Skip the auto shop - Our mechanics make house calls

Why Do Diesel Vehicles Emit Black Smoke?

diesel 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.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

More related articles

How to Find the Keyless Code on a Ford Explorer or Mercury Mountaineer
Many Ford Explorers and Mercury Mountaineers were manufactured with an option known as the Ford keyless entry keypad. Some models refer to it as SecuriCode as well. It...
How to Buy Fuel Treatments
Adding a fuel additive to your gas tank when you fill up is one way to clean vital engine parts of deposits, improve the performance of your engine,...
How to Get a Louisiana Driver's Permit
s licensing program. The first step in this program is to obtain...


Related questions

Q: Car won't shift,smoke coming from underneath,leaking transmission fluid.

You will need to have the transmission leak pinpointed to isolate the source of the issue. If one of the lines are leaking then it can be repaired but if the front pump seal is leaking then the transmission would...

Q: Manifold overheat!! Its (KILLING ME!)

Hello. Thanks for writing in. The vehicle's malfunction with the exhaust system has likely caused the catalytic converter to fail. Unless there is a foreign object in the tail pipe clogging the airflow, it is the catalytic converter which is...

Q: It is a 320d and it doesn't start by itself in the morning unless I tow it.

Check the battery in the car. The battery may not have the amount of charge that is needed to start the diesel engine. A diesel engine requires more battery power to start it as the compression is higher than a...