See Figure 1
The Central Fuel Injection (CFI) system is a single point, pulse time modulated injection system which was used on 1986-90 2.5L engines. Fuel is metered into the air intake stream according to engine demands by one or two solenoid injection valves, mounted in a throttle body on the intake manifold. Fuel is supplied from the fuel tank by a single low-pressure pump. The fuel is filtered and sent to the air throttle body, where a regulator keeps the fuel delivery pressure at a constant 39 psi (269 kPa) on high-pressure systems, or 14.5 psi (100 kPa) on low-pressure systems. One or two injector nozzles are mounted vertically above the throttle plates and connected in parallel with the fuel pressure regulator. Excess fuel supplied by the pump, but not needed by the engine, is returned to the fuel tank by a steel fuel return line.
The fuel charging assembly controls air/fuel ratio. It consists of a typical carburetor type throttle body, and has one or two bores without venturis. The throttle shaft and valves control engine air flow based on driver demand. The throttle body attaches to the intake manifold mounting pad.
A throttle position sensor is attached to the throttle shaft. It includes a potentiometer (or rheostat) that electrically senses throttle opening. A throttle kicker solenoid fastens opposite the throttle position sensor. During air conditioning operation, the solenoid extends to slightly increase engine idle speed.
Cold engine speed is controlled by an automatic kick-down vacuum motor. There is also an all-electric, bimetal coil spring which controls cold idle speed. The bimetal electric coil operates like a conventional carburetor choke coil, but the fuel injection system uses no choke. Fuel enrichment for cold starts is controlled by the computer and injectors.
The fuel pressure regulator controls critical injector fuel pressure. The regulator receives fuel from the electric fuel pump and then adjusts the fuel pressure for uniform fuel injection. The regulator sets fuel pressure at 39 psi (269 kPa) on high pressure systems, or 14.5 psi (100 kPa) on low pressure systems.
The fuel manifold (or fuel rail) evenly distributes fuel to each injector. Its main purpose is to equalize the fuel flow. One end of the fuel rail contains a relief valve for testing fuel pressure during engine operation.
The fuel injectors are electromechanical devices. The electrical solenoid operates a pintle or ball metering valve which always travels the same distance from closed to open to closed. Injection is controlled by varying the length of time the valve is open.
The computer, based on voltage inputs from the crank position sensor, operates each injector solenoid two times per engine revolution. When the injector metering valve unseats, fuel is sprayed in a fine mist into the intake manifold. The computer varies fuel enrichment based on voltage inputs from the exhaust gas oxygen sensor, barometric pressure sensor, manifold absolute pressure sensor, etc., by calculating how long to hold the injectors open. The longer the injectors remain open, the richer the mixture. This injector ON time is called pulse duration.