Excelsior-HENDERSON HCX 1999, Service Handbook

Page: 59 / 474

ENGINE CONTROL SYSTEM
6999-0003 7/15/99
4-9
OPTIMIZING COMBUSTION
Oxygen content in the exhaust is a measure of combus-
tion efficiency. The number two exhaust header is
equipped with an oxygen sensor that provides feedback
to the ECM about how much oxygen is present in the
exhaust. At startup, the ECM ignores feedback from the
oxygen sensor. This is called open loop. As the engine
and the sensor warm to operating temperature, the ECM
uses the oxygen sensor feedback to maximize combus-
tion. This is called closed loop. When accelerating or
decelerating, the ECM ignores the oxygen sensor feed-
back, changing to open loop operation.
At operating temperature, the oxygen sensor measures
oxygen in the exhaust and transmits a “lean” signal for
high oxygen content (< 0.5V) or a “rich” signal for low
oxygen content (> 0.5V). The ECM compensates by
increasing or decreasing the injection duration based on
these signals. This causes the calculated air-to-fuel ratio
to rise and fall about the ideal air-to-fuel ratio for the cur-
rent conditions.
The ECM begins closed loop operations with an initial
air-to-fuel ratio about which to make fueling adjust-
ments. As the engine runs and conditions change, the
ECM learns and stores a new average air-to-fuel ratio.
The difference between the initial value and the new
average is called the
long term fuel trim . The ECM
allows the long term fuel trim to increase and decrease
within certain limits. Long term fuel trim values beyond
these limits indicate an electrical or mechanical failure in
the fuel injection system. Use the Examinator diagnostic
tool to reset the long term fuel trim after work is done on
the fuel injection system.