"Fuel delivery fault" covers a family of very different failures. The 5-gas + OBD picture narrows them quickly once you know what each looks like.
Weak fuel pump / dirty filter
- Normal at idle (demand is low).
- Goes lean under load / high rpm.
- STFT swings positive during load.
- Fuel pressure drops under flow.
Leaking injector
- Rich at idle (fuel dripping into cylinder).
- High HC if fuel doesn't burn (fouls plug).
- STFT negative (ECU trimming out fuel).
- Cylinder balance test shows the affected cylinder dominating.
Stuck-closed / clogged injector
- Lean on affected cylinder.
- Elevated HC — misfire from lean ignition.
- Cylinder drop test identifies which cylinder.
- Injector flow test confirms.
Fuel pressure regulator
- Stuck low: universal lean, bogging under load.
- Stuck high: universal rich, sooty plugs.
- Leaking diaphragm: fuel drawn via vacuum line, classic rich idle with no injector fault.
Confirmation path
- Fuel pressure gauge: key-on, idle, snap-throttle.
- Injector balance / click test.
- Individual STFT per cylinder where supported.
- Spark plug colour comparison across cylinders.