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Diesel Exhaust Fluid - DEF

In 2010 the EPA mandated enhanced pollution controls for diesel engines. Many manufacturers adopted the "Selective Catalytic Reduction" SCR system which reduces the release of Nitrogen Oxide. This system works by injecting small amounts of DEF into the exhaust system, which converts the harmful Nitrogen Oxide to form simply Nitrogen and water (since nearly 80% of the atmosphere consists of Nitrogen anyway, it is environmentally friendly).

DEF is compromised of mostly deionized water, with a 32.5% concentration of aqueous urea. Urea is a compound of nitrogen that turns to ammonia when heated. DEF is not a fuel additive, it is a tank of liquid that during engine operation, is injected into the exhaust system.

Since many RV'ers use diesel trucks as tow vehicles, DEF is an appropriate topic for discussion in a RV oriented website. As well, there is actually some good news for vehicles equipped with DEF systems.

DEF allows the engine manfacturer to tune their engines for maximum fuel economy yet maintain low emissions. Inded, a DEF equipped Diesel engine may very well be cleaner burning than the typical gasoline engine. While the EPA does not require fuel mileage estimates for commercial vehicles (3/4T pickups and larger), I have found the following fuel milege:


Driving ConditionsBest MPGNote:
70MPH Highway22MPHSummertime - Pickup Bed covered.
65MPH Highway24MPHSummertime - Pickup Bed covered.
60MPH Highway26MPHSummertime - Pickup Bed covered.
60MPH Highway Towing13MPHSummertime - Towing 10,000lb RV.
City8MPH Towing and not Towing.


This is a huge improvement over vehicles that do not use DEF, and in reality, might be enough of a reason to upgrade your vehicle.

DEF typically burns around 2% of the fuel burn rate. For example, 50 Gallons of fuel will burn perhaps 1 Gallon of DEF. Since the typical pickup has around a 5 Gallon DEF tank, that is equivalent to 250 Gallons, or around 4,500 miles between DEF fills. In fact, when I have my pickup serviced by the dealer, they typically top off the DEF (unless they forget), so I rarely have to fill the truck with DEF myself.

However, certain conditions will cause DEF to be consumed at a higher rate:

  • Towing. Expect higher DEF consumption when towing.
  • Exhaust Brake. When using the vehicle Exhaust Brake in conjunction with towing, DEF consumption will be higher.
  • Age. Older DEF may lose potency, so it may be consumed at a higher rate.

When purchasing DEF, it is desireable to purchase the freshest DEF you can buy, as it can go stale. Not necessarily will it go bad, like in spoiled meat, but it may burn at a faster rate, and in extreme conditions, it might cause damage to the DEF system. Some vehicle warranties may not cover damage from old DEF.

Also, DEF should be stored in moderate temperatures (86Deg or less), and out of the direct sunlight as either of these conditions can shorten the shelf life of DEF. Manufacturers of DEF such as Peak state the DEF can have as high as a 2yr shelf-life if stored properly.

So, how should you buy DEF? First, go to a retailer that has a high turn-around. Avoid any DEF that has dust on the upper surfaces, as it has obvously sit around for a long time. Also, if the store displays DEF near the front entrance having many windows, then the sunlight could shorten the DEF life.

You can also read the date code on some brands of DEF. Always ensure you will consume the DEF prior to the 1yr manufacturing date anniversary. Date codes may be hard to read, but Peak (Blue DEF) was nice enough to give me a "magic date decoder ring" for the asking.

The date code for the Blue DEF box shown at the right is "CP18327 6 5291". Decoding the date code as is follows:

In this example, the mfg date was Feb 7, 2017, with the expiration date being 1 or 2yrs from that date, depending on storage conditions.

You can also test DEF with a DEF specific hygrometer or refractometer. As DEF gets older, it gets weaker. A hygrometer/refractometer will ensure you have full strength DEF (the specified 32.5% urea).






Last reviewed and/or updated May 10, 2017