A contaminated or aged MAF sensor typically under-reports airflow. The ECU trusts the sensor and adds less fuel than the engine actually needs. The signature is subtle because fuel trims partially compensate.
The pattern
- LTFT persistently positive at low load (ECU adding fuel).
- LTFT approaches zero or goes negative at WOT where MAF reads closer to correct.
- Calculated airflow from MAP / rpm / volumetric efficiency does not match reported MAF g/s.
- Gas signature: mild lean; O₂ slightly elevated; HC normal.
Confirmation
- OBD MAF g/s vs expected: use VE × rpm × displacement calculation or compare to a known-good vehicle of the same type.
- Tap test on suspected MAF — watch for momentary rpm change.
- Clean with MAF-specific cleaner only (no contact cleaner!) and retest.
- Replace if cleaning doesn't restore readings within tolerance.
// vacuum leak look-alike
Under-reporting MAF and vacuum leak both lean the mixture. The distinguishing test is rpm response: vacuum leak normalises at high rpm, MAF under-reporting persists across the load range.