Nissan 240SX/Altima 1993-1998 Repair Guide

General Information


See Figure 1

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Fig. Fig. 1: Closed loop control provides the best air/fuel mixture ratio for driveability and emission control

The amount of fuel injected from the fuel injector is determined by the ECM. The ECM controls the length of time the valve remains open (injection pulse duration). The amount of fuel injected is a program value in ECM memory. The program value is preset by the engine operating conditions. These conditions are determined by input signals (for engine speed and intake air) from both the camshaft position sensor and the mass air flow sensor.

In addition, the amount of fuel injected is compensated to improve engine performance under the following various operating conditions:

Fuel is increased:

During warm-up
When starting the engine
During acceleration
During hot engine operation
When the selector lever is changed from N to D on an automatic transaxle/transmission equipped vehicles
During high load, high speed operation

Fuel is decreased:

During deceleration
During high speed operation

The mixture ratio feedback system provides the best air/fuel mixture ratio for driveability and emission control. The three-way catalyst can then better reduce CO, HC and NOx emissions. This system uses an oxygen sensor in the exhaust to determine if the engine is rich or lean. The ECM adjusts the injection pulse width according to the sensor voltage signal. This maintains the mixture ratio within the range of stoichiometric (ideal air/fuel mixture). When in mixture ratio feedback, the engine is said to be in closed loop control.

On some models, a rear oxygen sensor is located downstream of the three-way catalyst. Even if the switching characteristics of the front oxygen sensor shift, the air/fuel ratio is controlled to stoichiometric by the rear oxygen sensor.

Under the conditions below, the ECM will switch to open loop operation to maintain stabilized fuel combustion.

Deceleration and acceleration
High load and high engine speed operation
Engine idling
Malfunction of the oxygen sensor or its circuit
Insufficient activation of the oxygen sensor or its circuit under low engine coolant temperature
High engine coolant temperature
After shifting from N to D
During warm-up
When starting the engine

The mixture ratio feedback control system monitors the mixture ratio signal transmitted from the front oxygen sensor. The feedback signal is then sent to the ECM. The ECM controls the basic mixture ratio as close to the theoretical (optimal) mixture ratio as possible. However, the basic mixture ratio is not necessarily controlled as originally designed. Both manufacturing differences and characteristic changes (such as injector clogging) during operation directly affect mixture ratio.

Accordingly, the difference between the basic and theoretical mixture ratios is monitored in this system. This is then computed in terms of injection pulse duration to automatically compensate for the difference between the two ratios.

Fuel trim refers to the feedback compensation value compared against the basic injection duration. Fuel trim includes short-term fuel trim and long-term fuel trim.

Short-term fuel trim is the short-term fuel compensation used to maintain the mixture ratio at its theoretical value. The signal from the front oxygen sensor indicates whether the mixture ratio is rich or lean, compared to the theoretical value. The signal then triggers a reduction in the fuel volume if the mixture ratio is rich, and increases the fuel volume if it is lean.

Long-term fuel trim is overall fuel compensation carried out long-term to compensate for continual deviation of the short-term trim from the central value. Such deviation will occur due to individual engine differences, wear over time, and changes in the usage environment.

Two types of fuel injection timing are used. Sequential multi-port fuel injection refers to fuel that is injected into each cylinder during each engine cycle, according to the firing order. This system is used when the engine is running.

Simultaneous multi-port fuel injection refers to fuel that is injected simultaneously into all four cylinders, twice each engine cycle. In other words, pulse signals of the same width are simultaneously transmitted from the ECM. The injectors will then receive the signals two times for each engine cycle. This system is used when the engine is being started and/or if the system is operating in the fail safe mode.

Fuel shutoff, where fuel to each cylinder is cut, occurs during deceleration or operation of the engine at excessively high speeds.