A technician is diagnosing a vehicle with a complaint of poor acceleration and black smoke from the exhaust, especially under load. The scan tool shows a Mass Air Flow (MAF) sensor reading of 5 grams/second (g/s) at idle and 50 g/s at wide-open throttle (WOT). The engine specification for this 3.0L V6 engine typically calls for approximately 3-5 g/s at idle and around 120-150 g/s at WOT. Which of the following is the most likely cause?
- AA faulty Engine Coolant Temperature (ECT) sensor.
- BA restricted intake air filter.
- CA leaking fuel injector.
- DA faulty oxygen (O2) sensor.
Show answer & explanationAnswer & explanation
Correct answer: B. A restricted intake air filter.
The MAF sensor reading at idle (5 g/s) is within the normal range, but the WOT reading (50 g/s) is significantly lower than the specified 120-150 g/s. This indicates a severe restriction in the engine's ability to draw in air, primarily noticeable under high load. A restricted intake air filter would severely limit the amount of air entering the engine, leading to a low MAF reading at WOT, causing a rich condition (black smoke) as the ECU still tries to inject fuel based on a higher anticipated airflow for the throttle position.
Why the other options are wrong
- A. A faulty ECT sensor could cause a rich condition if it reports a constantly cold engine, but it wouldn't directly cause a low MAF reading at WOT.
- C. A leaking fuel injector would cause a rich condition and potentially black smoke, but it wouldn't cause a low MAF reading. The MAF sensor measures air, not fuel.
- D. A faulty O2 sensor could cause a rich condition, but it wouldn't directly cause a low MAF reading at WOT. The MAF reading itself points to an airflow issue.
Low WOT MAF Diagnosis
A significantly lower-than-specified Mass Air Flow (MAF) sensor reading at wide-open throttle (WOT), especially when idle readings are normal, indicates an intake air restriction. This causes a rich condition (black smoke) as the engine receives less air than the ECU expects for the commanded throttle.
- WOT MAF should be high, proportional to engine size.
- Low WOT MAF suggests intake air restriction.
- Causes rich condition (black smoke) due to air/fuel imbalance.
Memory trick: Low 'WOT MAF' means 'No Air' getting 'In', causing 'Black Smoke'.