Fuel Delivery


Fuel Delivery Troubleshooting


Fuel Trim. Fuel Delivery Problem
  • Disconnected fuel injector creates leaner condition than disconnected spark plug on that cylinder.
    • Total fuel trim ~ +24% (2000 Lexus RX300 with a wideband O2 sensor)
    • Total fuel trim ~ +20% (2004 Dodge Neon with a narrow band O2 sensor)
  • Disconnected spark plug in one cylinder creates less lean condition than disconnected fuel injector.
    • Total fuel trim ~ +8% (2000 Lexus RX300 with a wideband O2 sensor)
    • Total fuel trim ~ +7%-8% (2004 Dodge Neon with a narrow band O2 sensor)
  • Rev the engine to 1500 to 2000 RPM and hold it steady for half a minute or so. If the fuel trim readings do not change much, the lean fuel condition is more likely due to a fuel delivery problem  than a vacuum leak.
    • Rule of Thumb. With fuel related misfire,  STFT goes more than 15 percent positive, often into the mid 20 percent range or more and the downstream O2 voltage below 350mV (lean).
    • At WOT front O2 has to show above 900mV to indicate a good fuel supply to the injectors. Below 100mV can indicate fuel supply problem as well as bad MAF.
    • Bad fuel supply would have good VE and lean front O2 sensors.
Excessive Fuel Flow
  • Fuel pressure regulator stuck closed
  • Leaky fuel injector
  • Excessive fuel pressure
    • Bad pressure regulator
    • Restricted fuel return line
  • Raptured fuel pressure regulator diaphragm
  • Leaking EVAP Purge solenoid
  • PCV leak (fuel or oil vapors) ???
Weak Fuel Flow
  • Wrong / bad fuel (e.g. E85 instead of E89)
  • Fuel pump (weak or non-operational)
  • Restricted fuel filter
  • Leaky fuel pressure regulator
  • Injector flow (sticking or not opening)


Fuel Composition
  • High ethanol/alcohol content?
    • GM. If live data shows ‘alcohol content’ around 27%, try to do ‘Fuel composition reset’ using Tech2. It should be close to 1%.




Fuel Pump Testing

  • Fuel Pressure Gauge (low pressure side)
    1.  Connect fuel pressure gauge. Look at the  fuel pressure specs and compare it.
    2.  When starting the car, pressure should build up to specs values quickly.
    3.  With open throttle, fuel pressure should increase (like 10psi). If it deceases, fuel system has a problem, i.e. bad fuel pump. If car has pressure regulator with vacuum line, take off vacuum line to simulate wide open throttle.
    4.  After releasing the throttle, pressure will drop lower than it was at idle, and then quickly stabilize back to specs. Its due to high vacuum at decel event.
  • Downstream O2 Sensor
    • As the fuel volume drops off and the system goes lean the downstream sensors will go lean as well.  Because both banks will be similarly affected you can rule out a single cylinder concern as the cause of the lean running condition.
  • High Pressure Fuel Pump Testing
    • Use oscilloscope and fuel rail pressure sensor. If you know how many plunger does the fuel pump have, and the speed at which it turns, you can see if one pumping is missing (IATN).


Fuel Injector Testing

Balance Test
    • Connect a fuel gauge or pressure transducer.
    • Pressurize the fuel system.
      • Make sure that the fuel pressure stays steady.
    • Operate each injector by a scanner or manually and record a pressure drop.
      • Ideally, each such injector’s test should show the same pressure drop.
      • A variation of 2psi or more is a cause for concern.
Waveform Analysis

  • Oscilloscope connection
    • Black lead – chassis ground;
    • Red lead – injectors pulse ground.
  • Current
    • As a rule of thumb, injectors draw around 950mA
    • Shorted injectors will have a steep rising ramp
    • Lower than normal current ramp
      • Bad injector
    • No current ramp
      • Bad Control Module
      • Bad connection
  • Injector’s Power Feed
    • On some vehicles, you need to crank the engine to turn injectors relay ON, otherwise it will be OFF.
  • Injector’s Ground Level
    • Look for a little raise in ground voltage at ON time due to raising current
  • Pulse
    • On running engine pulse usually within a range of 3-5ms.
    • During cranking it may be 20ms or higher.
    • Duty Cycle.
      • On most vehicles at idle the duty cycle is 2.9% – 3%.
  • Clean switching (OFF/ON/OFF)
  • Pintle Activity
    • Look at current waveform to see pintle opening
      • Should open at the same place all the time, otherwise may be intermittently sticking
    • Look at ground side voltage to see a pintle closing
      • Should have a little notch/hump at the downward slope after EMF spike
  • Fuel flow (with the help of a pressure transducer hooked to the fuel rail)


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