has a heater to maintain the DEF; however, most if not all tanks do not heat the fill pipe leading to the tank.
So if you suspect the DEF in your vehicle’s tank is bad, what should you do? The only true way to know would be to pull a sample and send it to a laboratory for testing. However, many manufacturers have become American Petroleum Institute-certified, a voluntary program that monitors the quality of DEF. Remember, DEF is a clear, colorless liquid. If you see any color, haze, phase separation or sediment in your DEF container, this should be an immediate red flag.
How much DEF your truck or tractor will consume relates directly to how much fuel it typically uses. The more efficient the motor, the less DEF will be required. My diesel truck has an 8-gallon tank. I generally need to refill the tank about every 9,000 to 10,000 miles, so it really isn’t too bad to deal with at this point.
So, as much as I hate change, DEF is definitely here to stay. Just like unleaded gas, it will soon be known as “regular.” 
Andy Overbay holds a Ph.D. in ag education and has more than 40 years of hands-on dairy and farming experience.
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Andy Overbay
- Extension Agent
- Virginia Cooperative Extension
- Email Andy Overbay
