Multiport Fuel Injection (MFI) System

Fig. 105 Fuel Injector:

FUEL INJECTOR
The fuel injector (one) is small solenoid that allows fuel to be injected into the intake manifold. The injector receives the ground circuit from the ECU in terms of "Injection Duration". The voltage applied to the solenoid's windings pulls the needle valve up allowing fuel to flow through the nozzle.
WARNING: The positive circuit is always "Hot" at the injector terminals when the battery is connected.

Fig. 106 Fuel Injection Control System:

BASIC FUEL INJECTION CONTROL
The amount of fuel injected or the amount of injector duration is determined by a programmable value mapped in the ECU's ROM memory. In other words the ECU writes a program based for injection duration based on input signals sent by the crank angle sensor and the air flow meter.

Fig. 39 Fuel Enrichment:

FUEL INJECTION COMPENSATION
Fuel injector duration is compensated to improve engine performance under various conditions, for instance;
1. Increased Duration (more fuel)
a. During engine warm-up.
b. While starting engine.
c. During acceleration.
d. Hot-engine operation.
2. Reduced Duration (less fuel)
a. During deceleration.

Fig. 11 Closed Loop Control:

MIXTURE RATIO FEEDBACK
The mixture ratio feedback operation is provided to control the air fuel mixture at the stoichiometric point. This allows the three-way catalyst to operate most efficiently. The ECU adjusts the injector duration based on a voltage signal sent by the oxygen sensor mounted in the exhaust stream. When the ECU is controlling the air-fuel mixture this is called "Closed-Loop Operation". The system is considered in "Open-Loop Operation" (not controlling A/F mixture) when the ECU senses the following conditions:
1. Deceleration.
2. Engine starting.
3. Engine idling.
4. O2 sensor or circuit malfunction.
5. Low O2 sensor temperature.
6. High speed or high load operation.
7. When the ECU senses a rich condition for more than 10 seconds.