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Thread: Idle drops low when coming to a stop

  1. #11
    Senior Member
    Join Date:  May 2011

    Location:  Norton, MA

    Posts:    847

    My VIN:    767 (3.0L EFI/EDIS)

    Here's the log and tune from yesterday. I ran with VEAL on the first leg, smoothed it out a bit, then ran without VEAL on the way home. This log and tune are from the trip home.

    The fuel table is still a bit bumpy. I'm not actually sure if that's OK, or if I want it to be perfectly smooth. I know what an idea table is supposed to look like, but I'm not sure if that is actually you have in the real world.

    I had no stalling, although the RPMs dropped to the 500s at times. I have A/C idle up working, and I did drive with the A/C on for most of the trip. A/C idle up is set to turn off the A/C below 700 RPMs, which may have saved me a few times, but I'm not totally sure.

    I'd like the idle not to drop quite that low, but I don't really know how to stop that.

    Also, I'm considering an accelerator pedal position sensor built from a pot, a 3D printed case and a rod or two. Mostly for logging purposes, but maybe I'll be accurate enough for MegaSqurit to use it. I just have to figure out exactly how I'm going to mount it.

    Tune: CurrentTune.msq
    Log (zipped; it's over an hour of driving): 2019-09-01_16.55.41.msl.zip

  2. #12
    '82 T3 FABombjoy's Avatar
    Join Date:  May 2011

    Location:  Lansing, MI

    Posts:    878

    My VIN:    10270

    Almost certain that you need more base throttle opening.

    Right at the end of the log you're at 194F, indicating warmed up. Your CL idle target RPM is 990 (base plus AC idle up). Your IAC is open over 100 steps.

    Your IAC drops to a base of 33 but even that isn't enough to prevent the RPM drop on throttle close.

    Typically you want to set idle when hot, lowest RPM target. Open base idle until the number of steps approaches 0.

    What kind of IAC do you have?
    You have 350 steps to close, but idle valve open is 200

    Is the sawtooth RPM input a feature of EDIS? All of your table targets 'buzz' around which seems strange.
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

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  3. #13
    Senior Member
    Join Date:  May 2011

    Location:  Norton, MA

    Posts:    847

    My VIN:    767 (3.0L EFI/EDIS)

    Since my throttle plates are currently completely closed (to compensate for a vacuum leak while my oil vapor separator cap was missing), I'd say that I need more base opening too. Before that it was very slightly open. I assume that's what you mean by "base idle", with the IAC steps being close to 0 (fully closed) when at warmed-up idle? I recall you mentioning this once before, many months ago,

    To be clear: in theory, idle should be the least amount of air the engine ever needs, which is why the IAC should be at 0 steps when at idle, right?

    A slightly annoying thing about the Mustang throttle body design is that you have to pull the throttle all the way open to turn the adjustment screw. It makes iteration slower, since you have to turn the car off if you don't want to revving at WOT -- you can't just slowly turn the screw while watching the RPMs change. Not a huge deal; just annoying.

    I have a a stepper IAC, I believe a pretty standard GM type. Where were you getting the 200 from? I know I have that in test mode, but that's only used for, well, testing, right? The 350 set for the Homing Steps int he Idle Control panel is correct, though (more than the number of steps to completely close the IAC). I can't seem to find where "all the way open" is set in TunerStudio. Early on I had a lot of trouble understanding test mode and IAC position until I finally removed the IAC housing from the throttle body and could just watch it moving in the housing (previously I was holding just the IAC in my hand; I lost a pintle doing that I accidentally reached the end of the range and it shot off the end of the shaft).

    Possibly related, but I have noticed that if I idle for a while, the IAC will switch to completely closed and not come out of it, and the idle will be a bit high (1000-1200 or so). If I turn the car off and on again it will work properly. I think that it's losing IAC calibration and restarting the ECU re-homes and fixes it, but I'm not really sure. It only really notice it when I'm screwing around in TunerStudio, so I'm not actually sure it happens in the real world very much.

    I'm not sure about the sawtooth RPM input thing. I vaguely remember something about a sawtooth signal sent to or from MS to indicate advance, but I don't remember anymore, and I can't find anything about that. To be honest, I being within 50-100 RPM of my target was normal, since I had nothing to compare it against.

    I'll try to find some time tonight to play with the throttle plate base opening position. If not tonight, I'll do it tomorrow.

    Thanks

    -- Joe

  4. #14
    Senior Member
    Join Date:  May 2011

    Location:  Norton, MA

    Posts:    847

    My VIN:    767 (3.0L EFI/EDIS)

    I found the IAC open and closed positions (200 and 0) -- I forgot it was in the Closed Loop Idle Settings, not the Idle Settings. I likely didn't update the homing position from 350 because I forgot to go back into that panel as I was fixated on the CLIS panel. So it's probably doing a BIT too much homing there; I can probably bring it down to 250 and be fine, but I should probably pop off the IAC housing at some point and be sure.

    -- Joe

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