When diagnosing black exhaust smoke from a boosted engine, which component should be checked first?

• A. Cylinder compression
• B. Injector condition
• C. Air filter restriction
• D. Cooling system condition

Answer: (C)

Black exhaust smoke from a boosted engine usually means the air–fuel mixture is too rich. In a turbocharged setup, the engine relies on a clean, adequate supply of air to match the fuel being injected. If the intake path is restricted, starting with a clogged air filter, the engine can’t draw enough air even though the turbo is delivering boost. That shortfall in air relative to the commanded fuel results in a rich mixture, which the exhaust manifests as black smoke. Checking the air filter restriction first is the quickest, simplest way to rule out a fundamental air flow issue before moving on to fuel delivery or engine internals.

If the air filter isn’t restricting flow, other potential causes include how the fuel is delivered or issues inside the engine, but they’re secondary steps. Cylinder compression problems tend to produce symptoms like power loss or misfires rather than just black smoke. Injectors can cause overfueling and smoke if they’re malfunctioning, but that should be investigated after confirming airflow isn’t restricted. The cooling system condition isn’t linked to the air–fuel balance and isn’t a driver of black smoke.