Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a complaint of poor engine performance, especially under load, and a noticeable 'whooshing' or 'hissing' sound from the engine compartment. The scan tool shows positive fuel trims (STFT and LTFT) on both banks, and a P0171 (System Too Lean Bank 1) code is present. A visual inspection reveals no obvious leaks. Which of the following is the MOST likely cause?
- AA faulty mass air flow (MAF) sensor reporting high airflow.
- BA clogged fuel filter.
- CA restricted exhaust system.
- DA vacuum leak in the intake system.
Show answer & explanationAnswer & explanation
Correct answer: D. A vacuum leak in the intake system.
Positive fuel trims and lean codes (like P0171) indicate the engine control module (ECM) is adding fuel because it detects a lean condition. A 'whooshing' or 'hissing' sound, especially under load, along with lean codes, strongly points to a vacuum leak in the intake system, allowing unmetered air into the engine.
Why the other options are wrong
- A. A MAF sensor reporting high airflow would cause the ECM to command more fuel, leading to rich conditions and negative fuel trims, the opposite of the symptoms described.
- B. A clogged fuel filter would cause low fuel pressure, leading to lean conditions and positive fuel trims, but it wouldn't produce a 'whooshing' sound from the engine compartment.
- C. A restricted exhaust would typically cause high back pressure, reduced power, and potentially rich conditions or misfires, not lean codes and a hissing sound.
Vacuum Leak Symptoms
Unmetered air entering the engine's intake system, causing a lean air-fuel mixture and specific diagnostic indicators.
- Leads to positive fuel trims (ECM adds fuel).
- Often causes lean DTCs (e.g., P0171, P0174).
- Can produce an audible 'hissing' or 'whooshing' sound.
- Affects engine idle quality and performance.
Memory trick: Lean and hissy, vacuum's amiss-y!