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Torque Technical BulletinsMASTER MECHANIC VERIFIED6 MIN READ
Fuel Trim Tracing: Diagnosing Lean & Rich Conditions via OBD Telemetry
Technical Specifications & Target Criteria:
Author:Alex Ramirez, Senior Diagnostic Lead
Fuel trim represents the ECM correcting injector pulse width to maintain the stoichiometric 14.7:1 air-fuel ratio. Total fuel trims exceeding +15% indicate unmetered intake air (vacuum leak) or failing fuel delivery.
If fuel trim is +20% at idle but drops to +3% at 2,500 RPM, the cause is a vacuum leak. Conversely, if trims remain high under boost/load, the issue is a failing high-pressure fuel pump (HPFP) or clogged injectors.
Fuel System Checklist
1
Log Short-Term and Long-Term Fuel Trim on both cylinder banks.
2
Check Mass Air Flow (MAF) sensor grams-per-second at wide-open throttle.
3
Perform smoke leak test of intake manifold gaskets and PCV diaphragm.
4
Measure high-pressure fuel rail pressure under wide-open throttle acceleration.
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