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Thread: What are all parameters that trigger IAC Ramp Down?

  1. #1

    Default What are all parameters that trigger IAC Ramp Down?

    Trying to keep IAC from ramping down until I want it to. Proving to not be easy. What are all the things that trigger IAC Ramp Down? My IAC settings are:
    GM LSx
    Idle Speed: 1050 RPM
    RPM Above Idle To Start Ramp: 1500 RPM
    RPM Above Idle To Re-enable Idle Control: 200 RPM
    IAC Hold Position: 15%
    Ramp Decay Time: 3.5 sec

    Was driving steady 2% throttle at ~2150 RPM (1st gear) when IAC Ramp Down began to initiate three times in a 10 second window. The first two times the system decided to abort and went back up to IAC Hold Position. The third time it did a complete Ramp Down to 0% IAC Position before I even let off the throttle. Don't know the underlying reason why it made any of those decisions.

    Obviously I'm under the RPM threshold to start the Ramp Down so a Ramp Down could occur. But it's not clear to me what's the secondary decision making that's saying do & don't. I thought maybe it was deceleration, but my throttle never wavered from 2% in this example.
    Last edited by Qwktrip; 05-24-2020 at 09:01 PM.

  2. #2

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    I just assumed that the IAC being at the Hold Position and TPS returning to zero % would start it once the ramp RPM was hit.
    351W, 800HP stock shortblock assy

  3. #3

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    My experience is that it's more complicated than that. It has other background rules that are a great mystery.

  4. #4

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    RPM Above Idle To Start Ramp: 1500 RPM. That's part of the problem. It allows idle ramp down to begin at 2550 RPM. My settings:
    Idle Speed: 900 RPM
    RPM Above Idle To Start Ramp: 475 RPM
    RPM Above Idle To Re-enable Idle Control: 50 RPM
    IAC Hold Position: 16%
    Ramp Decay Time: 4 sec

  5. #5

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    Appreciate the input, but that's not going to work for me. Manual transmission and engine snaps fast. Something like that would work with gentle cruising, but not when driving aggressive. Got to have that higher RPM start of ramp down to catch an engine in free fall from high RPM.
    Last edited by Qwktrip; 05-31-2020 at 12:36 AM.

  6. #6
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    Just out of curiosity, have you tried idling in "Alpha-N Idle Fuel"? With any TPS Position % signal issue, the ECU remains in IAC Ramp Down mode (above idle), which is Open Loop mode, and the Idle Spark control retards the timing to assist returning to idle.
    May God's grace bless you in the Lord Jesus Christ.
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  7. #7

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    Quote Originally Posted by Qwktrip View Post
    Appreciate the input, but that's not going to work for me. Manual transmission and engine snaps fast. Something like that would work with gentle cruising, but not when driving aggressive. Got to have that higher RPM start of ramp down to catch an engine in free fall from high RPM.
    I have a manual as well. The ramp down doesn't work like you think it does (if I'm reading your post correctly).

    Say you're at 3,000 RPM in neutral and let off. At that point your IAC is at 15% (the Hold Position). Once it gets to 2,550 it will ramp down. That means it will close to 0% because it wants to get RPM down to idle. Now RPM falls like a rock.

    When it gets to 1,250 the idle control re-enables so that's when it tries to catch it.

    If you change your RPM Above Idle To Start Ramp to, say 500, it'll keep the IAC at 15% until 1,550 which means it will decrease RPM much slower.
    Last edited by zwede; 06-01-2020 at 09:32 AM.

  8. #8

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    I see what you're saying. You got me thinking harder about that now and coming up with a new plan to try out. Thanks.

  9. #9

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    Quote Originally Posted by Danny Cabral View Post
    Just out of curiosity, have you tried idling in "Alpha-N Idle Fuel"? With any TPS Position % signal issue, the ECU remains in IAC Ramp Down mode (above idle), which is Open Loop mode, and the Idle Spark control retards the timing to assist returning to idle.
    No, I haven't. So I already have a really good tune in Speed Density. Would Alpha-N erase all that work? Literally the only thing I'm battling with is engine return to idle. Everything else is good. Same battle I had with my last engine using this same throttle body and nearly identical cam specs. But I'm a lot smarter this time around, so I don't have any of the other tuning problems I had with my last engine. This tune is pretty much spot on except for the one issue.
    Last edited by Qwktrip; 06-04-2020 at 12:42 AM.

  10. #10

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    Followed your advice, zwede, and lowered the IAC Ramp Down to trigger below cruise RPM. Also listened to allan5oh, who encouraged me to make a custom Advanced Table for IAC Offset. Basically I adapted the concept of a "Throttle Cracker" with a custom Advanced 2D Table of IAC Offset based on RPM vs. MAT.

    The result is a dynamic IAC Hold Position that blends smoothly into the IAC Ramp Down strategy. The two of those working together resulted in fantastic control of engine descent to idle today. Just got one drive under my belt so far, but hopefully it will continue to behave and I'll be done with all this hassle and finally be able to put down the laptop.
    Last edited by Qwktrip; 06-05-2020 at 04:22 PM.

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