Nissan Sentra/Pulsar/NX 1982-1996 Repair Guide

Mixture Ratio Feedback System



The need for better fuel economy, combined with increasingly strict emission control regulations, required a more exact control of the engine air/fuel mixture. The manufacturer has developed this system which is installed on most 1984 and later models. The system consists of the following sensors.

Air Temperature Sensor
Air/Fuel Mixture Solenoid
Crank Angle Sensor
Inhibitor Switch
Neutral/Clutch Switch
Oxygen Sensor
Throttle Valve Switch
Vacuum Sensor
Vehicle Speed Sensor
Water Temperature Sensor

The principle of the system is to control the air/fuel mixture exactly, so that a more complete combustion can occur in the engine and more thorough oxidation and reduction of the exhaust gases can occur in the catalytic converter. The object is to maintain a stoichiometric air/fuel mixture, which is chemically correct for theoretically complete combustion.

It should be noted that proper operation of the system is entirely dependent on the oxygen sensor. Thus, if the sensor is not replaced at the correct interval or if the sensor fails during normal operation, the engine fuel mixture will be incorrect, resulting in poor fuel economy, starting problems or stumbling and stalling of the engine when warm.


See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: ECM connector terminal identification-all except California emission equipped models

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Fig. Fig. 2: ECM connector terminal identification-California emission equipped models

Make sure the ignition switch is OFF before disconnecting any harness connectors. Disconnect the negative battery cable before disconnecting the ECU harness connector. Voltage spikes can damage the electronic components. Make sure the ECU (Electronic Control Unit) connector is fully seated during inspection.

Do not condemn the electronic engine controls before fully checking the basics. Many electronic parts are replaced only to find out it is something simple. Electronic parts are usually not returnable. Make sure the component is defective before purchasing.

  1. Check the engine for air and vacuum leaks. Most engines should draw about 15-20 inch Hg. at idle. The needle should be steady.
  3. Check the air filter for clogging.
  5. Inspect the dipstick, AB valve hose, air induction hose, intake manifold gasket, valve cover gasket, EGR gasket and oil filler cap for leaks.
  7. Check the EGR valve seat and operation. A malfunction EGR can cause no or abnormal idle.
  9. Check the fuel supply to the carburetor and make sure the fuel is at the proper octane. Old fuel can cause driveability problems that are difficult to diagnose.
  11. Check the fuel filter for clogging.
  13. With the engine idling, check the fuel level through the glass window on the carburetor bowl. The level should be at the middle dot. If not correct, remove the top of the carburetor and adjust the float level as outlined in Fuel System .
  15. Perform ignition tests with an oscilloscope. All firing voltages should be in the same range. Check the secondary ignition system with a spark tester.
  17. Check and adjust the ignition to the proper specification in Engine Electrical .
  19. Check all engine related electrical harness connectors, including the ECU. Loose or corroded terminals can cause many driveability problems.