Nissan Stanza/200SX/240SX 1982-1992 Repair Guide

Mixture Ratio Feedback System


The need for better fuel economy coupled to increasingly strict emission control regulations dictates a more exact control of the engine air/fuel mixture. Datsun/Nissan has developed a Mixture Ratio Feedback System in response to these needs. The system is installed on all 200SX, 1984 and later Stanza models and 240SX models.

The principle of the system is to control the air/fuel mixture exactly, so that 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. The stoichiometric ratio is 14.7:1 (air to fuel). At that point, the converter's efficiency is greatest in oxidizing and reducing HC, CO, and NOx into CO 2 , H 2 O, O 2 , and N 2 .

Components used in the system include an oxygen sensor, installed in the exhaust manifold upstream of the converter, a three way oxidation reduction catalytic converter, an electronic control unit, and the fuel injection system itself.

The oxygen sensor reads the oxygen content of the exhaust gases. It generates an electric signal which is sent to the control unit. The control unit then decides how to adjust the mixture to keep it at the correct air/fuel ratio. For example, if the mixture is too lean, the control unit increases the fuel metering to the injectors. The monitoring process is a continual one, so that fine mixture adjustments are going on at all times.

The system has two modes of operation: open loop and closed loop. Open loop operation takes place when the engine is still cold. In this mode, the control unit ignores signals from the oxygen sensor and provides a fixed signal to the fuel injection unit. Closed loop operation takes place when the engine and catalytic converter have warmed to normal operating temperature. In closed loop operation, the control unit uses the oxygen sensor signals to adjust the mixture. The burned mixture's oxygen content is read by the oxygen sensor, which continues to signal the control unit, and so on. Thus, the closed loop mode is an interdependent system of information feedback.

Mixture is, of course, not readily adjustable in this system. All system adjustments require the use of a CO meter. Thus, they should be entrusted to a qualified technician (ASE certified) with access to the equipment and special training in the system's repair. The only regularly scheduled maintenance is replacement of the oxygen sensor at 30,000 mile intervals. This procedure is covered in the following section.

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.