See Figures 1 and 2
The Multi-port Fuel Injection (MFI) system is controlled by an Electronic Control Module (ECM) which monitors engine operations and generates output signals to provide the correct air/fuel mixture, ignition timing and engine idle speed control. Input to the control unit is provided by an oxygen sensor, coolant temperature sensor, detonation sensor, hot film air mass sensor and throttle position sensor. The ECM also receives information concerning engine rpm, road speed, transmission gear position, power steering and air conditioning.
Some later model vehicles are equipped with a Sequential Fuel Injection (SFI) system. This system operates basically the same and consists of the same components as the MFI system.
With MFI, metered fuel is timed and injected sequentially through the injectors into individual cylinder ports. Each cylinder receives 1 injection per working cycle (every 2 revolutions), just prior to the opening of the intake valve. In addition, on V6 engines, the SFI system incorporates a Computer Controlled Coil Ignition (CI) system that uses an electronic coil module that replaced the conventional distributor and coil used on most engines. An Electronic Spark Control (ESC) is used to adjust the spark timing.
The injection system uses solenoid-type fuel injectors, 1 at each intake port, rather than the single injector found on the earlier throttle body system. The injectors are mounted on a fuel rail and are activated by a signal from the electronic control module. The injector is a solenoid-operated valve which remains open depending on the width of the electronic pulses (length of the signal) from the ECM; the longer the open time, the more fuel is injected. In this manner, the air/fuel mixture can be precisely controlled for maximum performance with minimum emissions.
Fuel is pumped from the tank by a high pressure fuel pump, located inside the fuel tank. It is a positive displacement roller vane pump. The impeller serves as a vapor separator and pre-charges the high pressure assembly. A pressure regulator maintains 34-47 psi (240-315 kPa) in the fuel line to the injectors and the excess fuel is fed back to the tank.
Engine idle is controlled by an Idle Air Control (IAC) valve, which provides a bypass channel through which air can flow. It consists of an orifice and pintle which is controlled by the ECM through a stepper motor. The IAC provides air flow for idle and allows additional air during cold start until the engine reaches operating temperature. As the engine temperature rises, the opening through which air passes is slowly closed.