Computers and Control Systems: Description and Operation

Multiport Fuel Injection (MFI) System Diagram:

GENERAL INFORMATION

The Multiport Fuel Injection System consists of sensors which detect the engine conditions, the ENGINE CONTROL MODULE (ECM) / POWERTRAIN CONTROL MODULE (PCM) which controls the system based on signals from these sensors, and actuators which operate under the control of the ECM /PCM . The ECM /PCM carries out activities such as fuel injection control, idle air control, and ignition timing control. In addition, the ECM /PCM is equipped with several diagnostic test modes which simplify troubleshooting when a problem develops.

FUEL INJECTION CONTROL
The injector drive times and injection timing are controlled so that the optimum air/fuel mixture is supplied to the engine to correspond to the continually-changing engine operation conditions. A single injector is mounted at the intake port of each cylinder. Fuel is sent under pressure from the fuel tank to the fuel injectors by the fuel pump, with the pressure being regulated by the fuel pressure regulator. The regulated fuel is distributed to each of the injectors. Fuel injection is normally carried out once for each cylinder for every two rotations of the crankshaft. The firing order is 1-2-3-4-5-6. Each cylinder has a dedicated fuel injector. This is called multiport. The ECM /PCM provides a richer air/fuel mixture by carrying out "open-loop" control when the engine is cold or operating under high load conditions in order to maintain engine performance. In addition, when the engine is under normal operating temperature after warming-up, the ECM /PCM controls the air/fuel mixture by using the heated oxygen sensor signal to carry out "closed-loop" control. The closed-loop control achieves the theoretical air/fuel mixture ratio where the catalytic converter can obtains the maximum cleaning performance.

IDLE AIR CONTROL
The idle speed is kept at the optimum speed by controlling the amount of air that bypasses the throttle valve in accordance with changes in idling conditions and engine load during idling. The ECM /PCM drives the idle air control (IAC) motor to keep the engine running at the pre-set idle target speed in accordance with the engine coolant temperature and A/C and other electrical load. In addition, when the A/C switch is turned off and on while the engine is idling, the IAC motor adjusts the throttle valve bypass air amount according to the engine load conditions to avoid fluctuations in the engine speed.

IGNITION TIMING CONTROL
The ignition power transistor located in the ignition primary circuit turns ON and OFF to control the primary current flow to the ignition coil. This controls the ignition timing to provide the optimum ignition timing with respect to the engine operating conditions. The ignition timing is determined by the ECM /PCM from engine speed, intake air volume, engine coolant temperature, and atmospheric pressure.

DIAGNOSTIC TEST MODE
- When an abnormality is detected in one of the sensors or actuators related to emission control, the Malfunction Indicator Lamp (SERVICE ENGINE SOON or Check Engine Lamp) illuminates to warn the driver.
- When an abnormality is detected in one of the sensors or actuators, a diagnostic trouble code corresponding to the abnormality is stored in the ECM /PCM .
- The RAM data inside the ECM /PCM that is related to the sensors and actuators can be read with the scan tool. In addition, the actuators can be controlled by scan tool MUT-II (MB991502) under certain circumstances.

OTHER CONTROL FUNCTIONS

- Fuel Pump Control
Turns the fuel pump relay ON so that current is supplied to the fuel pump while the engine is cranking or running.

- A/C Compressor Clutch Relay Control
Turns the compressor clutch of the A/C ON and OFF.

- Fan Relay Control
The radiator fan and condenser fan speeds are controlled in response to the engine coolant temperature and vehicle speed.

- Generator Output Current Control
Prevents generator output current from increasing suddenly and idle speed from dropping at times such as when the headlights are turned on.

- Evaporative Emission Purge Control
(Refer to Emission Control System - Evaporative Emission Control System - General Information)

- EGR Control
(Refer to Emission Control System - Exhaust Gas Recirculation (EGR) System - General Information)