General System

System Diagram:

This vehicle is equipped with a 1.8L, DOHC V6 cylinder gasoline engine and computer controlled electronic fuel injection system. The system maintains an air/fuel mixture as close as possible to the ideal stoichiometric ratio for all operating conditions. During normal operation, the computer monitors various engine sensors, including an exhaust gas oxygen sensor, and operates injectors sequentially for varying lengths of time, depending on amount of fuel required.

FUEL INJECTION
Fuel is supplied to the fuel rail and injectors at high pressure by the fuel pump in the tank. A fuel pressure regulator varies fuel pressure depending on manifold vacuum. When fuel demand is high as indicated by low manifold vacuum, pressure is increased slightly. Injectors are electric solenoid valves that get power when the engine is cranking or running and turn "ON" when the Engine Control Unit (ECU) completes their circuits to ground. The amount of fuel sprayed depends on the injector "pulse width" (length of time the injectors are turned "ON" during each injection cycle). Injectors are operated sequentially, which means injectors turn "ON" in the same sequence as engine firing order, so that injection occurs at each cylinder while the intake valves for that cylinder are open.

STARTING
During cranking, all injectors energize at the same time once for each ignition pulse (3 times for each revolution of the crankshaft). This provides a richer mixture for easy starting. Once started, injectors operate sequentially following engine firing order.

DE-CHOKE CONTROL
If the engine won't start or is hard starting due to excess fuel in manifold or cylinders, the ECU will stop injection under the following conditions:
^ Ignition switch in "START" position (engine cranking signal to ECU).
^ Engine speed less than 500 rpm ( engine not running).
^ Throttle sensor signal indicates throttle wide open.

If these conditions exist concurrently, no fuel will be injected. This allows air to be pumped through the cylinders, purging excess fuel.

WARM-UP
When the engine is running cold, the fuel injection system operates in what is known as "OPEN LOOP" mode. In this mode, the ECU determines the amount of fuel needed depending on the engine coolant temperature, throttle position sensor signal, and the amount of air flow as measured by the air flow meter. A specific pulse width is assigned for certain air flow rates at various temperatures and throttle openings. These are the "base" pulse width values stored in the computer memory, which provide a slightly rich air/fuel ratio for good driveability while the engine is cold and during warm-up.


NORMAL WARM ENGINE OPERATION
When the engine warms up, the fuel injection system switches to "CLOSED LOOP" operation. In this mode, the computer monitors several sensors, including an exhaust gas oxygen sensor. Signals from the oxygen sensor tell the ECU if the air/fuel ratio is rich or lean. The ECU then modifies the base pulse width accordingly to increase or decrease the amount of fuel being injected, depending on the need at that moment. Injector pulse width is adjusted several times each second to maintain the air/fuel ratio as close as possible to ideal.

IDLE SPEED CONTROL
Idle speed is controlled by the Bypass Air Control (BAC) valve. The BAC valve regulates how much measured air is allowed to bypass the throttle valves, depending on engine temperature and signals from the ECU. The BAC valve has two parts, the "Air Valve" which is open when cold to provide cold start fast idle, and the Idle Speed Control (ISC) valve which is electrically operated by the ECU. In the air valve, a temperature sensitive wax pellet is heated by engine coolant. As the engine warms up, the pellet expands against the air valve, closing it. When the engine is warm, idle speed is controlled by the ECU through duty cycle signals to the ISC valve (the ratio of "ON" time vs."OFF" time).

DECELERATION FUEL CUT-OFF
The ECU recognizes when the engine is decelerating by monitoring the engine speed and an idle switch in the throttle position sensor that tells the ECU when the throttle is closed. When the throttle is closed and the engine rpm is above a certain speed the ECU will leave the injectors turned "OFF" and no fuel in injected. This reduces emissions, prolongs catalytic converter life, and improves fuel economy.

