If your vehicle is showing black smoke coming from the exhaust, here are the most likely causes and the parts you'll need to fix each one.
A severely dirty air filter restricts airflow into the engine. This starves the engine of oxygen, causing the air-to-fuel ratio to become too rich (too much fuel, not enough air), which burns as black smoke.
The MAF sensor measures incoming air to calculate fuel delivery. If it is dirty, failing, or was accidentally left unplugged after an air filter change, the engine computer will default to a rich fuel mixture to prevent stalling.
Oxygen sensors monitor the exhaust to adjust the fuel mixture. A sluggish or dead upstream O2 sensor can falsely report a lean condition, prompting the engine computer to overcompensate by dumping excess fuel into the cylinders.
If a fuel injector is worn out or stuck open, it will spray raw, unmetered fuel into the combustion chamber. This extreme rich condition causes heavy black smoke and can wash oil off the cylinder walls.
A failed fuel pressure regulator can allow fuel pressure to spike. This high pressure forces more fuel through the injectors than the computer intends, resulting in a rich mixture and black smoke.
The engine computer adds extra fuel when the engine is cold (like a choke). If the coolant temperature sensor fails and constantly reads freezing temperatures, the engine will stay in this rich 'warm-up' mode indefinitely.