General System Description

PURPOSE
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.

FUEL SUPPLY COMPONENTS
The fuel supply is stored in the fuel tank. An electric fuel pump, which is located in the fuel tank with the fuel level sensor sending unit, pumps fuel through an in-line fuel filter to the engine. The pump provides fuel at a pressure greater than is needed by the injectors. The fuel pressure regulator keeps fuel available to the injectors at a regulated pressure. The unused fuel is returned to the fuel tank by a separate line.

FUEL METERING SYSTEM COMPONENTS
The fuel metering system is made up of the following parts:

^ The fuel supply components (fuel tank, pump, and lines).
^ The fuel pump electrical circuit.
^ The intake manifold assembly including:
- The fuel injectors.
- The fuel pressure regulator.
^ The upper manifold assembly including:
- The throttle valve
- The idle Air Control (IAC) valve.
- The throttle Position (TP) sensor.

FUEL PUMP ELECTRICAL CIRCUIT
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.