On 1988-94 2.8L and 3.1L engines, the Multi-port Fuel Injection (MFI) system is available. The MFI system is controlled by a computer control module (ECM/PCM), which monitors engine operation 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/PCM also receives information concerning engine rpm, road speed, transmission gear position, power steering and air conditioning.
The 2.3L engines also use a Multi-port Fuel Injection (MFI) system, however, this system incorporates an Alternating Synchronous Double Fire (ASDF) method of injection. Fuel is delivered to the engine by individual fuel injectors mounted in the intake ports of the cylinder head as in an all multi-port type injection system. The difference is that the Electric Control module (ECM) pulses the fuel injectors in pairs. Alternate pairs are pulsed every 180° of crankcase revolution. This system provides better fuel vaporization and improved emission control.
On 1995-96 3.1L engines, a Sequential port Fuel Injection (SFI) system is used for more precise fuel control. With SFI, metered fuel is timed and injected sequentially through injectors into individual cylinder ports. Each cylinder receives one injection per working cycle (every two revolutions), just prior to the opening of the intake valve. The main difference between the two types of fuel injection systems is the manner in which fuel is injected. In the multi-port system, all injectors work simultaneously, injecting half the fuel charge each engine revolution. The control units are different for SFI and MFI systems, but most other components are similar.
- Both systems use Bosch injectors, on 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 computer control module (ECM/PCM); the longer the open time, then 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 40.5-47 psi (271-315 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.