Diesel exhaust fluid: Fluid-injection for NOx reduction

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Diesel exhaust fluid: Fluid-injection for NOx reduction

If your shop doesn’t routinely service diesel-equipped vehicles, you may not be fully aware of the use of DEF (Diesel Exhaust Fluid). DEF consists of a mixture of high-purity urea and deionized water.

Controlled by a “dosing” module, DEF is injected into the diesel engine’s exhaust stream. DEF consists of a mixture of 32.5% high-purity synthetic urea and 67.5% deionized water.

DEF dosing is controlled by the engine’s ECU. When heated, DEF splits into ammonia and carbon dioxide, which is then atomized and vaporized. Once DEF enters the exhaust, the water in the DEF vaporizes, leaving ammonia molecules to travel to the catalytic converter where it neutralizes NOx molecules. This reaction converts NOx to harmless nitrogen and water, substantially cleaning up diesel emissions.

The EPA has mandated that all on-road diesel-equipped vehicles manufactured after January 2010 must reduce NOx emissions. The most common method to achieve this reduction is by injecting DEF into the exhaust path. All diesel-equipped vehicles that feature an SCR (Selective Catalytic Reduction) system utilize DEF injection.

The use of SCR systems reportedly provides an additional benefit of a 3% to 5% increase in diesel fuel economy. This is a result of enhanced combustion as exhaust temperature rises, especially in the heavy-duty applications. Cummins reportedly has gained substantial mileage improvements with their SCR system.


DEF usage rate

A “typical” DEF capacity on a full-size light truck should provide a driving range of somewhere in the neighborhood of 5,000 to 7,000 miles or so (depending on the make, model, size of the DEF tank, driving habits, etc.). The SCR system in Chevy and GMC light trucks uses DEF at a reported rate of about 1% to 1.25% of the vehicle’s diesel fuel usage. Citing the Chevy Silverado HD as an example, a full tank of 5.3 gallons of DEF provides a driving range of about 4,970 miles.

When the DEF on-board tank is approaching empty, the vehicle’s onboard warning system will alert the driver with enough lead-time to replenish the DEF tank. Using the Chevy Silverado as an example, the first warning will occur at a remaining range of about 1,000 miles with additional warnings at the 300 mile and 0 range. Below a range of 300 miles, the warning (possibly along with a speed limiting warning) will appear every time the engine is re-started.

Again, using the GM vehicle as an example, if the DEF tank is allowed to run empty, at the next engine start, vehicle speed will be reduced to a maximum of 55 mph and eventually to a miserable 3 mph to 5 mph. If the DEF tank is not refilled, no damage will result from running the DEF tank empty.

If the DEF tank is run dry, according to GM, when refilling, more than one gallon of DEF must be added to release the speed limitation. You must then wait about 30 seconds with the engine at idle for the exhaust fluid message to clear.

If the vehicle is driven before the message clears, the speed limiting may remain in effect. If the exhaust fluid empty message clears while driving, the vehicle must be brought to a complete stop in order to release the speed limitation.

Ford diesel vehicle SCR design is slightly different. An alert is illuminated when the system determines an 800-mile-to-go status (Ford claims that their system requires a refill every 7,500 miles). If the DEF tank runs dry, about a 50% loss of power will be realized during that drive cycle (not just after the next ignition-on cycle).

Instead of (or in addition to) refilling DEF tanks when an early warning is provided, it’s wise to top-off a DEF tank at the same time that the engine oil is changed (for example, every 3,000 to 4,000 miles). In other words, you need to keep on top of the DEF level status. Running the system dry will result in severe driver inconvenience, in terms of entering a limp mode (either loss of power or severely limited vehicle speed).

Dodge Ram diesel light trucks feature an 8.2-gallon DEF tank. According to Chrysler, the anticipated usage rate is about one gallon of DEF for every 50 gallons of diesel fuel used.

The Ram early-warning system initiates when there is about two gallons of DEF remaining in the tank. A “Low DEF Refill Soon” warning is displayed, along with an audible chime. When only one gallon of DEF remains, a message will be displayed that says “Refill DEF — Engine Will not Start in XXX Miles” (the number of miles displayed is calculated by the system’s controller). If the warnings are ignored and the tank runs dry, a “Refill DEF — Engine Will Not Start” message appears. This indicates that the engine will not start on the next key-on cycle. At least two gallons of DEF must be added in order to re-start the engine.


