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warm-up, so the computer must operate
“open loop” at that time.
Acceleration, deceleration and idle
conditions
As long as the engine and oxygen
sensor are hot, the computer can
operate “closed loop” for best economy
and least emissions During the drive
conditions listed above, the computer
may have to ignore the sensor and run
“open loop,” relying on internal program-
ming for fuel delivery instructions. During
idle, for example, the oxygen sensor may
cool down and stop sending a signal. A
different situation can occur during wide-
open-throttle acceleration. The computer
sometimes adds additional fuel (on
purpose) for temporary acceleration
power. The computer knows it is running
“rich” so it ignores the sensor signal until
the wide-open-throttle condition is over.
Other fuel control functions
Various systems may have the computer
controlling other aspects of air or fuel
delivery for performance enhancements.
These may include...
• Switchable air intake path length for
best high or low RPM performance.
• Variable valve timing.
• Using a “cold start” injector to aid start-
up.
• Controlling fuel pressure.
Refer to electronic control system
description in vehicle service manual for
details.
How the computer controls Idle
Speed
Early Systems:
Idle speed is mechanically set, but the
computer can increase it by a fixed
amount. A computer controlled solenoid
can open a small air passage bypassing
the closed throttle plate. The additional
air flow increases the idle speed by a
fixed amount. This idle speed boost is
necessary when engine loading results
from air conditioning, power steering or
similar demands. (Otherwise the engine
might stall.) Signals from such systems
tell the computer when engine loading is
about to occur. Some engines have
more than one such throttle air bypass.
Later Systems:
Throttle position and RPM sensors tell
the computer when the vehicle is idling.
(Sometimes an idle position switch on
the throttle is used.) The computer
watches RPM and adjusts an idle speed
control device on the vehicle to maintain
the desired idle condition. Note that this
is another example of “closed loop”
operation. The computer performs an
action (activating an idle control device),
then watches the results of its action
(engine RPM) and readjusts as
necessary until the desired idle speed is
achieved.
Idle speed is controlled by adjusting
throttle bypass air as in the early
systems. The difference is that the
computer can change idle speed by
varying amounts instead of a fixed
amount. One method uses a small
electric motor to adjust an air valve
opening in the bypass passage. The
other way uses a solenoid switched with
a “duty cycle” type signal from the
computer. Refer to Duty Cycle definition
in the Glossary (Section 14).
How the computer controls Spark
Advance Timing
You set spark timing in a non-computer
controlled ignition by using a timing light
and adjusting the distributor at idle RPM.
During vehicle operation, timing is
changed by either engine vacuum
(vacuum advance function) or by engine
RPM (centrifugal advance function.)
These spark timing changes are done
mechanically inside the distributor.
Computer controlled ignitions using a
distributor still have you set spark timing
by using a timing light and adjusting the
distributor at idle RPM. The timing
changes which occur during vehicle
operation, however, are controlled
electronically. The computer looks at
sensors to determine vehicle speed,
engine load and temperature. (RPM,
throttle position, coolant temperature
and manifold pressure, vane air flow or
Summary of Contents for Code Scanner CP9025
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