Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a persistent P0171 (System Too Lean Bank 1) diagnostic trouble code (DTC). The fuel trims at idle are +20% short-term and +15% long-term. At 2500 RPM under load, the fuel trims drop to +5% short-term and +3% long-term. Which of the following is the MOST likely cause of this condition?
- AA clogged catalytic converter.
- BA vacuum leak.
- CA restricted fuel filter.
- DA faulty fuel pressure regulator.
Show answer & explanationAnswer & explanation
Correct answer: B. A vacuum leak.
A vacuum leak allows unmetered air into the engine, causing a lean condition that is most pronounced at idle when manifold vacuum is highest. As engine speed and load increase, the effect of the vacuum leak becomes less significant relative to the total airflow, causing fuel trims to decrease.
Why the other options are wrong
- A. A clogged catalytic converter typically causes back pressure issues, affecting engine performance and potentially causing DTCs like P0420/P0430, but not directly leading to fuel trim patterns like this.
- C. A restricted fuel filter would cause a lean condition that worsens with increased fuel demand (higher RPM/load), not one that improves.
- D. A faulty fuel pressure regulator, depending on its failure mode, could cause a rich or lean condition, but a consistent lean condition worsening at idle and improving at higher RPMs is less typical for this component than a vacuum leak.
Vacuum Leak Diagnosis
A vacuum leak introduces unmetered air into the engine, causing a lean condition that is more pronounced at idle due to higher manifold vacuum. Fuel trims will be significantly positive at idle and decrease at higher RPMs under load.
- Causes positive fuel trims (lean condition).
- Effect is more pronounced at idle.
- Fuel trims decrease as RPM/load increases.
Memory trick: Lean on the vacuum, not the fuel pump.