DEF tank fill location

Location of the DEF fill point varies depending on the make and model of the vehicle. On the Chevy/GMC models, the DEF fill point is located next to the fuel fill point on full-size vans; and underhood on the passenger side of full-size trucks. DEF fill points are identified with a blue cap. On Ford F-Series trucks, the DEF fill point is located adjacent to the diesel fuel fill on the driver side.

Commercial/specialty vehicle DEF fill locations can vary widely, depending on the model. Utility box trucks may feature a DEF fill at any convenient location, usually on an exterior wall of the box area. Tow trucks might feature the fill point behind the cab. For any diesel vehicle that falls outside of the “normal” consumer light truck or van, you may need to search (or look in an owner’s manual).

Dodge Ram DEF fill locations are found underhood.

All DEF fill caps, regardless of make or model, should be light blue in color and should be labeled “DIESEL EXHAUST FLUID.” When filling the DEF tank, DO NOT overfill. In freezing temperatures the DEF mixture will expand when frozen. DEF tanks are designed to accommodate this expansion, but only if you fill to the advised level. If a DEF tank does become damaged and leaks, there’s no real hazard, since it’s not toxic or flammable. It’ll just make a stinky ammonia-smelling mess.

DEF prices

For light truck applications, DEF is commonly available in one-gallon and 2.5-gallon containers. I’ve seen one-gallon containers going from anywhere between $7.74 to as much as $12.25 (with the higher prices usually found at car dealerships). NOTE: I purchased one gallon of Peak’s BlueDef at a local parts store for $7.96 on April 30, 2013.


Once DEF temperature reaches approximately 12 degrees Fahrenheit, it will freeze. However, this won’t leave the vehicle stranded. The engine is able to start, and the vehicle may be driven normally. As engine coolant warms, the DEF will thaw and will once again flow properly. While there have been claims that urea will become toxic at 118 degrees F, this isn’t so. Actually, urea isn’t toxic at any temperature. If it becomes too hot, the shelf life may simply decrease (as ammonia begins to form).

DO NOT add any additives, such as anti-gel, to any DEF tank. DEF will expand by about 7% when frozen and will revert to a liquid state when heated during engine operation. DON’T treat DEF as you would diesel fuel. You don’t need, nor should you ever add, any modifiers/additives. Keep the DEF pure!

Home-brew alternatives?

Just because the DEF solution is comprised of a mixture of urea and water, this does not mean that you can whip up your own batch. The urea used in DEF is a very high-purity synthetic, and is not urine. Also, the water used in DEF must be extremely pure and deionized. The bottom line: no, you can’t make your own DEF by mixing urine and tap water. DEF is a sophisticated blend designed specifically for SCR systems.

Allowing impurities such as calcium, copper, magnesium, etc. (which would/could be present in urine and tap water), would introduce impurities that can kill active sites in the catalytic converter (the ammonia itself won’t harm the sites). Continued use will eventually make the converter inoperative, requiring converter replacement.

Also, and this is IMPORTANT to know and to relate to your customers: DO NOT add ANYTHING to the DEF tank except actual DEF. If someone, by accident or ignorance, dumps diesel fuel treatment/booster into the DEF tank, this will kill the entire SCR system, resulting in a repair bill of thousands of dollars. PAY ATTENTION. You wouldn’t add orange juice to your fuel tank or Pepsi to your cooling system, so don’t add anything but the correct fluid to the DEF tank.

Along the lines of DEF purity, consider the quality of the DEF. The major brands (Peak’s BlueDef, ACDelco, Motorcraft, etc.) offer the highest quality both in terms of purity and proper urea-to-water percentages. However, there are a few bargain-priced brands out there that feature a higher water content, which isn’t as efficient and won’t last as long as anticipated. In other words, stick with the major brands.

Vehicle manufacturers have designed DEF fill ports at a smaller-diameter size in order to prevent accidental introduction of diesel fuel into the DEF tank. While the standard diesel fill point (for light trucks) accommodates a 22 mm nozzle, the DEF fill point is sized for 19 mm.


Keep it clean

Whenever handling DEF, the fluid must be kept clean. If a funnel is used, it must be absolutely clean and free of any contaminants. It’s best to dedicate a funnel (and label it) for DEF only. Spare DEF should be stored in its original container, with the lid/cap secured, to avoid airborne contaminants in the shop. Do not transfer DEF to another container that previously held anything else (oil, windshield wiper fluid, coolant, etc.). Avoid cross-contamination! It doesn’t take any extra time to handle DEF properly.

