Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairHard
A technician is diagnosing a vehicle with a rough idle, poor fuel economy, and a 'Check Engine' light illuminated. DTC P0171 (System Too Lean Bank 1) is stored. Live data shows a positive fuel trim of +20% at idle and +15% at highway speeds. A vacuum gauge connected to the engine shows a steady reading of 15 inHg at idle. What is the most likely cause of this condition?
- AA clogged fuel injector
- BA dirty Mass Air Flow (MAF) sensor
- CAn exhaust restriction
- DA leaking intake manifold gasket
Show answer & explanationAnswer & explanation
Correct answer: D. A leaking intake manifold gasket
DTC P0171 indicates a lean condition. Positive fuel trims (+20% at idle, +15% at highway) confirm the ECM is adding fuel. A vacuum leak (like a leaking intake manifold gasket) has a greater impact at idle than at higher RPMs because the engine vacuum is highest at idle, pulling more unmetered air. This explains why the fuel trim is higher at idle. A steady 15 inHg vacuum is slightly lower than ideal (typically 17-21 inHg), which could be indicative of a vacuum leak.
Why the other options are wrong
- A. A clogged fuel injector would cause a lean condition, but typically would not have a varying effect between idle and highway speeds in this manner, and might cause a misfire.
- B. A dirty MAF sensor could cause a lean condition if it under-reports airflow, but it would typically affect fuel trims more consistently across the RPM range, or even more severely at higher airflow (highway speeds), not predominantly at idle.
- C. An exhaust restriction would typically cause a loss of power, poor acceleration, and possibly high engine vacuum at idle, but generally leads to rich conditions and high back pressure, not a lean condition with positive fuel trims.
Vacuum Leak Diagnosis
Identifying leaks in the engine's intake system that allow unmetered air to enter, causing a lean condition.
- Causes positive fuel trims (ECM adds fuel).
- Impact is typically greater at idle due to higher engine vacuum.
- Can cause rough idle, misfires, and P0171/P0174 codes.
Memory trick: Lean at idle? Think vacuum leak, it's sucking air!