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DEF tips to keep a diesel-powered vehicle running smoothly

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DEF tank on a truck (Image courtesy of American Petroleum Institute)

With 2020 being dominated by challenges caused by COVID-19, some diesel-powered vehicles have likely been out of service for a long period of time. This includes buses, motor coaches, trucks and other fleet vehicles that have been idle for many months due to school closures, the cancelation of events and reduced road travel.

As operators of various fleets manage the maintenance needs of vehicles that are out of service or those to be put back into service, there are many items that need to be considered. One of the easiest items to overlook is diesel exhaust fluid (DEF) that is already in the vehicle. DEF has a limited life and may need to be changed before a vehicle goes back into service.

The American Petroleum Institute (API) recommends that you determine when the diesel exhaust fluid was put in the vehicle because the storage life is 12 months in optimal conditions. When it has been stored in the vehicle past 12 months, it is recommended that it be drained and replaced. API also recommends changing the DEF if a vehicle sat unused in summer heat for a prolonged period of time or if the diesel exhaust fluid may have been contaminated in any way.

In addition, API suggests changing the engine oil if you believe moisture may have built up in the engine; plus check other critical fluids and vehicle components — including the tires — before going back out on the road.

Also, remember to evaluate the DEF on shelves in the shop if it has been a long time since the last delivery. Order new fluid if necessary as you don’t want to replace old DEF in your vehicle with product that is expired. While diesel exhaust fluid is not considered a hazardous material but should be disposed of in accordance with local regulations.

Managing DEF in wintertime

Made from a mixture of technically pure urea and purified water, DEF freezes at 11 degrees Fahrenheit and -11 degrees Celsius and needs to be properly maintained and dispensed to preserve its quality. Like water, diesel exhaust fluid will expand up to 7% when frozen and can damage the storage tank if it is full or nearly full when it freezes. Keeping a tank that you think may freeze less than full is a good idea.

If it freezes in the vehicle, do not put any additives in the tank to help it melt because the fluid needs to remain pure for it to work correctly. Don’t worry; on-spec DEF is specifically formulated to allow the fluid to thaw at the proper concentration to keep your vehicle operating smoothly.

In addition to cold, there are other things to consider when purchasing, storing and handling DEF. Drivers accustomed to purchasing the fluid in containers should look at the expiration date on the bottle and be sure to use it before this date. If a date is not present, ask for the most recently delivered products. Also, check the label for recommended storage temperatures and be sure to look for the API certification mark on the bottle as well. Many diesel engine manufacturers recommend that drivers use API-licensed DEF.

Purchasing for shop use

API has found that the biggest misconception by fleet managers is the belief that if the urea concentration of their DEF is on spec, then the fluid meets the required quality. While it is true that the concentration is very important, there are many other important quality characteristics built into the ISO 22241 specification.

Those responsible for procuring DEF should confirm their suppliers are providing a fluid that meets the entire ISO quality standard. One way to do this is to ensure that their supplier is providing a Certificate of Analysis (or Quality) with every shipment that addresses all of the quality characteristics that the specification requires. You can also check to see if the DEF they are buying is licensed through API’s real-time directory of licensees on the API website.


DEF Storage Tank (Image courtesy of American Petroleum Institute)

Tips for managing DEF in shops

For shops, the handling, storage and dispensing of DEF is very important so that off-spec fluid doesn’t reach the marketplace. Temperature during transport or at the point of storage or sale can harm the shelf life of DEF sold in containers. Make sure the stock is rotated to use the oldest product first. Proper storage temperatures in a shop is also vital. Storing in temperatures above 86 degrees Fahrenheit will limit the shelf life of the fluid over time.

Some additional things to consider in storing and handing DEF include the following:

  • Bulk storage tanks should be dedicated for DEF. Don’t switch products in the bulk tank without thoroughly rinsing the tank with distilled, de-ionized water or on-spec DEF.
  • A closed loop system for transferring the fluid from a drum or bulk tank is recommended so contaminants don’t get into the DEF. This is particularly important in a shop or construction site that has dust or dirt in the air.
  • Use dedicated equipment for dispensing DEF. Don’t use funnels, pitchers, hoses, etc. that are used for other fluids when putting DEF in a tank.
  • Anything used for dispensing the fluid should be cleaned with distilled or de-ionized water and followed by a DEF rinse. Don’t use tap water for cleaning.

For shops and drivers, it’s important to know what you are putting into your DEF tank. The quality of the fluid going into your vehicle is as important as the quality of the engine oils or fuels used in your vehicles. Use of API-licensed Diesel Exhaust Fluid will ensure that it meets the high standards required by engine and vehicle manufacturers.

The Guest Author is Jeffrey Harmening, manager – EOLCS/DEF/MOM, American Petroleum Institute.

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Keep your truck running smoothly by using high-quality diesel exhaust fluid

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