What is Diesel Exhaust Fluid? Who Needs It?

Here’s a little quiz:

When have emissions rules ever produced better-running trucks?

If you answered, “How about the 2008 Tier 2 or 2010 Clean Air Act standards,” you can stop reading. You probably already know everything in this post.

For the rest of you, the answer is, “When it became clear that diesel exhaust fluid, or DEF, was a key part of the solution to the emissions problem.

Everyone who’s ever driven behind an older bus or semi belching smoke and diesel exhaust knows the problem: Diesel engines are dirty and smelly.

That’s the price we’ve paid for power and torque that gasoline engines can’t equal.

DEF brings that price way down.

Here’s why.

The Diesel Penalty…

Combustion occurs in a gasoline engine’s cylinders when a spark ignites a mixture of fuel and air.

A diesel doesn’t use a spark. Instead, it greatly compresses a leaner fuel-air mix.

It’s the high degree of compression that causes the fuel to ignite.

Unfortunately, the compression-ignition process also produces emissions that are higher in dangerous nitrogen oxides (NO) and particulates.

…And the Solution

To meet air quality standards calling for lower NO levels, manufacturers adopted an approach called “selective catalytic reduction,” or SCR.

With SCR, a small amount of fluid is sprayed into the exhaust stream. The chemical reaction that occurs converts NO emissions into harmless nitrogen and water.

In fact, what comes out of the tailpipe of a diesel using SCR may well be cleaner than what goes into the engine in the first place.

That was helpful in 2010. It’s essential in 2017 when regulations call for a 90% reduction in allowable nitrogen oxides from exhaust.

The magic responsible for the reduction is the DEF: a 32.5% solution of ammonia in ionized water. And it’s a powerful (and somewhat misunderstood) solution to a serious problem.

What You Need to Know

DEF lives in a reservoir in your vehicle, which usually holds about five gallons.

In GM’s heavy duty trucks, customers get about 1,000 miles per gallon of DEF. Heavy hauling or towing can increase the rate of DEF consumption just as it does with fuel.

In consumer vehicles, DEF can be consumed at a rate of roughly 2.5 gallons per 800 miles of driving.

The DEF tank must be kept at least partially full at all times. Many diesels won’t run at more than 4-5 MPH if the tank runs dry. Others won’t run at all.

Fortunately, it shouldn’t be a problem, since most manufacturers provide a gauge, digital readout or “low fluid level” warning on the IP.

And keeping the tank topped off is easy and relatively inexpensive.

Manufacturers have built DEF service into their scheduled maintenance specs. So the fluid can be topped off when owners bring their vehicles in for scheduled service.

And DIY-ers can easily maintain DEF levels themselves, much as they would washer fluid. The fluid is readily available at dealerships, trucks stops and auto parts stores, usually at about $5/gallon.

Truck stops often have DEF pumps on their fuel islands.

The Upside

Maybe the biggest misperception about DEF is that it’s a drag on performance.

In fact, the opposite is the case. Manufacturers have tuned their diesels such that DEF actually optimizes combustion.

That means better fuel efficiency, more power, less engine wear and high reliability.

And there are other myths:

DEF evaporates quickly. (In fact, it evaporates very slowly.)

It’s a drag on mileage. (In fact, it offers better mileage than most other emissions reductions systems. Manufacturers estimate a mileage bump of as much as 5%.)

It’s toxic. (In fact, it’s less toxic than many of the other fluids used in a vehicle. And it’s been used for years in commercial trucking and the ag industry.)

Some Caveats

While DEF is good for newer engines with DEF systems, it’s useless in older diesels that lack such systems.

And DEF should never be added directly to fuel. It’s corrosive and can reduce your fuel system to worthless scrap.

That’s why some manufacturers provide under-the-hood access to the system. Others provide a separate filler neck next to the fuel cap.

Either way, if you suspect your fuel has been contaminated with DEF, don’t even turn the ignition to accessories, which can activate the fuel system.

Instead, have the vehicle towed, without starting, to a repair shop. Have them flush the fuel tank, then get back to enjoying the diesel power, torque, and reliability which DEF enables.