INTAKE AIR SYSTEM - VARIABLE RESONANCE INDUCTION SYSTEM (VRIS)

Variable Resonance Induction System (VRIS):

The intake manifold is divided in two sections. The separate sections are joined by two common runners, each with a shutter valve. The shutter valves are opened and closed at different engine speeds to take advantage of harmonic oscillations in the intake manifold runners. This helps "pack" more fuel and air into the cylinders during each cylinder cycle (improved volumetric efficiency), increasing torque and horsepower over a broader rpm range.

IGNITION TIMING CONTROL
Ignition timing is also controlled by the computer. The ECU monitors signals from sensors in the distributor. From these signals, the ECU can calculate crankshaft position and engine speed. The ECU uses this information, compared with information from other engine sensors, to determine the correct spark advance for the engine speed and load at any given time. The ignition coil "fires" when the ECU interrupts the low voltage signal to a power switching transistor (igniter). The igniter turns the primary coil circuit "OFF" causing the ignition coil to discharge. For more information on the ignition coil, igniter, and other secondary ignition components, refer to IGNITION SYSTEM

SELF-DIAGNOSTICS
The EGI system has self-diagnostic capability. When a fault is detected in a monitored component and/or circuit, the ECU will store a numbered code in its memory (malfunction code) and the Malfunction Indicator Lamp (MIL) will light and stay on while the engine is running, alerting the driver to seek service. The technician can then access the ECU fault memory and display trouble codes identifying the faulty component(s) or circuit(s), to aid in troubleshooting and repair.

FAIL-SAFE MODE
To provide a margin of safety and maintain driveability in the event of certain system failures, the ECU has a fail-safe mode. In this mode, the ECU will substitute a fixed "in range" signal for that of the failed sensor. This allows the vehicle to be operated until repaired, although driveability will probably be affected. The ECU automatically switches to fail-safe mode if a fault is detected in any of these inputs:
^ Airflow meter
^ Atmospheric pressure sensor
^ EGR position sensor
^ Feedback system
^ Intake air thermosensors
^ Oxygen sensor
^ Throttle sensor
^ Water thermosensor

INPUTS/OUTPUTS
Individual components function as either input devices or output devices. Input devices modify or generate voltage signals that the ECU "reads". Output devices are components operated or controlled by the ECU. Not all of these devices are monitored by the on-board diagnostic system. Some input signals come from devices that are not specifically ECU sensor devices. For example, the A/C amplifier signals the ECU when the air conditioning is turned "ON", for idle speed control (to compensate for the additional load on the engine). In the case of many output devices (relays and solenoids), the ECU's output function is simply to make or break the ground circuit to turn a device "ON" or "OFF". The following is a list of input and output devices and signals.

INPUTS
^ A/C switch
^ Airflow meter
^ Atmospheric pressure sensor (built into the ECU)
^ Crankshaft pulley sensor (crank angle and engine speed)
^ Distributor sensors
-G1 signal (crankshaft position)
-Ne1 signal (engine speed)
^ EGR position sensor
^ Electrical load inputs
-Headlight switch
-Rear window defroster switch
-Heater - A/C blower switch
^ Idle switch
^ Ignition switch ("START" position - engine cranking)
^ Intake air thermosensor
^ Knock sensor
^ Oxygen sensor
-Left side
-Right side
^ Power steering pressure switch
^ Stop light switch
^ Test connector
^ Throttle sensor
^ Vehicle speed sensor
^ Water thermosensor (engine fan)
^ Water thermosensor (EGI)

OUTPUTS
^ A/C relay/cut-off
^ Bypass air control (BAC) valve
-Idle speed control (ISC) valve
^ Fuel injectors/injection timing
^ Igniter
^ Malfunction indicator lamp (MIL)
^ Oxygen sensor heater - right side
^ Oxygen sensor heater - left side
^ Solenoid valves
EGR - Vacuum side
EGR - Vent side
VRIS #1 [1]
VRIS #2 [1]
Pressure regulator control
Purge control

[1]Variable Resonance Induction System