Fuel Delivery System

Intake Manifold Assembly:

PURPOSE
The function of the fuel metering system is to deliver the correct amount of fuel to the engine under all operating conditions.

OPERATION
Fuel is delivered to the engine by individual bottom feed type multiport fuel injectors mounted in the lower intake manifold near each cylinder.
The Powertrain Control Module (PCM) pulses the fuel injectors in pairs. Alternate pairs are pulsed every 180° of crankshaft revolution. This is called Alternating Synchronous Double Fire (ASDF) injection.
The PCM uses two injector driver circuits, each controlling a pair of injectors. The current in each circuit is allowed to climb to a peak of 4 amps and then is reduced to 1 amp to hold the injector open. This happens very quickly.

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.

ALTERNATING SYNCHRONOUS DOUBLE FIRE (ASDF) FUEL INJECTION
For better fuel vaporization and improved emission control, fuel is injected at the intake valves while they are in the closed position. To do this, the fuel injectors are alternately pulsed in pairs. Injectors for cylinders 1 and 4 are electrically paired, as are the injectors for cylinders 2 and 3. Each pair of injectors is pulsed twice (double fire), once on the compression stroke and once on the exhaust stroke for each cylinder.

FUEL METERING SYSTEM COMPONENTS
The fuel metering system consists of the following parts:
^ Fuel supply components (fuel tank, pump, pipes).
^ Fuel pump electrical circuit.
^ Intake manifold assembly, consisting of:

- Lower manifold assembly, including:
^ MFI fuel injectors.
^ Fuel pressure regulator assembly.

- Upper manifold assembly, including:
^ Throttle valve.
^ Idle Air Control (IAC) valve.
^ Throttle Position (TP) sensor.

FUEL SUPPLY COMPONENTS
The fuel supply is stored in the fuel tank. An electric fuel pump, located in the fuel tank with the gauge sending unit, pumps fuel through an in-pipe fuel filter to the lower manifold assembly fuel passage. The pump provides fuel at a pressure greater than is needed by the injectors. The presure regulator, attached to the end of the lower manifold assembly, keeps fuel available to the injectors at a regulated pressure. Fuel, in excess of injector needs, is returned to the fuel tank by a separate return pipe.