A vehicle has a persistent 'Check Engine Light' with DTC P0171 (System Too Lean, Bank 1). The technician observes that the short-term fuel trim (STFT) is +25% and long-term fuel trim (LTFT) is +20% at idle, but both drop significantly towards 0% at higher RPMs/load. What is the MOST likely cause?
- AA faulty Mass Air Flow (MAF) sensor reporting high airflow
- BA clogged fuel injector on Bank 1
- CA vacuum leak in the intake manifold
- DA restricted exhaust system
Show answer & explanationAnswer & explanation
Correct answer: C. A vacuum leak in the intake manifold
Positive fuel trims (+25% STFT, +20% LTFT) indicate the PCM is adding fuel to compensate for a lean condition. The key clue is that this lean condition is prominent at idle but improves at higher RPMs/load. This behavior is characteristic of a vacuum leak. At idle, a vacuum leak introduces a significant amount of unmetered air relative to the total airflow, causing a lean condition. At higher RPMs, the engine draws much more air through the throttle body, making the unmetered air from the leak a smaller percentage of the total, thus reducing its impact on fuel trims.
Why the other options are wrong
- A. A faulty MAF sensor reporting high airflow would cause the PCM to inject too much fuel, resulting in a rich condition (negative fuel trims), not a lean condition (positive fuel trims).
- B. A clogged fuel injector would cause a lean condition, but it would typically worsen at higher RPMs/load when more fuel is demanded, not improve.
- D. A restricted exhaust system would typically cause poor engine performance across all RPMs and could lead to rich conditions or misfires, but not specifically a lean condition that improves at higher RPM.
Fuel Trim Diagnosis (Lean at Idle)
High positive fuel trims (STFT/LTFT) at idle that decrease at higher RPMs often indicate a vacuum leak.
- Positive fuel trims mean PCM is adding fuel (lean condition).
- Vacuum leaks introduce unmetered air, most impactful at idle.
- Diagnosis often involves smoke tests or listening for leaks.
Memory trick: When the engine's 'LEAN' and the 'TRIMS' are high, check for 'VACUUM' leaks or 'FUEL' supply nigh.