If your vehicle is showing black smoke coming from the exhaust tailpipe, here are the most likely causes and the parts you'll need to fix each one.
A severely dirty air filter restricts the amount of air entering the engine. This creates a 'rich' condition where there is too much fuel and not enough air, resulting in unburned fuel exiting as black smoke.
The MAF sensor measures incoming air to calculate fuel delivery. If it is dirty or failing, it can overestimate airflow, causing the engine computer to inject excessive fuel. This is sometimes caused by over-oiling aftermarket reusable air filters.
Oxygen sensors monitor the exhaust to adjust the air/fuel mixture. If a sensor fails and falsely reads a 'lean' (too much air) condition, the engine computer will compensate by dumping excess fuel into the cylinders.
A fuel injector that is stuck open or leaking will spray raw fuel into the cylinder constantly, regardless of what the engine computer commands. This leads to a severely rich mixture, black smoke, and potential engine damage from oil dilution.
If the fuel pressure regulator fails, it can allow too much fuel pressure to reach the injectors. The higher pressure forces excess fuel into the engine during normal injector pulses.
If this sensor fails and tells the computer the engine is always freezing cold, the system will stay in 'warm-up' mode. This commands a continuously rich fuel mixture, similar to leaving the choke on in an older vehicle.
On older, non-fuel-injected vehicles, a stuck choke, heavy float, or improperly adjusted carburetor jets will dump excessive fuel into the intake manifold.