Programmed Fuel Injection (PGM-FI) System consists of three sub-systems: air intake, electronic control, and fuel.Air Intake System
The system supplies air for all engine needs. It consists of the air cleaner, air intake pipe, throttle body, idle control system, fast idle mechanism, and intake manifold. A resonator in the air intake pipe provides additional silencing as air is drawn into the system.THROTTLE BODY
The throttle body, either a side-draft type or down-draft type, depending on the engine, with the primary air horn at the top. To prevent icing of the throttle valves and air horn walls, under certain atmospheric conditions, the lower portion of the throttle body is heated by engine coolant. A throttle sensor is attached to the primary throttle valve to sense changes in throttle opening. A dash pot is used to slow the throttle as it approaches the closed position.IDLE CONTROL SYSTEM
The air/fuel ratio during idling is controlled by the electronic control unit and various solenoid valves such as Electronic Air Control Valve (EACV), fast idle valve and air boost valves. With the exception of the A/C idle control solenoid valve, these change the amounts of air bypassing into the air intake manifold. The A/C control solenoid valve opens the throttle when the air conditioner is turned on by signals sent from the ECU.BYPASS CONTROL SOLENOID VALVE (BPCSV)
When engine rpm is below 5000 rpm, the BPCSV directs air flow through the long intake path for higher torque. At higher speeds, intake air flow is through the shorter path to reduce resistance to air flow.IDLE ADJUSTER (BYPASS CIRCUIT)
Fuel cut-off takes place at a set position or angle of the throttle valve. If the throttle valve is moved to adjust idle speed, this position or angle will be changed and the system may not cut off fuel supply. To solve this problem, the throttle body contains an adjustable bypass circuit. This circuit is designed to control the amount of air bypassing into the intake manifold without changing the position of throttle valve. The idle speed usually does not require adjustment. When the idle control system is in operation, the idle adjustment screw has no effect on the idle speed.FAST IDLE MECHANISM
To prevent erratic running when the engine is warming up, it is necessary to raise the idle speed. The air bypass valve is controlled by a thermowax plunger. When the thermowax is cold, the valve is open. When the thermowax is heated, the valve is closed. With the engine cold and the thermowax consequently cold, additional air bypasses into the intake manifold so that the engine idles faster than normal. When the engine reaches operating temperature, the valve begins to close, reducing the amount of air bypassing into the manifold.Electronic Control System
In order to get fuel into the cylinders at the correct instant and in correct amount, the control system must perform various separate functions. The Electronic Control Unit (ECU, or on 1992-later cars, Engine Control Module, ECM), the heart of the PGM-FI, uses an eight-bit microcomputer and consists of a CPU (Central Processing Unit), memories, and I/O (Input/Output) ports. Basic data stored in the memories are compensated by the signals sent from the various sensors to provide the correct air/fuel mixture for all engine needs.ELECTRONIC CONTROL UNIT (ECU)
The unit contains memories for the basic discharge duration at various engine speeds and manifold pressures. The basic discharge duration, after being read out from the memory, is further modified by signals sent from various sensors to obtain the final discharge duration. Other functions also include:
CRANK ANGLE SENSOR (TDC/CYL SENSORS)
The sensors and distributor are designed as an assembly to save space and weight. The entire unit consist of a pair of rotors, TDC and CYL, and a pickup for each rotor. Since the rotors are coupled to the camshaft, they turn together as a unit as the camshaft rotates. The CYL sensor detects the position of the No. 1 cylinder as the base for the Sequential Injection whereas the TDC sensor serves to determine the injection timing for each cylinder. The TDC sensor is also used to detect engine speed to read out the basic discharge duration for different operating conditions.
