Subaru LEGACY 2002 Quick Reference Index page 212

Fuel Injection (Fuel System)
3. AIR-FUEL RATIO FEEDBACK FACTOR
The ECM creates this factor utilizing the front oxygen sensor signal. When the signal voltage is
low, the air-fuel ratio is richer than the stoichiometric ratio. The ECM then makes the fuel injection
duration shorter by modifying the factor. When the voltage is high showing that the mixture is lean,
the ECM modifies the factor to make the injection duration longer. In this way, the air-fuel ratio is
maintained at a level close to the stoichiometric ratio at which the three-way catalyst acts most
effectively.
Injection duration increment signal
Injection duration decrement signal
(1) Front catalyst
(2) Exhaust gas
(3) Front oxygen (A/F) sensor
4. LEARNING FEATURE
The air-fuel ratio feedback control includes a learning feature which contributes to more accurate
and responsive control.
! In the air-fuel ratio feedback control, the ECM calculates the necessary amount of correction
based on data from the oxygen sensor and adds the result to the basic duration (which is stored
in the ECM's memory for each condition defined by the engine speed and various loads.)
! Without a learning feature, the ECM carries out the above-mentioned process every time. This
means that if the amount of necessary correction is large, the air-fuel ratio feedback control be-
comes less responsive and less accurate.
! The learning feature enables the ECM to store the amount of correction into memory and add
it to the basic fuel injection duration to create a new reference fuel injection duration. Using the
reference duration as the basic duration for the injection a few times later, the ECM can reduce the
amount of correction and thus make its feedback control more accurate and responsive to chang-
es in the air-fuel ratio due to difference in driving condition and sensor/actuator characteristics that
may result from unit-to-unit variation or aging over time.
CONTROL SYSTEM
Injection duration increases
Injection duration decreases
(4) Fuel injector
(5) Combustion chamber
FU-32
B2H0989C