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  • Thanks for that.


    This is good sum-up from David Aiketgate

    Resetting the iacv (stepper motor) merely tells the ecu that the throttle butterfly is at 3.9% open.(Don't quote me on the actual figure)
    If your throttle is actually a way off 3.9% open, the IACV is confused and varies between step settings trying to achieve the correct idle.
    Hence the idle rising and falling, or being way too high..

    This is complicated because the stepper motor/IACV actually does have stepped settings and the ecu will try to get you to one of the steps. Step 21, I believe.

    If your base idle is wrong then this is an endless loop of the ecu using the stepper settings to try to get a set of correct readings.

    If you vaguely followed that, what you need to do is get the engine fully up to temperature after a good blast, come home let it idle, but disconnect the IACV plug. Let the idle settle for a few minutes and use torque app or pscan to see what the base idle is. It should be 850-875rpm. (Assuming no other reason ie vac leak, sensor problem.)
    If the idle is substantially above or below this then the stepper motor/iacv will constantly hunt or fail to get the idle correct.

    You need to adjust the idle set screw on the throttle body.
    (I know there are a lot of people who say you must not alter this as it's factory set, but it is adjustable. Its just that the manufacturer didn't want amateurs fiddling with it, hence the non tamper cover that might still be in place) Adjustment may be necessary over time due to wear, or especially if the throttle body has been swapped.

    So, reset the base idle to 850 using the set screw on the throttle body, no more than 1/4 turn at a time. After each adjustment wait until the idle settles. Once you've got reasonably close, reconnect the IACV and see what happens for about 5 minutes. If the idle settles to a stable level near 875, great!
    If not then you'll need to adjust the idle screw a small amount until you achieve a stable idle somewhere between 850-890rpm. Sometimes you might think you are nearly there and the idle will suddenly change. This is the ecu making the stepper motor move to the next step. Hopefully you will be on step 21 already but if not you'll have to keep adjusting, until you achieve 875 and step 21..

    Once you are happy switch off and reset the TPS, if you still have the torque/ pscan connected you'll see the tps sensor change to 3.9% from whatever it was.

    I went through all this a couple of years ago and the idle has been rock solid since.
    David
    :shrug:

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