Most 1990 vehicles, incorporate the TDC sensor, CYL sensor and Crank sensor in a single assembly. The function of these components does not change, but if any single sensor is defective, the entire distributor must be replaced as an assembly.MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP SENSOR)
The sensor converts manifold air pressure readings into electrical voltage signals and sends them to the ECU/ECM. This information with signals from the crank angle sensor is then used to read out the basic discharge duration from the memory.ATMOSPHERIC PRESSURE SENSOR (PA SENSOR)
Like the MAP sensor, the unit converts atmospheric pressures into voltage signals and sends them to the ECU. The signals then modify the basic discharge duration to compensate for changes in the atmospheric pressure.COOLANT TEMPERATURE SENSOR (TW SENSOR)
The sensor uses a temperature-dependent diode (thermistor) to measure differences in the coolant temperature. The basic discharge duration is read out by the signals sent from this sensor through the ECU. The resistance of the thermister decreases with a rise in coolant temperature.INTAKE AIR TEMPERATURE SENSOR (TA SENSOR)
This device is also a thermistor and is placed in the intake manifold. It acts much like the water temperature sensor but with a reduced thermal capacity for for quicker response. The basic discharge duration read out from the memory is again compensated for different operating conditions by the signals sent from this sensor through the ECU/ECM.THROTTLE ANGLE SENSOR
This sensor is essentially a variable resistor. In construction, the rotor shaft is connected to the throttle valve shaft such that, as the throttle valve is moved, the resistance varies, altering the output voltage to the control unit.OXYGEN SENSOR
The oxygen sensor, by detecting the oxygen content in the exhaust gas, maintains the stoichiometric air/fuel ratio. In operation, the ECU/ECM receives the signals from the sensor and changes the duration during which fuel is injected. The oxygen sensor is located in the exhaust manifold.
The sensor is a hollow shaft of zirconia with a closed end. The inner and outer surfaces are plated with platinum, thus forming a platinum electrode. The inner surface or chamber is open to the atmosphere whereas the outer surface is exposed to the exhaust gas flow through the manifold.
Voltage is induced at the platinum electrode when there is any difference in oxygen concentration between the two layers of air over the surfaces. Operation of the device is dependent upon the fact that voltage induced changes sharply as the stoichiometric air/fuel ratio is exceeded when the electrode is heated above a certain temperature.
Some 1990 models, use a heated oxygen sensor. The heater stabilizes the sensor's outputs and allow the sensor to heat quicker after the engine as been started.STARTER SWITCH
The air/fuel mixture must be rich for starting. During cranking, the ECU/ECM detects signal from the starter switch and increases the amount of fuel injected into the manifold according to the engine temperature. The amount of fuel injected is gradually reduced when the starter switch is turned OFF .Fuel System
The fuel pump is a compact impeller design and is installed inside the fuel tank, thereby saving space and simplifying the fuel line system.
The fuel pump is comprised of a DC motor, a circumference flow pump, a relief valve for protecting the fuel line systems, a check valve for retaining residual pressure, an inlet port, and a discharge port. The pump assembly consists of the impeller (driven by the motor), the pump casing (which forms the pumping chamber), and cover of the pump.PRESSURE REGULATOR
The fuel pressure regulator maintains a constant fuel pressure to the injectors. The spring chamber of the pressure regulator is connected to the intake manifold to constantly maintain the fuel pressure at 36 psi (248 kpa) higher than the pressure in the manifold. When the difference between the fuel pressure and manifold pressure exceeds 36 psi (248 kpa), the diaphragm is pushed upward, and the excess fuel is fed back into the fuel tank through the return line.FUEL INJECTOR
The injector is of the solenoid-actuated constant-stroke pintle type consisting of a solenoid, plunger, needle valve and housing. When current is applied to the solenoid coil, the valve lifts up and pressurized fuel fills the inside of the injector and is injected close to the intake valve. Because the needle valve lifts and the fuel pressure are constant, the injection quantity is determined by the length of time that the valve is open, i.e., the duration the current is supplied to the solenoid coil. The injector is sealed by an O-ring and seal ring at the top and bottom. These seals also reduce operating noise.RESISTOR
The injector timing, which controls the opening and closing intervals, must be very accurate since it dictates the air/fuel mixture ratio. The injector must also be durable. For the best possible injector response, it is necessary to shorten the current rise time when voltage is applied to the injector coil. Therefore, the number of windings of the injector coil is reduced to reduce the inductance in the coil. This, however, makes low resistance in the coil, allowing a large amount of current to flow through the coil. As a result, the amount of heat generated is high, which compromises the durability of the coil. Flow of current in the coil is therefore restricted by a resistor installed in series between the electric power source and the injector coil.MAIN RELAY
The main relay is a direct coupler type which contains the relays for the electronic control unit power supply and the fuel pump power supply. This relay is installed at the back of the fuse box.