Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a 'Check Engine' light illuminated and a Diagnostic Trouble Code (DTC) P0171 (System Too Lean, Bank 1). The fuel trims are consistently positive at idle and under load. Which of the following is the MOST likely cause?
- AA faulty fuel pressure regulator causing excessively high fuel pressure.
- BA clogged fuel filter restricting fuel flow.
- CA contaminated Mass Airflow (MAF) sensor reporting higher than actual airflow.
- DA vacuum leak in the intake manifold.
Show answer & explanationAnswer & explanation
Correct answer: D. A vacuum leak in the intake manifold.
A vacuum leak introduces unmetered air into the engine, leading to a lean condition. The engine's computer attempts to compensate by adding more fuel, resulting in positive fuel trims.
Why the other options are wrong
- A. Excessively high fuel pressure would cause a rich condition, leading to negative fuel trims, not positive.
- B. A clogged fuel filter would typically cause a lean condition, but often presents with low fuel pressure and more pronounced issues under load, not necessarily consistently positive trims at idle.
- C. A contaminated MAF sensor reporting higher than actual airflow would cause the PCM to inject too much fuel, leading to a rich condition and negative fuel trims.
Lean Condition (P0171)
A lean condition occurs when there is too much air or not enough fuel in the air-fuel mixture, often indicated by positive fuel trims.
- DTC P0171 or P0174 indicates system too lean.
- Positive fuel trims mean the PCM is adding fuel.
- Common causes include vacuum leaks, low fuel pressure, or faulty MAF sensor.
Memory trick: Lean means 'less fuel', so the computer adds more. Positive trims are the clue!