Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairHard
A technician is diagnosing a vehicle with a complaint of poor engine performance, particularly hesitation during acceleration, and a stored DTC P0171 (System Too Lean, Bank 1). Using a scan tool, the technician observes that Short Term Fuel Trims (STFT) are consistently positive (+20% to +25%) and Long Term Fuel Trims (LTFT) are also significantly positive (+15% to +20%) at idle and under light load. Which of the following is the MOST likely cause of these readings?
- AA restricted catalytic converter.
- BA clogged fuel injector on bank 1.
- CA vacuum leak after the Mass Air Flow (MAF) sensor.
- DA leaking exhaust manifold gasket on bank 1.
Show answer & explanationAnswer & explanation
Correct answer: C. A vacuum leak after the Mass Air Flow (MAF) sensor.
Significant positive fuel trims (both STFT and LTFT) indicate the PCM is adding fuel to compensate for a lean condition. A vacuum leak after the MAF sensor allows unmeasured air into the engine, causing a lean condition that the PCM tries to correct by increasing fuel delivery.
Why the other options are wrong
- A. A restricted catalytic converter would primarily affect exhaust flow and engine power, but it would not directly cause a lean condition that the PCM attempts to correct with positive fuel trims.
- B. A clogged fuel injector would cause a lean condition in a specific cylinder, not necessarily a lean condition across the entire bank that the PCM can compensate for with overall fuel trim adjustments.
- D. A leaking exhaust manifold gasket before the oxygen sensor could allow ambient air into the exhaust stream, making the PCM think the engine is lean and causing it to add fuel, but a post-MAF vacuum leak is a more direct cause of unmetered air leading to positive fuel trims.
Positive Fuel Trims
Positive fuel trims indicate that the engine control module (ECM) is adding fuel to the engine to compensate for a lean condition (too much air or too little fuel for the combustion process).
- STFT (Short Term Fuel Trim) reacts quickly to changes.
- LTFT (Long Term Fuel Trim) makes slower, more permanent adjustments.
- Significant positive trims (e.g., >+10%) suggest a persistent lean condition.
Memory trick: Lean on the trims, look for unmetered air.