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Why Do You See Black Smoke From Diesel Engines When Accelerating?

Rusty diesel fuel gauge against a green background

Step on the accelerator of your diesel-powered vehicle and a plume of black smoke may emit from the tailpipes. That’s a natural byproduct of diesel power, but it raises valid concerns about pollution and the air we breathe. There’s a reason why you see black smoke from diesel engines when accelerating, but there is also a clean solution consumers and fleet owners should readily employ.

The Science Behind the Smoke

Polluting carBlack smoke represents partially burned fuel. It’s much more apparent in older diesel engines than in modern common-rail diesel engines. This is because older vehicles don’t have computers that regulate emissions output like newer engines do.

Any diesel vehicle emitting copious amounts of black smoke may not have the right fuel-air mixture. This problem can be resolved by maintaining the air cleaner system. Specifically, adjusting the air injector timing or replacing the exhaust gas recirculation valve may solve the problem. However, if both are working fine, then the fuel injectors may be plugged or worn.

Another cause may be carbon buildup, which can be solved by using a fuel treatment with detergents. Choosing diesel fuel with added detergents will help avoid future buildup. Employ these solutions and the amount of black smoke your diesel engine emits should noticeably decrease.

An Environmental Solution

Even if your vehicle’s black smoke problem is under control, it will still produce more emissions than a gasoline engine. This is unfortunate because diesels are more efficient and durable than gas engines.

To counter nitrous oxide emissions — which contribute to acid rain and form ground-level ozone that’s harmful to humans, animals and plants — you can use diesel exhaust fluid (DEF). It’s a blend of approximately two-thirds purified or ionized water and one-third automotive-grade urea, which is a synthetic form of urine. The blend is colorless and odorless, so it’s nontoxic. Here’s how it works:

  1. DEF isn’t added directly to diesel fuel. Instead, add it to a blue-covered tap, which is typically located next to the fuel receptacle or under the hood.
  2. The DEF is sprayed into the diesel exhaust streams, lowering the exhaust’s nitrous oxide content.
  3. The water vaporizes, leaving ammonia molecules, which then move to the catalytic converter.
  4. Once in the converter, the nitrous oxide molecules are converted to nitrogen and water vapor, which are less toxic.
  5. Nitrogen and water vapor are then emitted through the tailpipes.

Modern diesel vehicles typically supply warning notices on the instrument panel that alert owners when diesel exhaust fluid is getting low. Although your vehicle will still run without it, you’ll want to stay in the habit of adding DEF to eliminate black smoke filled with harmful nitrous oxide.

Check out all the diesel exhaust fluids available on NAPA online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on why you see black smoke from diesel engines when accelerating, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Pixabay.


Matthew C. Keegan View All

Matt Keegan has maintained his love for cars ever since his father taught him kicking tires can be one way to uncover a problem with a vehicle’s suspension system. He since moved on to learn a few things about coefficient of drag, G-forces, toe-heel shifting, and how to work the crazy infotainment system in some random weekly driver. Matt is a member of the Washington Automotive Press Association and is a contributor to various print and online media sources.

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