ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Hard
A vehicle exhibits an intermittent misfire and hesitation under acceleration. The technician uses a scan tool and notices that the short-term fuel trim (STFT) is consistently negative (-15% or more) for Bank 1. What does this indicate?
- AThe engine is running rich and the ECM is removing fuel.
- BThere is an exhaust leak before the upstream oxygen sensor.
- CThe engine is running lean and the ECM is adding fuel.
- DThe oxygen sensor for Bank 1 is faulty and biased lean.
Show answer & explanationAnswer & explanation
Correct answer: A. The engine is running rich and the ECM is removing fuel.
A consistently negative short-term fuel trim (STFT) indicates that the engine control module (ECM) is detecting an excessively rich air-fuel mixture and is actively trying to remove fuel to compensate. If the ECM is removing a significant amount of fuel (-15% or more), it suggests a problem causing the engine to run rich, leading to potential misfires and hesitation.
Why the other options are wrong
- B. An exhaust leak before the upstream O2 sensor would introduce outside air, causing the O2 sensor to read lean, and the ECM would add fuel, resulting in positive fuel trims.
- C. Positive fuel trims indicate the ECM is adding fuel to compensate for a lean condition.
- D. If the O2 sensor was faulty and biased lean, the ECM would try to add fuel, resulting in positive fuel trims.
Negative Short-Term Fuel Trim (STFT)
A scan tool reading indicating the engine control module (ECM) is actively reducing the amount of fuel delivered to compensate for a detected rich air-fuel mixture.
- Measured as a percentage, typically +/- 20%.
- Negative values mean fuel removal, positive values mean fuel addition.
- Consistently high negative values (e.g., -15% or more) indicate a significant rich condition.
Memory trick: STFT high, lean is the cry. STFT low, rich is the flow!