Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A technician observes that a vehicle's short-term fuel trims (STFT) are consistently negative (-15% to -20%) at idle and under light load, but return to near zero (+/-5%) at wide-open throttle (WOT). Which of the following is the MOST likely cause?
- AA leaking fuel injector.
- BA restricted air filter.
- CA weak fuel pump.
- DA faulty mass airflow (MAF) sensor.
Show answer & explanationAnswer & explanation
Correct answer: D. A faulty mass airflow (MAF) sensor.
Consistently negative fuel trims mean the engine control module (ECM) is reducing fuel delivery because it perceives a rich condition. If this happens at idle/light load but corrects at WOT, it suggests an issue where the MAF sensor is over-reporting airflow at lower flow rates. At WOT, the ECM often uses pre-programmed fuel maps (open loop) or relies less on MAF input, causing the trims to normalize.
Why the other options are wrong
- A. A leaking fuel injector would cause a rich condition and negative fuel trims, but this condition would likely persist or worsen at WOT, not normalize.
- B. A restricted air filter would cause a lean condition (less air than expected) or reduced power, leading to positive fuel trims or no change, not negative trims.
- C. A weak fuel pump would cause a lean condition and positive fuel trims, especially under load, not negative trims.
MAF Sensor Over-reporting
A MAF sensor that over-reports airflow at lower RPMs will cause negative fuel trims (rich condition) as the ECM tries to compensate. Trims may normalize at WOT when the ECM uses open-loop fueling.
- Causes negative fuel trims.
- Often more pronounced at idle/low RPM.
- May normalize at WOT.
- Check MAF sensor readings against known good values.
Memory trick: Negative trims, MAF's big lie.