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

P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
P2159 OBD-II Trouble Code: Vehicle Speed Sensor B Range/Performance
Diagnostic Trouble Code (DTC): P2159 P2159 code definition Vehicle Speed Sensor B Range/Performance Related Trouble Codes: P2158: Vehicle Speed Sensor B P2160: Vehicle Speed Sensor B Circuit Low P2161:...
Auto Safety Tips
Driving is more than a way to get from point A to point B. Owning and driving a car can also be a highly enjoyable experience. Whether a person is driving...


Related questions

Q: Smoke coming out of exhaust

Hi, the blue tint you mentioned is in fact from oil burning. Over time the internal engine components, specifically the piston o-rings and the valves, wear out and oil starts seeping into the cylinders and burning with the air and...

Q: Car is loud when accelerating

Hello. Based on your description, it does sound like this is an exhaust leak. Because exhaust pipes are made of metal, moisture will cause them to rust. Although it seems like this problem happened out of nowhere, it has actually...

Q: Engine stalls and serving tick over 2.5 diesel

Hi there. If the crank sensor is used only for misfire detection and NOT spark timing, the vehicle should start and run with the engine light on. Usually the crank sensor or the reluctor ring may have been damaged and...