Black smoke from a diesel engine is caused by an unbalanced air-to-fuel ratio. Either not enough oxygen is being added to burn the fuel, or too much fuel is in the mix. Black smoke is one of the most common smoke issues for diesel vehicles.
The black smoke emitted from a diesel vehicle is full of particles, such as carbon and soot, that are not being burned as fuel. If the air-to-fuel ratio was correct, these particles burn normally as fuel. Because a lack of proper air or fuel affects the performance of the vehicle, it is likely to lose significant gas mileage as long as the smoke is a problem.
The most common root causes of a bad air-to-fuel ratio are bad injectors, a bad injector pump, a bad or dirty air filter, a faulty EGR valve, or a bad turbocharger. These are some of the easier fixes for black smoke. Some other more complex engine issues that may cause black smoke include incorrect timing, incorrect valve clearance, bad fuel, carbon buildup in combustion space, bad cylinder compression, carbon buildup in the intake manifolds and other engine tuneup-related problems.
The best way to prevent black smoke is to keep up with maintaining basic engine tuneup components such as filters and worn parts.