ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Medium
A technician is diagnosing a vehicle with a rough idle and a strong sulfur-like odor from the exhaust. After checking for diagnostic trouble codes (DTCs) and finding none, the technician observes live data showing that the short-term fuel trim (STFT) for Bank 1 is consistently around +15% to +20%, while the long-term fuel trim (LTFT) is also significantly positive. Which of the following is the most likely cause of these readings and symptoms?
- AA vacuum leak.
- BA restricted exhaust system.
- CA leaking fuel injector.
- DA faulty oxygen sensor reporting lean.
Show answer & explanationAnswer & explanation
Correct answer: A. A vacuum leak.
A vacuum leak introduces unmetered air into the engine, causing a lean condition. The engine control unit (ECU) compensates by adding more fuel, resulting in positive short-term and long-term fuel trims. The rich condition from over-compensation can lead to a sulfur smell.
Why the other options are wrong
- B. A restricted exhaust system can cause various driveability issues but typically does not directly lead to consistently high positive fuel trims.
- C. A leaking fuel injector would cause a rich condition, leading to negative fuel trims as the ECU tries to reduce fuel.
- D. A faulty oxygen sensor reporting lean would cause the ECU to add fuel, resulting in positive fuel trims, but the sulfur smell is more indicative of a rich condition from the compensation, and a vacuum leak is a primary cause of lean conditions that trigger this response.
Positive Fuel Trim
Positive fuel trim indicates the engine control unit (ECU) is adding fuel to compensate for a lean condition, often caused by unmetered air or insufficient fuel delivery.
- STFT and LTFT measure fuel adjustments.
- Positive trim means ECU is adding fuel.
- Common causes include vacuum leaks, low fuel pressure, or dirty MAF sensor.
Memory trick: Fuel is 'PLUS'ed when there's 'LEAN' air.