The Ugly Truth About Rolling Coal : Black Smoke

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Zach is one of the founders of 8020 Media and a lead writer for DieselIQ. He’s been in the automotive industry for over a decade and has published more than 400 articles for DieselIQ, TuningPro, BMWTuning, & more. His blend of automotive knowledge, writing & research skills, and passion make him an excellent resource for fellow diesel owners. His expertise goes beyond writing and includes a deep knowledge of Cummins and Powerstroke engines, as well as nearly 10 years of DIY experience. Zach is also experienced with tuning and has a wealth of technical knowledge that he brings to every article he writes.

Often times we get questions about why someone’s well running diesel truck isn’t “rollin’ coal” or making black smoke. The simple answer is, the fact that you are not seeing any black smoke is a good thing. A properly running diesel engine should not have any smoke at all. Black smoke is an indication of restricted air among other things.

Newer common-rail trucks can make more than 1,000 horse power while being nearly smoke-free.

It can be entertaining to see black smoke coming from the diesel trucks at a diesel sled pulling competition, but on the street, it is unnecessary and usually indicative of restricted air, over fueling or a turbocharger that can’t keep up with the demand for air.

When you see black smoke coming from a diesel sled pulling trucks it is because sled pullers use lots of fuel, and lots of air. The turbos they use are so huge that is takes a long time to get them to spool up. You need a lot of fuel to spool up a large turbo. So, at the line, these trucks have to “roll coal” before the green light for quite some time in order to get the boost to acceptable levels before the light turns green. When a lot of fuel is added at low RPM before boost is built, the engine can’t burn all the fuel, so you get smoke. 

A properly running diesel engine in good condition should produce no visible smoke from the exhaust, under most operating conditions. A short puff of smoke when an engine is accelerated under load may be acceptable, due to the lag before the turbocharger speed and air flow is able to match the volume of diesel injected into the cylinders. That would only apply to older technology diesel engines, but with modern type diesels, no smoke at all should be evident.

Black smoke is commonly emitted from diesel engines. It indicates poor and incomplete combustion of the diesel fuel  – too much fuel or not enough air.. 

The black smoke is full of particulates that are basically large diesel particles that normally would be burned as fuel. Any way you look at it, a diesel truck on the street emitting black smoke is not going to be getting optimal performance or fuel mileage.

Black smoke can occur across the entire operating range, but is usually the worst under full power, or during the lag before the turbocharger boosts air supply to match the fuel usage such as in the early stages of acceleration and during gear changes. Moderate turbo lag smoke is acceptable; otherwise black smoke should be hardly visible in a correctly running engine.

The causes for black smoke emitted from diesel engines can include the following:     

  • Incorrect fuel injection timing 
  • Dirty or worn fuel injectors 
  • Over fueling 
  • Faulty turbocharger, or turbo lag
  • Faulty or dirty exhaust gas recycling (EGR) system
  • Incorrect valve clearance 
  • Incorrect fuel to air ratio 
  • Dirty or restricted air cleaner systems 
  • Over loading the engine 
  • Poor fuel quality 
  • Cool operating temperatures 
  • High altitude operation 
  • Excessive carbon build-up in combustion and exhaust spaces 

Some of these issues are simple and inexpensive to fix. 

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