Explanation of possible cause for a FUEL SYSTEM LEAN CODE The Fuel System Lean code is set when: Short Term Compensation multiplied by Long Term Adaptive reaches a predetermined value. The actual number varies make model, but in general this will be highly positive when the condition set. Usually either short term or long term will be at +20 % or higher. The adaptives become higher when the PCM increases the injector pulse width in attempt to make the oxygen sensor switch uniformly about the PCM's oxygen sensor goal voltage. This means that the oxygen sensor is producing a voltage that is close to zero volts ( 0v ) consistently indicating a high oxygen (lean) state in the exhaust. The causes of the oxygen sensor reading a lower voltage usually falls into three main categories: 1 )Too little fuel, 2) Severe engine misfire, 3) The PCM is being provided with the improper information. THE FOLLOWING IS A GENERIC LIST OF POSSIBLE CAUSES. THE LIST IS PROVIDED AS AN EXAMPLE OF POSSIBILITIES. SOME OF THESE MAY NOT APPLY TO YOUR SPECIFIC VEHICLE. IT IS YOUR RESPONSIBILITY TO DECIDE WHICH COULD APPLY. THIS IS NOT A COMPREHENSIVE LIST : MERELY THE MORE COMMON CAUSES. 1) NOT ENOUGH FUEL -- there is an abundance of left over oxygen in the exhaust after combustion. -- This excessive could be caused by any of the following: TOO LOW OF FUEL PRESSURE-(including restricted fuel pump inlet filter, bad pump, defective pump, restricted fuel filter ) INADEQUATE FUEL VOLUME FROM FUEL PUMP ( Should deliver 1/2 liter in 7 second fuel pump actuation time) RESTRICTED INJECTORS-(both inlet screens and deposits at the injector pintle.) WRONG INJECTORS DEFECTIVE PURGE SYSTEM- (purge system is the source of a vacuum leak) 2) SEVERE ENGINE MISFIRE- although OBDII equipped vehicles have misfire detection; earlier vehicles and heavy duty truck do not. If the cylinder misfires the oxygen will simply pass through the cylinder rather than have combustion occur and the amount of oxygen in the exhaust will be high. 3) PCM IS BEING PROVIDED IMPROPER INFORMATION- If the PCM does not receive proper sensor readings it will not deliver the proper injector pulse width for conditions. The following is a list of sensor problems that could cause a fuel system lean code: COOLANT SENSOR CALIBRATION (sensor is reading much warmer than the actual engine temperature). OXYGEN SENSOR CIRCUIT PROBLEM- ( this includes O2, O2 sensor wiring including the signal, sensor ground, and heater circuit wires the O2 sensor must also be installed tight.) MAP SENSOR CALIBRATION--One of the most important readings for testing a MAP sensor is the voltage reading obtained with the DRBIII with the ignition on and the engine off. This voltage reading is important because the PCM uses this for calculating barometric pressure. If the PCM does not "know" the correct air density from the MAP sensor (baro. reading) it will have to drastically change the fuel curve through the adaptives to get the proper response from the oxygen sensor. Below is a chart of key on MAP sensor readings for a given elevation. You should get to know what is normal for your area. Elevation (feet) Baro Reading (in.-hg.) MAP voltage 0 (sea level) 29,92 4.6v 29.67 500 29.42 29.17 1000 28.92 28.67 4.4v 1500 28,42 28.17 2000 27.92 27.67 2500 27.42 4.2v 27.17 3000 26.92 26.67 3500 26.42 26.17 4.0v 4000 25.92 25.67 4500 25.42 25.17 5000 24.92 3.8v VOLTAGE SENSE-- The PCM compensates the injector pulsewidth based upon charging system voltage. The higher the voltage=> the stronger the magnetic field opening the pintle valve in the injector. The PCM compensates for this condition. If the PCM sense a higher voltage than the voltage applied to the injectors the Fuel System Rich Code could also result.