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mass air flow sensors are used.) Then,
the computer adjusts timing according to
factory programmed instructions. The
computer sends a timing signal to an
ignition module which eventually creates
the spark.
A simpler version uses a conventional
distributor with mechanical timing. Here,
the computer controls a solenoid to
change the vacuum advance timing by a
fixed amount when desirable.
Some vehicles have a "knock" sensor.
The computer can "fine tune" the spark
timing if this sensor signals an engine
knock condition.
Newer ignition systems have no
distributor. They are called
Distributorless or Direct Ignition Systems
(DIS). Sensors for camshaft and
crankshaft position, in addition to the
sensors mentioned before, are used by
the computer to determine spark timing.
Multiple coils wired to the spark plugs
are fired directly from computer signals.
EGR Valve
The EGR valve lets exhaust gases re-
enter the intake manifold and mix with
the incoming air/fuel. The presence of
exhaust gases reduces combustion
temperatures in the cylinders and this
reduces poisonous NO
computer controls the flow of gases
through the EGR valve. The EGR system
is only used during warm engine cruise
conditions. A partially open EGR valve at
other times can cause stalling.
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emissions. The
x
Air Injection System
This system works with the catalytic
convertor. The computer takes outside
air from an air pump and directs it to the
exhaust manifold as necessary for best
emission performance. (The extra air
helps partially burned exhaust gases to
completely burn and reduce pollution.)
Fuel Evaporation Recovery System
A special canister collects vapors
evaporating from the fuel tank, prevent-
ing them from escaping into the
atmosphere and causing pollution.
During warm engine cruise conditions,
the computer draws the trapped vapors
into the engine for burning.
The computer controls other odd jobs
like handling "speed control" and
transmission torque convertor lock-up
and shifting functions. Detailed explana-
tions may be found in your vehicle
service manual.
The Glossary describes various sensors
and actuators used in computer
controlled engine systems. You can learn
more by reading these definitions.
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