Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A vehicle is brought to a shop with a complaint of reduced engine performance and a strong smell of raw fuel from the exhaust. A scan tool reveals a P0172 (System Too Rich, Bank 1) DTC and consistently negative fuel trims (-25% on both STFT and LTFT). Which of the following components is MOST likely to be the cause?
- AA leaking fuel injector on Bank 1.
- BAn exhaust leak before the upstream oxygen sensor.
- CA stuck open EVAP purge valve.
- DA faulty Mass Air Flow (MAF) sensor reporting low airflow.
Show answer & explanationAnswer & explanation
Correct answer: A. A leaking fuel injector on Bank 1.
A leaking fuel injector on Bank 1 would continuously add excess fuel to that bank, even when commanded off or at minimum pulse width. This directly causes a rich condition (raw fuel smell, P0172), and the ECM would attempt to compensate by drastically reducing fuel delivery (negative fuel trims). The -25% trims indicate a severe rich condition that the ECM is struggling to correct.
Why the other options are wrong
- B. An exhaust leak before the upstream O2 sensor would introduce ambient air, causing a false lean reading and positive fuel trims, not a rich condition.
- C. A stuck open EVAP purge valve would introduce unmetered fuel vapor, causing a rich condition and negative fuel trims, but the 'strong smell of raw fuel from the exhaust' points more strongly to liquid fuel from an injector.
- D. A MAF sensor reporting low airflow would cause the ECM to *under-fuel* the engine, leading to a lean condition (positive fuel trims), not rich.
Leaking Fuel Injector
A leaking fuel injector allows fuel to drip into the cylinder even when closed, causing a constant rich condition, misfires, raw fuel smell, and extreme negative fuel trims.
- Fuel continuously enters the cylinder.
- Causes rich combustion and high emissions.
- Can lead to engine damage (e.g., washed cylinder walls).
Memory trick: If the engine is drowning in fuel, an injector is likely stuck open.