Many modern diesel engines will not run well without DEF. Knowing what it is, how to keep it, and what ruins it protects your equipment from avoidable downtime.
Diesel exhaust fluid is a mixture of 32.5 percent urea and 67.5 percent deionized water. It is not a fuel additive and it does not go into the fuel tank. It is stored in its own tank on the vehicle or machine and injected into the exhaust stream, where it helps convert nitrogen oxides into harmless nitrogen and water in a system called selective catalytic reduction.
The 32.5 percent concentration is not arbitrary. It is the urea-water mix with the lowest freezing point.
Because DEF is consumed along with fuel, it pays to track it with the same discipline. Noting DEF use alongside diesel in each fueling record makes unusual consumption easier to notice and investigate.
Because DEF goes into its own tank, filling it is a separate step from fueling and a separate item on any checklist.
DEF quality is defined by the ISO 22241 standard, which covers the fluid's specification and also guidance for handling and storage. Equipment makers expect DEF that meets it, and many products are marketed as meeting that standard. When you buy DEF, make sure the product states it, and treat unlabeled or home-diluted fluid as a risk to an expensive aftertreatment system.
Modern equipment monitors DEF level and quality. If the tank runs empty or the fluid is off-spec, the machine can reduce power or enter a limited mode, which is why running out of DEF is as disruptive as running out of fuel.
If a machine shows a DEF quality warning, treat it seriously. Stop, check the fluid, and consult the manual instead of topping off with whatever is nearby.
Some equipment makers specify requirements beyond the standard, so check the owner's manual.
Heat is DEF's main enemy in storage. Guidance based on ISO 22241 puts the ideal storage range at roughly 12 degrees Fahrenheit to 86 degrees Fahrenheit, and advises avoiding prolonged storage above 77 degrees Fahrenheit, because the urea can decompose.
Shelf life depends on temperature, with cooler storage lasting longer. For most of the United States, a minimum of about one year is a reasonable expectation, and it can be shorter if the fluid sits somewhere hot. Keep containers out of direct sun and in a space that does not swing widely between day and night or season to season.
Do not rely on a calendar alone. If the DEF has been stored warm, have its concentration tested before putting it into a machine.
Where you store matters. In hot weather a shaded or insulated enclosure helps, and in cold weather a heated space avoids repeated freezing and thawing.
DEF freezes at about 12 degrees Fahrenheit. Frozen DEF is not spoiled. Once it thaws, a frozen 32.5 percent solution does not leave solids behind and does not lose its effectiveness. Check your equipment manual for how the machine handles freezing.
Do not top up a partly frozen tank with a different concentration or add anything to speed the thaw. Let it thaw naturally or through the vehicle's own system.
Plan DEF supply around the season. If you expect a slow winter or an idle machine, buy less at a time so stock does not age past its useful life.
DEF fails most often because something else got into it. Tap water contains minerals that can foul the catalyst, so only deionized water should ever be used to adjust DEF. Dirty funnels, shared hoses and open containers all introduce contaminants.
Material compatibility matters as well. DEF should be stored in high-density polyethylene or stainless steel. It reacts with copper and some other metals, so brass and copper fittings should stay out of the DEF path. A color change toward blue is a sign of copper contact.
Never put DEF in the diesel tank, and never put diesel in the DEF tank. Either error can cause costly repairs, and the manufacturer's procedure for recovery should be followed. If the concentration drifts, it is generally cleaner to replace the fluid than to adjust it.
Before using a container that has sat in a shop, look it over. A dirty, damaged or bulging container is a reason to set it aside. Wipe the cap area clean before opening, and keep the funnel or nozzle dedicated to DEF.
DEF is sold in several package sizes, from small jugs to larger totes and bulk delivery. The right one depends on how much you use. Low-volume users are well served by sealed jugs, while steady users benefit from totes or bulk, because there is less handling and fewer chances to introduce dirt.
Whatever the package, read the label for the quality standard it states, check that the seal is intact, and note any date code. Keep a dedicated hose, pump and funnel for DEF and never use them for anything else. Open containers should be used promptly, because DEF exposed to air can pick up dust and evaporate water.
Bulk DEF delivery works well for fleets and equipment owners who use it steadily. A properly sized tank with a vent that limits evaporation helps hold concentration. A dedicated pump and hose for DEF only keeps cross-contamination out of the equation.
DEF can be added to a diesel delivery at the same visit. See fleet fueling for how vehicle programs are planned, and job site equipment fueling for machines that work away from the yard.
Ask your supplier for the safety data sheet and keep it with your records.
Only with deionized water, and only to correct concentration. Tap water introduces minerals that can damage the system, so most users avoid adjusting it at all.
Shelf life depends on temperature. A year is a reasonable minimum for typical storage in most of the United States, with shorter life in heat.
DEF is not a fuel. Still, follow the supplier's safety data sheet for handling and clean up spills, since the fluid can leave residue and crystals.
No. Engines equipped with selective catalytic reduction do. Check your owner's manual or the equipment label.
Tell us what you run and where it sits, and we will come back with a delivery plan.