Fuel Always Lean
Components controlled by the MCU system in this test are the oxygen sensor, fuel control solenoid, MCU module, and thermactor air valve. Components which are not controlled by the MCU system, but could cause code fault are: faulty carburetor, low fuel pressure (kinked fuel line or faulty fuel pump), vacuum leaks (EGR valve and gasket, head gasket or cracked hoses), bowl vent, and/or canister purge solenoid. Anything which adds air or restricts fuel should be investigated as possible cause.
After starting engine, idle engine for at least 2 minutes before testing. Disconnect jumper wire from "trigger socket".
- Turn key off. Disconnect harness connectors at MCU, fuel control solenoid and oxygen sensor. Turn all accessories off. Measure resistance between ground and MCU connector pins 3 and 23. If resistance is 1000 ohms or less in either case, repair short and retest. If resistance is greater than 1000 ohms, go to next step.
- Check continuity and measure resistance between MCU connector pin 3 and oxygen sensor connector, then between MCU pin 23 and fuel control solenoid connector (FCS). If resistance is greater than 5 ohms, repair wire and retest. If 5 ohms or less, go to next step.
- Reconnect MCU and oxygen sensor. Disconnect harness from FCS. Check resistance at FCS terminal. See View A in Fig 1 . Resistance should be between 28-66 ohms. If resistance is within range, proceed to next step. If resistance is not within range, replace FCS and retest.
- Reconnect FCS and connect positive lead of voltmeter to back of solenoid harness connector (circuit 687) and negative lead to ground. See View B in Fig 1 . Turn key on, but do not start engine. If voltage is more than 10 volts, go to next step. If voltage is 10 volts or less, repair open circuit and retest.
- Turn ignition off. Connect negative lead of voltmeter to back of solenoid harness connector (circuit 95) and positive lead to battery positive terminal. See View C in Fig 1 . Start engine and idle at 2500 RPM. Observe voltmeter after 55 seconds. If voltage is 10 volts or less, proceed to next step. If voltage is greater than 10 volts, replace MCU and retest.
- Disconnect and plug thermactor air supply hose at air pump. Raise engine speed to 2500 RPM and hold choke 3/4 closed to force the system rich. With voltmeter still connected to FCS, observe voltage after 55 seconds. If voltage is 10 volts or less, proceed to next step. If voltage is more than 10 volts, MCU system is operating properly. Check non-controlled components.
- Turn key off. Disconnect oxygen sensor connector and check resistance between harness side of connector and ground. The resistance should be checked at Circuit 94. Leave harness connected at MCU module. If resistance is 1000 ohms or less, replace MCU module. If resistance is greater than 1000 ohms, go to next step.
- With oxygen sensor disconnected, start engine. With engine idling, connect a jumper wire to harness side of oxygen sensor connector. Be sure this connection cannot contact ground. Connect other end of jumper to battery positive terminal, then raise engine speed to 2500-2800 RPM. Measure voltage at FCS after 55 seconds. If voltage is 10 volts or less, replace MCU and retest. If voltage is more than 10 volts, replace oxygen sensor and retest.