FYI: Urea is corrosive to metals such as copper and brass. DEF systems are typically plumbed with high density polyethylene.

Verifying DEF quality

All API certified DEF will feature an API symbol on the container. Use only API certified DEF.

The concern with DEF quality (beyond API certification) is the potential for water dilution. Just as water can enter a fuel system, it is possible for water (moisture) to enter the SCR system and slightly dilute the DEF. One indication that the DEF has been diluted is when the vehicle uses more DEF than normal (beyond the mileage use specified by the vehicle maker).

An easy field test to check for the purity of DEF is with the use of a hand-held refractometer. A refractometer that is designed for testing DEF will easily allow you to verify the water content.

A refractometer is a precision optical instrument designed to measure the concentration or mixture of water-soluble fluids. It measures refractive index, which is the speed at which light passes through a liquid. The more dense the liquid, the slower light will travel through it, resulting in a higher reading.

Applying a few drops of the vehicle’s DEF to the refractometer is all that’s needed. DEF refractometers (those units capable of measuring urea concentration) are available with single scale or dual scale readouts. The single scale provides a percentage of urea by weight. A dual scale unit also provides a refractive index scale readout, which is primarily used by manufacturers and distributors of DEF (a quality control aid). For shop technicians who simply want to check for water dilution, a single scale unit will suffice.

Citing two examples of DEF-capable refractometers, MISCO U.S. offers a digital single-scale unit (their DEF-201) that retails for around $350. The reading will provide a precise level of water content to +/– 0.1%. The dual-scale unit (DEF-202) retails for around $435. Either model is powered by a pair of AA batteries.

OTC has released their OTC-5025 DEF refractometer, which retails for around $114. The kit includes a refractometer, dropper, lens wipe cloth, instructions, screwdriver and storage case. The refractometer features a reticle (scale) that is enlarged through an eyepiece to measure light. The values on the scale have been established to specifically evaluate DEF condition. OTC’s unit features 0.5% line graduations, with a scale range of 15% to 40%.

Regardless of the brand you choose, a DEF-capable refractometer is a useful and must-have checking tool for any shop that services diesel-powered vehicles. This eliminates guesswork with regard to the quality (in terms of water dilution) of customer DEF tank contents.


Tips on refractometer use

Wait until fluid and tool temperature are stabilized at ambient temperature before taking a reading.

Storing DEF

Storage temperatures need to be held at between 12 degrees F and 86 degrees F. According to sources, this maintains optimal shelf life for up to two years. If DEF is allowed to freeze, once thawed, its efficiency returns to normal. Prolonged storage above 86 degrees F will cause hydrolysis to occur, reducing shelf life.


DEF is available in a variety of volumes, including one-gallon containers, 2.5-gallon containers, 55-gallon drums and 275-gallon totes.

DEF is also available at the pump, primarily at truck stops and diesel repair facilities. As time progresses, we’ll see more at-the-pump DEF availability.

Our understanding at this time is that DEF is sold in North America as DEF, Peak’s BlueDef, Valvoline’s Airshield, C-Blue, TerraCair and Air1.

The use of DEF is here to stay, with the number of applications increasing at a very steady rate.

If your shop services diesel engine-equipped vehicles, model year 2010 and newer, you’ll need to stock DEF, and you need to understand the need for storage and transfer cleanliness.

We offer this brief article as a primer on SCR systems and the use of DEF. So, the next time a customer asks for “muffler juice,” you’ll know what he’s talking about.   ●



6200 Grand Pointe Dr., PO Box 6020, Grand Blanc, MI 48439, (800) ACDelco (223-3526)


5083 Pottsville Pike, Reading, PA 19605, (877) 363-5843


3511 Silverside Rd., Suite 203, Wilmington, DE 19810, (877) 221-7335


16800 Executive Plaza Dr., Dearborn, MI 48126


Old World Industries, 4065 Commercial Ave., Northbrook, IL 60062-1851, (877) 845-0333


3499 Blazer Pkwy., Lexington, KY 40509, (859) 357-7777



3401 Virginia Rd., Cleveland, OH 44122, (216) 831-1000

Models: DEF-201 & DEF-202


655 Eisenhower Dr., Owatonna, MN 55060, (800) 533-6127

Model: OTC 5025 DEF Refractometer

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