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 14.7:1 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 Won't Run symptom. Refer to Engine Cranks But Does Not Run to determine if the problem is caused by the ignition system, PCM, or fuel pump circuit. Whenever it 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. Whenever a problem occurs in the fuel metering system, it 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 FT values (-100 percent to 100 percent) which can be monitored by using a scan tool. A momentary change to the fuel calculation is indicated by the Short Term FT value, while a prolonged change is indicated by the Long Term FT 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 0-100 percent, see DTC P0171 or DTC P0131 tables for items which can cause a lean system. Whenever both fuel trim values are between 0 to -100 percent, see DTC P0172 or 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:

The fuel metering system is made up of the following parts:

The accelerator control system is a cable type and there are no linkage adjustments. Therefore, the only specific cable for each application must be used. When work has been performed on the accelerator controls, always make sure that all components are installed correctly and that the linkage and cables are not rubbing or binding in any manner. The throttle should operate freely without binding between full closed and wide open throttle (WOT).

The intake manifold is made up of the upper and lower manifold assemblies. The upper manifold assembly (1) is a single casting that includes the upper portion of the air chamber (plenum) and the air inlet (fitted with a throttle valve). This assembly replaces the individual throttle body assembly and plenum components. The throttle valve is used to control air flow into the engine, thereby controlling engine output. The throttle valve is opened through the accelerator controls. During engine idle, the throttle valve is almost closed, and air flow control is handled by the Idle Air Control (IAC)(2) valve described later in this section. Vacuum ports located at, above, or below the throttle valve generate vacuum signals needed by various components. The nonadjustable, throttle shaft-driven, Throttle Position (TP) sensor (3) is attached to the upper manifold assembly opposite the throttle cam lever. The lower manifold (5) is a single casting that includes the intake runners and a longitudinal fuel passage (6) with intersecting fuel injector bores in each runner. This assembly replaces the individual fuel rail and manifold components used in other systems. Information on the fuel injectors (7) and pressure regulator (4) are covered separately.

When the key is first turned ON, without the engine running, the PCM energizes the fuel pump relay for a calibrated time (approximately 2 seconds). This builds up fuel pressure quickly. Whenever the engine is not started within two seconds, the PCM shuts OFF the fuel pump and waits until ignition reference pulses are present. As soon as the engine is cranked, the PCM energizes the relay which powers the fuel pump. As a backup system to the fuel pump relay, the fuel pump also can be energized by the fuel pump and the engine oil pressure indicator switch. The fuel pump and the engine oil pressure indicator switch is a normally open switch which closes when the oil pressure reaches about 28 kPa (4 psi). Whenever the fuel pump relay fails, the fuel pump and engine oil pressure indicator switch powers the fuel pump. An inoperative fuel pump relay can result in long cranking times. An inoperative fuel pump would cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance.

The fuel injector assembly is a solenoid-operated device, controlled by the PCM, that meters pressurized fuel to a single engine cylinder. The PCM energizes the fuel injector solenoid, which opens a ball valve, allowing fuel to flow past the ball valve, and through a recessed flow director plate at the injector outlet. The director plate has machined holes that control the fuel flow, generating a conical spray pattern of finely atomized fuel at the injector tip. Fuel from the tip is directed at the intake valve, causing it to become further atomized and vaporized before entering the combustion chamber. A fuel injector that is stuck partly open would cause loss of pressure after the engine is shut down, so long crank times would be noticed on some engines. Dieseling could also occur because some fuel could be delivered to the engine after the ignition is turned OFF.

The fuel pressure regulator assembly is a diaphragm-operated relief valve with fuel pump pressure on one side, and a regulator spring pressure and intake manifold vacuum on the other side. The regulator's function is to maintain a constant pressure differential across the injectors at all times. The pressure regulator compensates for engine load by increasing the fuel pressure as the engine vacuum drops. The fuel pressure regulator is serviced as a complete assembly. With the ignition ON and engine OFF (zero vacuum), fuel pressure should be 284-325 kPa (41-47 psi). Whenever the pressure is too low, poor performance and a DTC P0171 could result. Whenever the pressure is too high, excessive odor and a DTC P0172 could result.

The nonadjustable, throttle shaft-driven TP sensor is mounted on the upper manifold assembly opposite the throttle cam lever. The TP sensor senses the throttle valve angle and relays the information to the Powertrain Control Module (PCM). Knowledge of throttle angle is needed by the PCM to properly control the injector control signals (pulses).

Engine idle speed is controlled by the powertrain control module (PCM) through the IAC valve (1) mounted on the throttle body. The PCM sends voltage pulses to the IAC valve motor windings causing the IAC valve pintle (3) to move IN toward the seat, or OUT away from the seat, a given distance (a step, or count), for each pulse. The commanded location, steps away from the seated position, can be observed as a number of counts displayed on a scan tool. The pintle movement controls the airflow around the throttle valve (2), which in turn, controls engine idle speed: