Actron GM Code Scanner CP9001 - Manual (Page 29)

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The computer needs to know:
• ...what the engine operating
condition is.
Sensors used: coolant temperature,
throttle position, manifold absolute
pressure, mass air flow, RPM.
• ...how much air is coming into the
engine.
Sensors used: mass air flow or a
combination of manifold absolute
pressure, manifold air temperature,
RPM.
• ...how much fuel is being delivered.
The computer knows this by how
long it turns on the fuel injectors.
(The computer uses a “feedback
control” or “duty cycle” solenoid on
electronic controlled carburetors.)
• ...that everything is working the way
it should.
Sensor used: exhaust gas oxygen
sensor.
Note: Not all engines use every sensor
listed above.
Cold engine warm-up condition
An example of “Open Loop” opera-
tion...
The coolant temperature sensor tells
the computer how warm the engine is.
Factory engineers know what the best
air/fuel mixture is for the engine at
various operating temperatures. (More
fuel is needed for a cold engine.) This
information is permanently pro-
grammed into the computer. After the
computer knows the engine tempera-
ture, it determines the amount of air
coming in, then it will look at its
programming to find out how much fuel
to deliver and operate the fuel injectors
accordingly. (Engines with electronic
carburetors don’t do any of this. They
have a thermostatically controlled
choke just like non-computer engines.)
This process is an example of “Open
Loop” operation by the computer. The
control system performs an action
(expecting a certain result), but has no
way of verifying if the desired results
were achieved. In this case, the
computer operates a fuel injector
expecting a certain amount of fuel to
be delivered. (The computer assumes
everything in the fuel system is
operating as expected.) In open loop
operation, the computer has no way of
checking the actual amount of fuel
delivered. Thus, a faulty fuel injector or
incorrect fuel pressure can change the
amount of fuel delivered and the
computer would not know it.
The computer system is forced to
operate “open loop” because no
sensor type is available which can
measure air/fuel ratios when the
engine is cold.
Hot engine cruise condition
An example of “Closed Loop”
operation...
The computer watches the coolant
temperature and throttle position
sensors to tell when the engine is all
warmed up and cruising. As before,
the computer determines the amount
of air coming into the engine, then
delivers the amount of fuel that should
provide the optimum air/fuel mixture.
The big difference is that this time the
computer uses the oxygen sensor to
check how well its doing and re-adjust
things, if needed, to make sure the fuel
delivery is correct. For example: If the
oxygen sensor indicates a “rich”
condition, the computer will compen-
sate by reducing fuel delivery until the
oxygen sensor signals an optimum air/
fuel ratio. Likewise, the computer will
compensate for a “lean” condition by
adding fuel until the oxygen sensor
once again signals an optimum air/fuel
mixture.
This is an example of “Closed Loop”
operation. The control system
performs an action (expecting a
certain result), then checks the results
and corrects its actions (if necessary)
until the desired results are achieved.
The oxygen sensor only works when it
is very hot. During cold engine warm-
up, and sometimes at idle, the sensor
will be too cool to operate (no signal
sent). The computer must operate
"open loop" during this time because it
cannot use the sensor to check the air/
fuel ratio.