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Old 01-26-2011, 12:53 AM   #2
Scooby Newbie
Member#: 250463
Join Date: Jun 2010
Location: Evanston, Il
2010 Impreza 2.5i


Not stupid.... Uninformed?

On a non-drive-by-wire fuel injected car when you let off the gas the throttle plate closes completely. In order for the car to idle they manufactured a small port on either side of the plate and use an IACV assembly to connect the two. Inside the IACV is a small plunger that is driven by a motor. The plunger is moved back and forth to block off or clear the passage as needed for the amount of airflow required so that the car can idle. The motor is controlled by the ECU. The ECU uses data from all the other sensors (MAF, ECT, O2, etc.) to determine the amount of air the engine needs and then moves the plunger accordingly.

I can't quite remember exactly how it works but the ECU counts the number of polarity changes that happen inside the electric motor as the windings get switched and interprets that as counts or steps (the IACV is considered a stepper motor. Memory power seats work the same way). In the old days disconnecting the battery would make the car forget what "step" it was on and then when you tried to restart the car it would idle like crap until it learned where it was and where it needed to be.

There is no adjustment (the Evap sticker under the hood tells you this), the computer controls where it's set at. There is no power to be gained from messing with the IACV as it only controls the idle and is responsible for a very small port in the throttle body.
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Wrencher86 is offline   Reply With Quote