Diagnostic Aids

Using Tech 1, observe long term fuel trim values at different RPM and airflow conditions. If conditions for code exist, long term fuel trim values will be around 115. Check for:

• Oxygen Sensor Wiring
  
  Reference oxygen is supplied through sensor wiring. Check sensor wiring for breaks or contamination that may prevent reference oxygen from reaching sensor.
• Fuel Pressure
  
  System will go rich is pressure is too high. Control module will compensate for some increase, but code will set if pressure gets too high.
• Fuel Contamination
  
  Check for oil contaminated fuel.
• HEI Shielding
  
  An open ground in ignition system reference low circuit may result in electrical interference. Control module looks at this at reference pulses. Pulses result in a higher than actual engine speed signal. Control module then delivers too much fuel, causing system to go rich. Tachometer will also show higher than actual engine speed, which can help in diagnosing problem.
• MAP Sensor
  
  If an output causes control module to sense high manifold pressure (low vacuum), system will go rich. Disconnecting MAP sensor will allow control module to set a fixed MAP value. Substitute a different MAP sensor if rich condition is gone when sensor is disconnected.
• Pressure Regulator
  
  Check for leaking fuel in pressure regulator diaphragm by checking for liquid fuel.
• Throttle Position Sensor
  
  An intermittent TP sensor output will cause system to go rich due to false indication of engine accelerating.
• Engine Coolant Temperature Sensor
  
  Check for shifted sensor that could cause a rich exhaust, but set Code P0117.