Why Do Diesel Vehicles Emit Black Smoke? | YourMechanic Advice

Why Do Diesel Vehicles Emit Black Smoke?

There’s a common misconception among drivers of gasoline engine cars that diesel engines are “dirty,” and that black smoke is emitted from all of them. That’s actually not the case. Take a look at any well-maintained diesel car, and you won’t notice any black smoke from the tailpipe. It’s actually a symptom of poor maintenance and failing components, not a symptom of burning diesel on its own.

What is the smoke?

Black smoke from a diesel engine is actually unburned diesel. If the engine and other components were properly maintained, that material would actually be burned in the engine. So, you can immediately tell that any diesel engine spewing black smoke is not getting the fuel mileage it should.

What causes it?

The primary cause of black smoke from a diesel is an incorrect air to fuel ratio. Either there is too much fuel being injected into the engine, or there is too little air. Either way, the result is the same. Notably, some drivers actually pay to have their vehicles modified to do this. It’s called “rolling coal,” and you’ll see it primarily with diesel pickup trucks (it’s costly and wasteful, as well).

Another cause of this problem is poor injector maintenance, but there are several others, as well. These include the following:

  • Blocked or clogged air filter or air intake
  • Contaminated fuel (grit or wax for instance)
  • Worn camshafts
  • Incorrect tappet adjustment
  • Incorrect backpressure in the vehicle’s exhaust
  • Dirty/clogged fuel filter
  • Damaged fuel pump

Finally, you may notice black smoke from a diesel engine because the driver is “lugging” it. Essentially, this refers to staying in a high gear for too long. You’ll notice this most with big rigs on the interstate, but you can also see it with other diesel engines to an extent.


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Bryant

11 years of experience
407 reviews
Bryant
11 years of experience
Ford Fusion - Brake Pads Replacement (Front, Rear) - Indianapolis, Indiana
Bryant has done great work on my car before, the latest work is no exception. He's very flexible, and goes the extra mile to get you back on the road.
Scion iA - Brake Pads Replacement (Front) - Carmel, Indiana
This is my first time using this service. I am very happy with it. Bryant was very knowledgeable and did quality work. I will use this service more regularly.

Augustine

11 years of experience
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Augustine
11 years of experience
Volkswagen Passat - Brake Pads Replacement (Front, Rear) - Albuquerque, New Mexico
He’s amazing! Did a wonderful job on my brakes and was extremely knowledgeable, and a real fun guy to be around as well!

Jimmi

25 years of experience
68 reviews
Jimmi
25 years of experience
Mazda CX-9 - Brake Pads Replacement (Rear) - Orlando, Florida
Jimmi was wonderful. He was on time, knowledgeable, and completed work in a timely manner. He went above and beyond with helpful tips and recommendations. We will be using him for all our family vehicles.

Robert

3 years of experience
4 reviews
Robert
3 years of experience
Toyota RAV4 - Brake Pads Replacement (Rear) - Tacoma, Washington
Came right on time! Had all the necessary tools to finish the job. After the test drive, he gave me an update on my car. Topped off my fluids and disinfect the inside. Well recommend!!

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