Why Do Diesel Vehicles Emit Black Smoke?

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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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Jason

31 years of experience
148 reviews
Jason
31 years of experience
Land Rover Range Rover Sport - Brake Pads Replacement (Front, Rear) - Surprise, Arizona
Needed new brake pads on my Land Rover-and booked through “your mechanic”- this was my second time using this service and I have to say- these guys are awesome- Today, Jason came out to my work place- and completed the work quickly. Thank you!!
Honda Accord - Brake Pads Replacement (Rear) - Phoenix, Arizona
Jason was on time, self motivated, well educated in his craft, and very helpful. I am fully sold on your method of using the home service. Jason helped by creating a safer ride for my wife, saving me time, and saving money for my family. I will be a client forever. Dennis Laycock

Lee

18 years of experience
105 reviews
Lee
18 years of experience
Toyota Camry - Brake Pads Replacement (Rear) - Richmond, Virginia
Lee was professional and knowledgeable. The job did not take long he was a little late but like I said professional because he let know that he was running late. LEE IS AWESOME!!!!!!

Willie

23 years of experience
23 reviews
Willie
23 years of experience
Jeep Compass - Brake Pads Replacement (Front, Rear) - North Hollywood, California
I made an appointment to have my brake rotors/disc replaced. On the day I scheduled my car services- it ended up raining. Willie checked in, moved my appointment, and continued to communicate through out the day until he was able to come work on my car. He took great car to make the repairs ANNNDDD SAVED ME MONEY by only doing what was necessary (only one of the rotors needed replacing). A blessing.

Honorio

18 years of experience
782 reviews
Honorio
18 years of experience
Saturn Vue - Brake Pads Replacement (Front) - Sunnyvale, California
This was our second appointment to service our Honda and Saturn with Honorio and we will continue to do so in the future. We highly recommend Honorio.

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