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Why Quality DEF Matters

By on July 1, 2019

Homer’s Guide to Maximizing Performance

By: Homer Hogg, Director, technical service

Nitrogen Oxides (NOx) are a family of poisonous, highly reactive gases that form when fuel is burned at high temperatures. The Environmental Protection Agency (EPA) regulates nitrogen dioxide which is a form of NOx. The problem is that NOx causes acid rain and ozone. I’m referring to the ozone or smog in the air we breathe, not the ozone that protects us from the sun’s UV rays. NOx is known to cause chronic breathing problems and cancer.

To help offset the harmful effects of NOx, diesel engineers discovered that Urea, as it is called, provides a good solution to meet the EPA’s Nitrogen Oxides regulations. Since mass-produced Diesel Exhaust Fluid (DEF) hit the market in 2010, it has proven to be very effective in dramatically reducing the harmful effects of Nitrogen Oxides.

A Bit of Science

In an internal combustion engine, heat is added to fuel and oxygen. One of the negative results is NOx. DEF consisting of 32.5% urea (ammonia) and 67.5% deionized water is injected in the exhaust stream, just before the exhaust reaches a device called a catalyst. The heat from the catalyst and the urea mixes and causes a chemical reaction, which separates the nitrogen from the oxygen. The two harmless byproducts are simply oxygen and nitrogen. This process is called Selective Catalytic Reduction (SCR).

Many after-treatment maintenance problems are a direct result of poor-quality DEF. Pay close attention to the date of the product you buy and the urea percent. Most diesel engine manufacturers require that the DEF used in their SCR systems must meet ISO standard 22241. This helps assure that the product meets the 32.5% urea requirement. Another important hint is to be certain that the DEF is certified by the American Petroleum Institute. Simply look at the label or storage tank to ensure that the product is properly certified.

The SCR system is calibrated to work most effectively at the 32.5% urea and 67.5% water blend. If these percentages are not correct, you’ll see a light and a code set appear on your instrument panel. If the problem persists, your engine’s performance will derate, and you may need a service call or a tow. Another reason you want to be sure about quality is that DEF freezes at 12˚F. If the blend is not optimal, it will freeze at higher temps, which could cause some serious downtime and delays. To make matters worse, if the mixture is compromised, SCR components can be damaged. DEF’s shelf life is based on the ambient temperature. The product should remain usable for up to one year if it is stored between 10˚F and 90˚F. DEF can last up to two years if it’s kept at or below 75˚F.

What We See

Many trucks arrive at a TA Truck Service location with DEF quality codes set in the computer. Experience tells us that the majority of these alerts are a direct result of poor quality DEF or a corroded NOx sensor. DEF is very corrosive and when sprayed into the exhaust stream and exposed to extreme exhaust heat, it becomes severely caustic to the NOx sensor. Many times, the sensor can be cleaned but often it must be replaced.

What to Look For

Also keep a close eye on any wiring harness routed near the exhaust. Just before the exhaust reaches the DEF system, it must pass through the Diesel Particulate Filter system. This means that exhaust temperatures can exceed 1200˚F. A wiring harness that touches or is routed too closely to the exhaust will be quickly fried, causing serious problems and loss of performance. 

If you see white dust or crystals forming on your truck near a DEF component, you likely have a leak or spill. The DEF must be removed from any external surface, part or component immediately, or severe damage will occur. Always monitor the quality going into your DEF tank. If you experience any fault codes or problems, see a qualified technician at the closest shop. 

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