A technician is diagnosing a vehicle with a complaint of a rough idle and occasional stalling when the engine is warm. The scan tool shows no active DTCs, but the short-term fuel trim (STFT) fluctuates wildly between -15% and +15% at idle. When the engine is revved to 2500 RPM, the STFT stabilizes around 0%. Which of the following is the most likely cause?
- AA vacuum leak.
- BA weak fuel pump.
- CA faulty oxygen (O2) sensor.
- DA clogged fuel injector.
Show answer & explanationAnswer & explanation
Correct answer: A. A vacuum leak.
A vacuum leak introduces unmetered air into the intake manifold, causing a lean condition. At idle, this unmetered air has a significant impact on the air-fuel ratio, leading to large, erratic fuel trim corrections. As engine RPM increases, the amount of metered air becomes proportionally much larger than the unmetered air from the leak, minimizing its effect, and allowing fuel trims to stabilize.
Why the other options are wrong
- B. A weak fuel pump would cause a lean condition and primarily positive fuel trims, especially under load, but wouldn't typically cause wild fluctuations that stabilize at higher RPM due to a fixed pressure issue.
- C. A faulty O2 sensor could cause erratic fuel trims, but usually, it would be consistently inaccurate regardless of RPM, or stick to one value, triggering a DTC. If it were truly faulty and fluctuating wildly, it would likely set a code.
- D. A clogged fuel injector would cause a consistent lean condition for that cylinder and generally more stable, but still positive, fuel trims across the board, not wildly fluctuating ones that stabilize at higher RPM.
Vacuum Leak & Fuel Trims
A vacuum leak introduces unmetered air into the engine, causing a lean condition that is most pronounced at idle (high vacuum). The ECU attempts to compensate, leading to erratic or high positive fuel trims at idle, which often stabilize at higher RPM.
- Unmetered air enters intake manifold.
- Causes lean condition, especially at idle.
- Results in fluctuating/high positive fuel trims at idle, stabilizing at higher RPM.
Memory trick: Fuel trims 'vary' with 'RPM' when there's an 'air' problem 'leaking' in.