Test 6: Oxygen Sensor And Closed Loop Test
- With engine off, remove air cleaner and plug manifold vacuum hoses. Start engine and operate until temperature gauge pointer is at midrange (or for 4 minutes). Connect positive probe of digital voltmeter (or voltmeter with greater than 1 megohm impedance) to pin 17 at harness side of MCU connector J2 and negative probe to pin 1 of connector J2 (ground).
- Increase engine speed to 1200 RPM and maintain. Close choke valve and watch voltmeter (close valve for at least 15 seconds but not more than 30 seconds. Reading should be greater than .6 volts. If not, go to step 5). If so go to next step.
- Operate engine for at least one minute. Increase engine speed to more than 1200 RPM. Hold the choke valve closed for at least 15 seconds and turn the ignition off with valve closed. Observe the stepper motor metering pins. Pins should be in lean position (toward rear of engine). If so, go to next step. If pins are in a midway to full rich position replace MCU and retest stepper motor pin position.
- Start engine and operate for 40 seconds. Increase engine speed to more than 1000 RPM and load engine (turn A/C on, etc.) Observe stepper motor pin movement. Pins should move for and aft of a fixed position. If so, test is complete. If not, proceed to basic engine test.
- Disconnect oxygen sensor connector and test voltage at oxygen sensor. If greater than .6 volts, repair wiring harness between oxygen sensor and connector and MCU connector. If not, look for large air leak into manifold vacuum. If okay, replace oxygen sensor.