If your vehicle is showing thick black smoke blowing from the tailpipe, here are the most likely causes and the parts you'll need to fix each one.
A heavily blocked air filter restricts airflow into the engine, creating a 'rich' condition where there is too much fuel and not enough air being burned.
The MAF sensor measures incoming air. If it fails or is accidentally left unplugged after checking the air filter, the engine computer defaults to a safe, overly rich fuel mixture.
A failing O2 sensor can falsely report to the engine computer that the exhaust is too lean (not enough fuel), causing the system to overcompensate by dumping excessive fuel into the cylinders.
A fuel injector that is stuck open or leaking will spray raw, unmetered fuel directly into the engine cylinders, resulting in heavy black smoke and poor running conditions.
If this sensor fails and reads extremely cold, the engine computer thinks the car is warming up in freezing weather and continuously adds extra fuel to keep it running.
If the regulator fails, it can send fuel to the injectors at a much higher pressure than needed, forcing too much fuel into the engine and causing a rich mixture.