What should be checked first when black exhaust smoke is observed?

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

Answer: (C)

Black exhaust smoke points to a rich air–fuel mixture, meaning there isn’t enough air for the amount of fuel being burned. The quickest, most effective first check is the air intake path, starting with the air filter. A restricted air filter reduces airflow into the engine, so the mixture becomes richer and combustion is incomplete, producing dark, sooty smoke. Cleaning or replacing the air filter often resolves the issue. If the air filter is clear and smoke still appears, then you’d examine fuel delivery and injector condition for over-fueling, but the air filter remains the best initial check. The cooling system doesn’t directly cause black smoke.