DESCRIPTION
Electronically Fuel Controlled Carburetor (EFC) system is a computerized emission and fuel control system. The EFC system controls engine operation and lowers exhaust emissions while maintaining good fuel economy and driveability. Electronic Control Module (ECM) is the "brain" of the EFC system. The ECM controls many engine related systems to constantly adjust engine operation.
The EFC system is primarily an emission control system, designed to maintain an ideal air/fuel ratio of 14.7:1 under all operating conditions. When the ideal ratio is maintained, the catalytic converter can control Carbon Monoxide (CO), Hydrocarbon (HC) and Nitrogen Oxide (NOx) emissions.
Subaru EFC System. Scheme 24
OPERATION
The EFC system consists of the following components: Oxygen (O2) sensor, Electronic Control Module (ECM), 2 duty solenoid valves, vacuum sensor and solenoid, thermosensor, engine speed sensor and altitude compensator.
OXYGEN SENSOR (O2)
The O2 sensor is a concentrated cell that generates voltage according to the oxygen content within exhaust gases. The voltage created is greater when oxygen content is low, and lesser when oxygen content is high.
ELECTRONIC CONTROL MODULE (ECM)
ECM receives signals from O2 sensor, thermosensor and engine speed sensor to adjust Air/fuel ratio to proper level.
Air/fuel ratio is judged to be rich when voltage produced by O2 sensor is higher than a set specified level. As a result, ECM signals duty solenoid to pass a greater quantity of air into carburetor. This additional air leans air/fuel mixture.
Air/fuel ratio is judged to be lean when voltage produced by O2 sensor is lower than a set specified level. As a result, ECM issues signals to duty solenoid to pass a less air into carburetor. The elimination of air enriches air/fuel mixture.
DUTY SOLENOID VALVE
A duty solenoid, is installed in carburetor. Signals received from the ECM, causes duty solenoid valve to repeat opening and closing in short cycles. The valve is equipped with a control air bleed and a control fuel jet. When current flows through the valve, a plunger inside moves down. This opens port to control air bleed and closes port to control fuel jet.
When there is no current a spring moves plunger up. This closes port to air bleed and opens port to fuel jet. These ports are routed to both the slow and main ports on primary side of carburetor.
VACUUM (PRESSURE) SENSOR & SOLENOID VALVE
The vacuum sensor provides accurate sensing of changes within intake manifold. A solenoid valve is placed in pressure line between intake manifold and vacuum sensor. When vacuum solenoid valve is off, vacuum sensor measures vacuum (pressure) in the intake manifold. When solenoid valve is on, it senses atmospheric pressure. The ECM is feed this information and corrects air/fuel mixture. (Scheme 25)
Vacuum Sensor Solenoid Valve Other Solenoid Valves Are Similar. Scheme 25
ALTITUDE COMPENSATOR
Altitude compensator is used for compensation of air/fuel mixture in response to elevation by means of supplying additional air to control air passages. The solenoid valve opens when atmospheric pressure detected by pressure sensor is 12 psi (89 kPa.)
THERMOSENSOR & ENGINE SPEED SENSOR
These sensors provide coolant and engine RPM input to ECM so that better driveability is assured.
Air Injection System
Secondary (fresh) air drawn from air cleaner is supplied to exhaust ports of cylinder head. Also, a constant supply of secondary (fresh) air is supplied to exhaust port near upstream portion of oxidation catalyst by rear ASV. The ASV (rear) is mounted on exhaust pipe.
Catalytic Converter
All models equipped with EFC system use a 3-way catalytic converters. This type of converter is used to reduce CO, HC and NOx emissions which permits simultaneous oxidation and reduction.
DIAGNOSTIC TOOLS
The EFC system requires a circuit tester, stethoscope and a dry-cell battery for diagnosis. The circuit tester is used to measure voltage and resistance of check connectors. It is also used to measure resistance of duty solenoids and vacuum (pressure) sensor after lead connectors are unplugged. Stethoscope or vinyl tube is used to check operating sound from duty solenoid valves. The dry-cell battery is used to test O2 sensor and its wiring for defects.
Note. Insert circuit tester probes from the harness side of test connectors.
Float Chamber Ventilation (FCV), Idle-Up Solenoid, High Altitude Compensator (HAC) Vacuum Line Charging (VLC) Valves
- Check resistance between positive and negative terminals of valve. Standard resistance should be 16.2-19.8 ohms for FCV valve and 32.7-39.9 ohms for all others. If not within specifications replace valve(s).
- Check resistance between positive and negative terminals of valve and valve body. Resistance should be at least one milli-ohm. If not within specifications replace valve.
- Check vacuum passage for opening and closing operation while applying voltage to positive terminal of valve. Repeat test using negative terminal.
DIAGNOSTIC SYSTEM
ECM of EFC system is equipped with a self-diagnostic function. When trouble occurs, ECS (Electronic Control System) light is illuminated and the trouble code is displayed on O2 monitor light in code. (Scheme 26)
To read trouble codes, observe oxygen sensor monitor light. This light has two duration periods of illumination, one period long (1.2 seconds) and one period short (.3 seconds). The long period signifies the tens digit in a numbered code. Short period signifies the ones digit. Example: 3 long flashes and 5 short flashes, would signify a code 35.
It is possible to have more than one code present. In this case, the lowest number trouble code should be diagnosed first, the next highest numbered code second and so on (sequentially). Trouble codes will be repeated as long as the system is in diagnostic mode.
Oxygen Monitor Light. Scheme 26
Diagnostic Procedure
- Prior to diagnosis, ensure that all connectors, air hoses and vacuum hoses are properly connected. Also check all parts and electric wiring for scratches or damage. Be sure to turn ignition switch to "OFF" position before replacing a defective part. Reconnect all hoses and connectors before ignition switch is turned to "ON" position.
- Self-diagnosing function is available in 2 modes: Regular Mode and Diagnostic Mode. Regular Mode is a continual function. Diagnostic Mode is obtained by connecting Test Mode connectors. Diagnostic Mode connector must always be disconnected after performing tests.
Entering Diagnostic Mode
To activate diagnostic mode to receive trouble codes. Connect test connectors under dash (next to ECM box). (Scheme 26)
Exiting Diagnostic Modes
Regular mode is always active in EFC system. Diagnostic mode can only be selected by connecting test mode connectors. Disconnect test connector to exit diagnostic mode.
Clearing Trouble Codes
Trouble codes will be cleared from ECM memory only after malfunction has been repaired. Always perform diagnostic test after any repair to make sure that no other malfunction exists and that repairs solved original problem.
| Trouble Code | Component Affected |
|---|---|
| 11 | Ignition Pulse System (Engine OFF) |
| 12 | Back-up System |
| 14 | Duty Solenoid Valve System |
| 21 | Thermosensor System |
| 22 | VLC Solenoid Valve System |
| 23 | Pressure Sensor |
| 24 | Idle-Up Solenoid Valve |
| 25 | FCV Solenoid Valve Control |
| 32 | O2 Sensor |
| 33 | Car Speed Sensor System |
| 35 | Purge Control Solenoid |
| 41 | Main System Feedback System |
| 52 | Clutch Switch System |
| 62 | Idle-up System (Clearance Light and Rear Defogger) |
| 63 | Idle-up System (Heater Fan and Radiator Fan) |
LIST OF ECM TROUBLE CODES
