The EFC system is essentially an emissions control system which utilizes an electronic signal, generated by an exhaust gas oxygen sensor, to precisely control the air-fuel mixture ratio in the carburetor. This allows the engine to produce exhaust gases of the proper composition to permit the use of a three-way catalyst. The three-way catalyst is designed to convert the three pollutants (1) Hydrocarbons (HC), (2) Carbon Monoxide (CO), and (3) Oxides of Nitrogen (NO x ) into harmless substances.
There are two operating modes in the EFC system:
When the engine is cold, the system will be operating in the open loop mode. During that time, the air fuel ratio will be fixed at a richer level. This will allow proper engine warm-up. Also during this period, air injection (from the air injection pump) will be injected upstream in the exhaust manifold.
The computer has been programmed to monitor various component systems. If a malfunction of a system occurs, a fault code will be stored in the computer. If the problem is repaired or corrects itself, the computer will erase the code after thirty ignition cycles.
In order to read the computer and find the fault code, diagnostic readout tool C-4805 or equivalent will be needed. If a fault code appears on the read-out tool, perform a careful visual check of all vacuum lines and electrical connections before doing any repairs to the system. The diagnostic read-out tool can be used to check out three different problem modes. They are the diagnostic test mode, circuit actuation test mode and switch test mode. The diagnostic test mode is used to retrieve fault codes that are stored in the computer. The circuit actuation test mode is used to analyze various systems. The switch test mode is used to test various switch circuits.Diagnostic Mode
- Install the readout tool to the wiring connector which is located on the left shock tower.
- Position the read/hold switch on the tool to the read position.
- Open the carburetor switch by placing the fast idle screw on the highest step of the fast idle cam.
- Position the ignition switch in the RUN position and wait for fault code 00 to appear on the tool display screen.
- Move the read/switch to the hold position.
Record all fault codes that may appear. The display fault codes may be stopped by switching the tool to the read position. Fault codes will continue when the tool is switched back to the hold position.
CODE 00-indicates that the diagnostic readout tool is functioning and receiving power
- Put the system into diagnostic mode testing and wait for code 55 to appear on the tool test screen.
- Press the ATM function button on the tool to activate the display. If a specific ATM test is desired, hold the button down until the proper test code appears.
- The computer will continue to turn the selected circuit on and off for as long as five minutes or until the ATM button is pressed again or if the ignition switch is turned OFF .
If the ATM button is not pressed again, the computer will continue cycling the selected circuit for five minutes and then shut the system off. Turning the ignition switch
will also shut the test mode off.
Code description is as follows:
CODE 91-oxygen feedback solenoid activated
- Put the system into diagnostic mode testing and wait for code 55 to appear on the test screen.
- Check that both air conditioning and heated rear window switches are in the off position.
- Press the ATM button on the tool and immediately move the read/hold switch to the read position.
- Wait for code 00 to appear on the test screen.
Turn the air conditioning switch to the on position. If the computer is receiving input, the display will change to 88 when the switch is turned on and switch back to 00 when the switch is turned off. Repeat the test for the heated rear window switch.Code description is as follows:
CODE-00 air conditioning and heated rear window switches are off
The computer will automatically erase the code(s) thirty ignition cycles after the problem has been repaired. The code(s) may also be erased manually by disconnecting the negative battery cable for 30 seconds.