The function of the fuel metering system is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel is delivered to the engine by individual fuel injectors near each cylinder. The PCM uses the voltage inputs from several sensors in order to determine how much fuel to give the engine. Each injector is energized individually in the engine firing order, which is called sequential fuel injection (SFI). However, if the PCM loses the voltage input from the camshaft position (CMP) sensor, the PCM will energize the fuel injectors by using the alternating synchronous double fire (ASDF) method. The main control sensor is the oxygen sensor (O2S) located in the exhaust manifold. This sensor indicates to the PCM how much oxygen is in the exhaust gas, and the PCM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best mixture to minimize exhaust emissions is 14.6:1 which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a Closed Loop system.

Some failures of the fuel metering system will result in an Engine Cranks but Does Not Run symptom. Refer to Engine Cranks but Does Not Run to determine if the problem is caused by the ignition system, the PCM, or the fuel pump circuit. Whenever the fault is determined to be a fuel problem, the Fuel System Diagnosis table will be used. This includes the fuel injectors, the fuel pressure regulator, the fuel pump, and the fuel pump relay. A problem in the fuel metering system usually results in either a rich or lean O2S signal, which causes the PCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short and long term fuel trim values (-100 percent to +100 percent) which can be monitored using a scan tool. A momentary change to the fuel calculation is indicated by the short term fuel trim value, while a prolonged change is indicated by the long term fuel trim value. A value of 0 percent indicates that the fuel delivery requires no compensation to maintain the proper air/fuel ratio. A negative value significantly below 0 percent indicates that the fuel system is rich and the fuel delivery is being reduced (decreased injector pulse width). A positive value significantly more than 0 percent indicates that a lean condition exists and the fuel delivery is being increased (increased injector pulse width).

Important: Whenever both fuel trim values are between 0 percent to 100 percent, refer to DTC P0171 or to DTC P0131 for items which can cause a lean system. Whenever both fuel trim values are between 0 percent and -100 percent, refer to DTC P0172 or to DTC P0132 for items which can cause the system to run rich.

Listed below are examples of lean and rich conditions with the system in control and out of control.