DID YOU KNOW?

Why Diesel Exhaust Fluid (DEF) Crystallizes, and What You Can Do About It

"Did You Know?"
Diesel Exhaust Fluid becAme widely used in 2010 

Did you know there was a time when thick black smoke trailing from diesel trucks was just part of everyday highway traffic? That all started to change in 2010, when new emissions standards took effect.

Beginning with most 2010 model‑year heavy‑duty diesel engines, the Environmental Protection Agency (EPA) required the use of Diesel Exhaust Fluid (DEF) to dramatically cut harmful emissions. By 2013, nearly all new diesel engines were equipped with DEF systems. As a result, DEF quickly became a normal part of daily operations for trucking fleets, construction companies, farms, municipalities, and many other diesel users.

What exactly is DEF?

DEF is a non‑toxic mixture of 32.5% high‑purity urea and 67.5% deionized water. It’s used in diesel engines with Selective Catalytic Reduction (SCR) systems to reduce nitrogen oxide (NOx) emissions, by as much as 90%, by converting them into harmless nitrogen and water vapor.

In short, DEF helps modern diesel engines run cleaner and meet emissions regulations.

The most common DEF issue: crystallization

The biggest challenge with DEF isn’t toxicity or handling, it’s crystallization.

Crystallization occurs when DEF loses water, leaving behind urea that forms white, solid crystals. These deposits can:

• Clog injectors, lines, and filters
• Cause engine derates or reduced power
• Lead to unexpected shutdowns
• Result in costly repairs if the engine’s DEF system is affected

Crystallization doesn’t only happen inside engines. It can also clog DEF transfer pumps, nozzles, and valves.

Why does DEF crystallize?

Crystallization is most often caused by:

• Evaporation of water (DEF is 67.5% water)
• High heat (above 86°F)
• Freezing temperatures (below 12°F)
• Contamination from fuel, dirt, or improper containers
• Long periods of inactivity allowing DEF residue to dry

Even a small amount of dried DEF left in a pump or nozzle can start the process.

The good news: it’s easy to clean

While crystallization inside an engine can be expensive to fix, crystallization in DEF transfer equipment is much easier, and far less costly to address.

The safest and most effective way to clean DEF crystals from pumps and accessories is with deionized (DI) water. Since DEF is already mostly DI water, rinsing with it won’t introduce harmful impurities.

✔ Deionized water – best option
✔ Distilled water – acceptable alternative

These are ideal for cleaning:

• Pump heads
• Nozzles
• Valves
• DEF fittings and accessories

Did You Know?

A small amount of routine maintenance on your DEF transfer equipment can help protect much more expensive engine components.

• Give the DEF pump, hose and nozzle a good rinsing with warm deionized water or distilled water before long periods of inactivity.
• Give the DEF pump, hose and nozzle a good rinsing with warm deionized water or distilled water before expected times of extreme temperate.
• Keep DEF pumps sealed when not in use.
• Store your DEF transfer pumps in a clean, dry, and shaded environment, ideally between 12⁰F and 86⁰F (-11⁰C to 30⁰C) to prevent contamination and urea crystallization.

For detailed, step‑by‑step cleaning instructions, see our companion guide: