ASE A1 Engine RepairGeneral Engine DiagnosisHard
A vehicle exhibits a strong smell of fuel, black smoke from the exhaust, and significantly reduced fuel economy. A scan tool shows that the long-term fuel trim (LTFT) is consistently negative, around -15% to -20%. Which of the following is the MOST likely cause?
- AAn exhaust leak before the O2 sensor
- BA lean fuel condition
- CA leaking fuel injector
- DA restricted air filter
Show answer & explanationAnswer & explanation
Correct answer: C. A leaking fuel injector
A consistent negative long-term fuel trim indicates the engine control module (ECM) is trying to *reduce* fuel delivery because it detects a rich condition. A leaking fuel injector would continuously add excess fuel, creating this rich condition, leading to black smoke, reduced economy, and a fuel smell.
Why the other options are wrong
- A. An exhaust leak before the O2 sensor would allow outside air in, causing the O2 sensor to read lean, and the ECM to *add* fuel (positive fuel trims).
- B. A lean fuel condition would result in positive fuel trims, as the ECM tries to *add* fuel.
- D. A restricted air filter would cause a rich condition, but typically leads to positive fuel trims as the MAF sensor would read less airflow, and the O2 sensor would detect richness, causing the ECM to *reduce* fuel.
Negative Fuel Trim Interpretation
Negative fuel trim indicates the engine control module (ECM) is reducing fuel delivery because the oxygen sensor detects a rich condition.
- Consistently negative LTFT means the system is removing fuel.
- Caused by excess fuel (leaking injector, high fuel pressure) or misreported air (faulty MAF reading high).
- Symptoms include black smoke, fuel smell, rich exhaust.
Memory trick: Negative trim, rich fuel stream, find the leak or over-scheme.