A technician is diagnosing an engine with a severe lack of power and black smoke from the exhaust under acceleration. The engine also has a strong raw fuel smell. A scan tool shows consistently high positive fuel trim values (+20% on both short and long term). Which of the following is the MOST likely cause?
- AVacuum leak in the intake manifold
- BLeaking fuel injectors
- CFaulty Mass Air Flow (MAF) sensor reporting low airflow
- DClogged exhaust system
Show answer & explanationAnswer & explanation
Correct answer: C. Faulty Mass Air Flow (MAF) sensor reporting low airflow
High positive fuel trims indicate the PCM is adding a lot of fuel because it perceives a lean condition. If the MAF sensor is under-reporting airflow, the PCM injects less fuel than needed, making the engine run lean. To compensate, the PCM increases fuel trim. However, the black smoke and raw fuel smell suggest the engine is actually running rich. This contradiction (PCM thinks lean, but engine is rich) is a classic sign of an under-reporting MAF sensor. The PCM adds fuel based on the MAF's incorrect low reading, but the actual air entering is higher, leading to an over-rich condition and black smoke.
Why the other options are wrong
- A. A vacuum leak introduces unmetered air, causing a lean condition. The PCM would add fuel to compensate, resulting in high positive fuel trims. However, a lean condition would generally NOT produce black smoke or a strong raw fuel smell.
- B. Leaking fuel injectors would cause an actual rich condition (black smoke, raw fuel smell) and the PCM would try to compensate by reducing fuel, leading to negative fuel trims, not positive.
- D. A clogged exhaust system causes lack of power, but typically not black smoke (unless it leads to extreme rich conditions) or high positive fuel trims. It's more about restriction than air-fuel mixture.
Mass Air Flow (MAF) Sensor Malfunction
A MAF sensor malfunction involves incorrect measurement of air entering the engine, leading to inaccurate fuel delivery, performance issues, and abnormal exhaust emissions.
- If MAF under-reports airflow, PCM commands less fuel (lean), then adds fuel (positive trims), but engine becomes actually rich.
- If MAF over-reports airflow, PCM commands more fuel (rich), then subtracts fuel (negative trims), but engine becomes actually lean.
- Can cause various driveability issues, including lack of power, stalling, and poor fuel economy.
Memory trick: Black smoke, fuel smell, high positive trim? MAF's lying, it would seem.