Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
Fuel delivery
Fuel systems must deliver the right pressure and volume at the right time. Check key-on/engine-off pressure, running pressure, and pressure drop-off after shutoff. A pump can make correct pressure but poor volume - a restricted filter or weak pump shows up as low power under load. Test injectors for resistance, pulse width, and balance; a clogged injector leans one cylinder. Remember that fuel pressure specs differ for return-type and returnless systems, and that a vacuum-referenced regulator changes pressure with manifold vacuum.
Air induction
Air must be metered and clean. Inspect the air filter, intake ducting, throttle body, and MAF for restrictions and leaks. Unmetered air entering after the MAF or throttle body causes lean trims at idle. Check idle air control or electronic throttle operation for idle complaints.
Exhaust
The exhaust must flow. A restricted catalytic converter or collapsed pipe causes low power, back-pressure, and sometimes misfire or lean codes. Measure back-pressure at the O2 port or use a vacuum gauge; a rapid drop at steady throttle indicates restriction.
Traps
Do not diagnose a lean code as a bad O2 sensor without checking fuel trim and unmetered air first. Do not assume fuel pressure alone proves the pump is good. Verify with volume and pressure-drop tests.
Rule of thumb
If fuel pressure drops more than a few psi in the first minute after shutdown, suspect a leaking injector, check valve, or regulator - but confirm against the manufacturer's spec.