Fuel System Diagnosis and Repair
What the candidate must know
Gasoline fuel system diagnosis covers the tank, pump, filter, lines, pressure regulator, injectors, throttle body, and the electronic controls that meter fuel. Know how to test fuel pressure and volume, how to check for pressure drop (leaks, regulator), how to evaluate injector balance and spray pattern, and how to interpret fuel trims (short and long term) to separate vacuum leaks, fuel delivery faults, and sensor errors. Understand returnless vs. return-style systems and how the regulator works in each.
Traps in questions
- Low fuel pressure can be a weak pump, clogged filter, restricted line, or faulty regulator. Test pressure and volume; a pump can make pressure at idle but fail under load.
- High positive fuel trims at idle that improve at speed suggest a vacuum leak; trims high across the range suggest a fuel delivery or sensor problem. Match the trim pattern to the cause.
- Injector balance test identifies a weak or clogged injector; a clogged injector leans that cylinder and can cause a misfire. Do not condemn the injector without a balance or flow test.
- Returnless systems regulate pressure at the pump or with a pulse-modulated pump; do not look for a return line regulator that is not there.
- Fuel pressure that bleeds down after shutoff points to a leaking injector, check valve, or regulator - not necessarily a bad pump.
- Gasoline in oil (dilution) points to a leaking injector or rich condition; confirm with fuel trim and injector testing.
Worked example
A truck has a rough idle and positive fuel trims (+18% short term) that drop to +4% at 2500 rpm. That pattern says vacuum leak at idle - check intake gaskets and vacuum lines. A fuel delivery fault would keep trims high at speed.
Rule of thumb
Test pressure and volume. A pump that passes pressure at idle can still fail under load. Use fuel trims to separate vacuum leaks (idle-heavy) from delivery faults (across the range).