When a diesel truck blows black smoke it is called Rolling coal or rollin coal which is the practice of modifying a diesel engine to deliberately emit large amounts of grey or black diesel exhaust smoke which contains soot and incompletely combusted diesel.
The rolling coal or black smoke from the diesel exhaust when done purposely is most often used as a form of anti environmentalism protest.
However normally a diesel exhaust on a truck or any diesel engine should not smoke and if it does it's an indicator of something wrong.
A bad diesel injector can cause black smoke.
The diesel fuel injectors are responsible for delivering the diesel fuel to the combustion chamber.
If the diesel fuel injectors are not working properly, the fuel will not be delivered correctly and will not burn completely.
Black smoke in a diesel engine indicates unburnt diesel fuel that the engine is not burning properly.
The most common cause of the black smoke in a diesel engine is a result of an imbalance of the air to fuel ratio.
The imbalance of the air to fuel ration can be caused by dirty air filters, clogged fuel injections or other faulty engine components.
If you're getting a small amount of black smoke on the diesel engine exhaust it could simply be dirty injectors or a faulty injector or air or fuel filter.
Sometimes adding a detergent additive to your diesel fuel regularly.
A multifunctional treatment like Dee-Zol will clean out the deposits, reduce the amount of fuel burned incompletely, and can even extend the life of your DPF (because less soot is being produced at any one time).
In more serious cases and when you get a lot of black smoke coming from the diesel exhaust it could be a bad diesel fuel injector that could be sticking or need replaced.
The diesel fuel injectors are responsible for delivering the diesel fuel to the combustion chamber.
If the diesel fuel injectors are not working properly, the fuel will not be delivered correctly and will not burn completely.
As a result, black smoke will come out through your exhaust.
Diesel engines should not smoke.
Black or white smoke from your diesel exhaust may indicate an engine issue.
Incomplete combustion occurs and soot is formed when there is an overabundance of fuel (both diesel fuel and lube oil), insufficient residence time in the combustion zone, and/or non-availability of sufficient oxidants.
Over-fueling is the primary cause of black smoke from the exhaust of a heavy duty diesel engine.