See Figures 1 through 6
Although not all of the fuel injection systems covered by this information were originally known as Programmed Fuel Injection (PGM-FI), the term was later applied to most, if not all of them. PGM-FI, has in fact become the industry's standard term when referring to any of Honda's fuel injection systems, and it is used in this information for all covered types of fuel injection.
Honda's Programmed Fuel Injection (PGM-FI) Systems consist of three sub-systems: air intake, electronic control, and fuel. Some of the vehicles are equipped with a multi-point type injection and others (available in 1988-91) may be equipped with a dual-point injection system, which is essentially a throttle body system. The testing of components is basically the same for both systems, except where noted.
In an effort to conform to an industry-wide move to standardize terminology, Honda has changed the names of a few of the PGM-FI components. These changed names will be noted throughout, but it should be understood the function of these components remains the same, regardless of their names. Also, other (slight) changes to the PGM-FI system over its ten years of existence will be noted where they occur.
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, is a 2-barrel side-draft type with the primary air horn at the top. To prevent icing of the throttle valves and air horn walls under certain atmospheric conditions, the throttle valve's air horn walls and the lower portion of the throttle body are heated by engine coolant. A throttle sensor is attached to the primary throttle valve to sense changes in throttle opening. A dashpot 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 (ECU, or from 1993-on, Engine Control Module, ECM) and various solenoid valves such as idle control, fast idle and air conditioning idle control solenoid valves. With the exception of the air conditioning idle control solenoid valve, these change the amounts of air bypassing into the air intake manifold. The air conditioning control solenoid valve opens the throttle when the air conditioner is turned on by signals sent from the ECU/ECM.
The Idle Control Solenoid Valve (ICSV) is used to compensate for idle speed reduction due to electrical, or other loads on the engine. The valve does this by bypassing additional air into the intake manifold. This additional air will allow the idle speed to increase to its normal speed (700-800 rpm). The operation depends upon changes in the voltage at the FR terminal of the alternator for quick response. The valve also lowers the fast idle speed in steps during warm-up, after the coolant temperature has reached 131°F (55°C). To prevent erratic running after the engine first fires, the valve is opened during cranking and immediately after starting to provide additional air into the intake manifold.
The Fast Idle Control Solenoid Valve prevents erratic running when the engine is warming up, and a higher idle speed is needed. When the atmospheric pressure is 26.4 in. Hg (660mm Hg) or less, the valve opens to bypass additional air into the intake manifold.
The air conditioning Idle Control Solenoid Valve maintains an idle speed of 700-800 rpm when the air conditioner is turned on. The valve causes the air conditioning idle control diaphragm to open the throttle valve which raises the idle speed. The valve is also opened when coolant temperature is low (immediately after starting) thereby ensuring stable idling regardless of position of the air conditioning switch.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 the throttle valve. While the idle control system is in operation, the idle screw has no effect on the idle speed.
Usually idle speed adjustment does not require turning the idle adjustment screw since idle speed is adjusted automatically by the operation of the idle control system. Idle speed does not change by turning the idle adjust screw while the idle control system is in operation.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 is bypassed 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 the correct amount, the control system must perform various separate functions. The Electronic Control Unit (ECU, or for 1993-95, Engine Control Module, ECM), the heart of the PGM-FI, uses an eight-bit microcomputer and consists of a Central Processing Unit (CPU), memory storage, and I/O (Input/Output) ports. Basic data stored in the memory 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 ECU is called the Engine Control Module (ECM) on 1993-95 models.
The control unit contains (in memory) information stored for the basic duration ("on time") of the injectors at various engine speeds and manifold pressures. Depending on the conditions detected by the various sensors (coolant temperature, air temperature, etc.) the basic signal is then modified to deliver the correct air/fuel duration. Other related functions include:
Crank Angle (TDC/CKP/CYP) Sensors
1.5L models produced before 1993 do not utilize the cylinder position sensor (CYP).
The sensors and distributor are designed as an assembly to save space and weight. The entire unit on pre-1992 1.5L models consists of a pair of rotors, TDC and CRANK sensors, and a pickup for each rotor. All Civic models with a 1.6L engine and all 1.5L models produced 1993-95 and later are equipped with a third sensor, the CYP Sensor. Since the rotors are coupled to the camshaft, they turn together as a unit as the camshaft rotates. The CYP sensor detects the position of the No. 1 cylinder as the base for sequential fuel injection to each cylinder, whereas the TDC sensor serves to determine the injection timing for each cylinder. The CKP sensor determines the timing for fuel injection and ignition of each cylinder and also detects engine speed. The TDC sensor is also used to detect engine speed to read out the basic fuel injector duration for different operating conditions.Manifold Absolute Pressure (MAP) Sensor
This 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 fuel injector duration from the memory.Atmospheric Pressure (PA) Sensor
Like the MAP sensor, this unit converts atmospheric pressures into voltage signals and sends them to the ECU/ECM. The signals then modify the basic fuel injector duration to compensate for changes in the atmospheric pressure.Coolant Temperature (TW) Sensor
The sensor uses a temperature dependent diode (thermistor) to measure differences in the coolant temperature. The basic fuel injector duration is read out by the signals sent from this sensor through the ECU/ECM. The resistance of the thermister decreases with a rise in coolant temperature.Intake Air Temperature (IAT) 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 quicker response. The basic fuel injector duration is read from the memory and again modified to compensate for different operating conditions by the signals sent from this sensor through the ECU/ECM.Throttle Position (TP) Sensor
This sensor is essentially a variable resistor (potentiometer). 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 (O2S)
See Figures 7 and 8
Models produced 1992-95 have a heater unit attached to the same type oxygen sensor, thereby making it a Heated Oxygen Sensor (HO2S). The heater was added when Honda found it stabilized the sensor's output. The description following for the two units is in every other respect the same.
The oxygen sensor, by detecting the oxygen content in the exhaust gas, helps the ECU/ECM decide how much fuel is necessary to maintain the stoichiometric air/fuel ratio. During 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.
The sensor is located in the control box. The primary objective of this unit is to maintain the correct air/fuel ratio at idling. No adjustment of the IMA sensor is necessary as the feedback control is performed by the oxygen sensor even during idling.Starter Switch
The air/fuel mixture must be rich for starting. During cranking, the ECU/ECM detects the 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.
The fuel pump is an inline, direct-drive type. Fuel is drawn into the pump through a filter, flows around the armature through the one-way valve and is delivered to the engine compartment. A baffle is provided to prevent fuel pulsation. The fuel pump has a relief valve to prevent excessive pressure. It opens if there is a blockage in the discharge side. When the relief valve opens, fuel flows from the high pressure to the low pressure side. A check valve is provided to maintain fuel pressure in the line after the pump is stopped. This is to ease restarting.
The pump section is composed of a rotor, rollers and pump spacer. When the rotor turns, the rollers turn and travel along the inner surface of the pump spacer by centrifugal force. The volume of the cavity enclosed by these three parts changes, drawing and pressurizing the fuel.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 (40-47 psi or 280-330 kPa for 1992-95 models). When the difference between the fuel pressure and manifold pressure exceeds 36 psi (248 kPa) (or 40-47 psi / 280-330 kPa for 1992-95 models), the diaphragm is pushed upward, and the excess fuel is fed back into the fuel tank through the return line.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 was reduced to lower 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.