Nissan Pick-ups and Pathfinder 1989-1995

Mixture Ratio Feedback System (MRFS)



The need for better fuel economy coupled to increasingly strict emission control regulations dictates a more exact control of the engine air/fuel mixture. Nissan has developed a Mixture Ratio Feedback System (MRFS) in response to these needs.

The main function of the system is to control the air/fuel mixture exactly, so that more complete combustion can occur in the engine, and a 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 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 first started, and is still still cold. In this mode, the control unit ignores any signals from the oxygen sensor (because will not send reliable information until it is at operating temperature. Instead, the engine control computer calculates air/fuel mixture requirements based on input from other engine control sensors. 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 (along with the data from other engine sensors) to more precisely 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 special diagnostic equipment. Thus, they should be entrusted to a qualified dealer or Automotive Service Excellence (ASE) certified technician with access to the equipment and special training in this system's repair.

It should be noted that proper operation of the system is entirely dependent on the oxygen sensor. Thus, 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.