Fuel and Air Induction Systems Diagnosis
Fuel delivery basics
Pressure alone is not enough; you need pressure and volume under load. Test at idle and during snap throttle or a road load. Returnless systems regulate at the pump or with a pressure sensor, so check pump current draw and the fuel pump control module. Do not forget the relay, inertia switch, and filter.
Injection
Port injectors can be checked for resistance, balance, and pattern. A clogged injector leans one cylinder and shows as positive fuel trim plus a misfire. GDI systems add a cam-driven high-pressure pump, very high rail pressure, and intake valve carbon that causes cold-start misfires and rough idle.
Air induction
MAF, MAP, throttle body, and idle control all meter air. Unmetered air such as a split intake boot, a leaking PCV, or a stuck-open EVAP purge leans the mixture at idle and is worst when hot. A dirty throttle body on a drive-by-wire vehicle causes idle flare and stalling and often needs a relearn.
Traps
Condemning a fuel pump without a volume test; missing a restricted filter or kinked line; assuming a MAF code means a bad MAF when it often means a vacuum leak; contaminating the MAF hot wire; and overlooking ethanol content when evaluating trims.
Rule of thumb
Compare trims at idle and at 2,500 RPM. If the lean condition is much worse at idle and improves with RPM, suspect a vacuum leak. If it stays lean across the range, suspect fuel supply or a MAF that under-reports.