If your vehicle is showing black soot or dark smoke coming from the tailpipe, here are the most likely causes and the parts you'll need to fix each one.
A severely restricted air filter starves the engine of oxygen, causing it to run 'rich' with too much fuel. This unburned fuel exits the tailpipe as black soot.
The O2 sensor monitors exhaust gases to adjust the air-fuel mixture. If it fails and falsely reads a 'lean' condition, the engine computer will dump excess fuel into the cylinders, creating black smoke.
The MAF sensor measures incoming air. If it is dirty or failing, it can overestimate the amount of air entering the engine, causing the computer to inject too much fuel.
A fuel injector that is stuck open or leaking will drip excess fuel into the combustion chamber, leading to a rich mixture and black smoke. This is often most noticeable right at startup.
If an intercooler pipe pops off or leaks, pressurized air escapes before reaching the engine, but the computer still injects fuel for that lost air. This is a very common installation error after recent engine work.
If this sensor fails and tells the computer the engine is always freezing cold, the system will stay in a 'warm-up' mode, continuously injecting a rich, smoky fuel mixture.
A bad regulator can allow excessive fuel pressure to reach the injectors, forcing more fuel than necessary into the engine and causing a severe rich condition.