[Xist]

Black Smoke - this is the most common one and is really just an imbalance in the air to fuel ratio - too much fuel to not enough air. This means either too much fuel is being added to the mix or there's not enough oxygen being supplied to burn the fuel. 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 emitting black smoke is not going to be getting the optimal fuel mileage it should be getting.

Most common causes of black smoke are faulty injectors, a faulty injector pump, a bad air filter (causing not enough oxygen to be supplied), a bad EGR valve (causing the valves to clog) or even a bad turbocharger. Some of these are easy fixes.

https://www.bellperformance.com/blog...moke-explained

That sounds like they are not being maintained properly.