Fuel Metering System

DIAGNOSTIC CHARTS
Some failures of this system will result in the symptom, "Engine Cranks But Will Not Run". If this condition exists, see CHART A-3. This chart will determine if the problem is caused by the ignition system, ECM or fuel pump circuit. If it's determined to be a fuel problem. CHART A-5 or A-7 will be used. This includes the injectors, fuel pressure control valve, fuel pump and fuel pump relay. The fuel system wiring schematic diagram is included with CHART A-5. These diagnostic charts are located at Computers and Control Systems / System Diagnosis.

BLOCK LEARN
If a malfunction occurs in the fuel control system, it usually results in either a rich or lean Heated Oxygen Sensor (HO2S) signal condition. This condition is sensed by the HO2S and the ECM will change the fuel calculation (injector pulse width) based on HO2S input. The change made to the fuel calculation will be indicated by a change in the block learn values which can be displayed on a "Scan tool". The ideal block learn values are around 128 counts. If the HO2S senses a lean condition, the ECM will add fuel, resulting in a block learn value above 128 counts. If the HO2S senses a rich condition, the ECM will subtract fuel, resulting in a block learn value below 128 counts.

Block learn values can vary greatly, depending on temperature, altitude and individual engine tolerances. The programmed range for block learn values is 58 to 198. Values within this range indicate the system is in "control". If a system is operating near the extremes of the block learn range during normal vehicle operation, and there is a driveability complaint, the fuel metering system should be checked. Refer to the diagnostic trouble code 45 chart for potential causes of a rich condition (low block learn values). Refer to the diagnostic trouble code 44 chart for potential causes of a lean condition (high block learn values). If a driveability symptom exists, also refer to the particular symptom in Diagnosis By Symptom for additional items to check.