Fuel delivery faults can cause both lean and rich conditions. The key to diagnosis is understanding the gas signature and fuel trim behavior.
Lean Fuel Delivery
When insufficient fuel reaches the cylinders:
- O₂ high (excess air not burned)
- Lambda > 1.1
- CO & CO₂ low
- STFT positive (+10-25%)
- LTFT positive if chronic
Causes:
- Weak fuel pump
- Clogged fuel filter
- Faulty fuel pressure regulator (low pressure)
- Contaminated fuel (water, wrong type)
Rich Fuel Delivery
When too much fuel is supplied:
- CO high (>2%)
- O₂ very low (near 0%)
- Lambda < 0.9
- STFT negative (-10 to -20%)
- LTFT negative if persistent
Causes:
- Overpressure (bad regulator)
- Leaking injector (internal or external)
- Stuck-open purge valve (introducing vapors)
- Return line restricted (excess pressure on return-less systems)
Testing Fuel Pressure
Connect a gauge to the fuel rail service port. With the engine running, pressure should match manufacturer spec (typically 3-4 bar for many systems). Check for:
- Pressure that drops when engine revs (weak pump)
- Pressure that rises when return is pinched (clogged filter)
- No pressure at all (pump dead)
Injector Leak Test
After shutting off the engine, fuel pressure should hold for several minutes. If it bleeds down quickly, one or more injectors may be leaking externally or internally.
Connecting the Dots
Combine observations:
- Rich + high CO + negative trims → overpressure or leaking injector
- Lean + high O₂ + positive trims → weak pump or clogged filter
- Intermittent rich/lean swings → failing fuel pump (low pressure) or electrical issues
The 4D engine integrates these patterns and assigns a health score based on the severity of the fuel delivery issue.