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Thread: Surging idle when coasting

  1. #1
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    Default Surging idle when coasting

    I need some help with a surging idle. I can't figure it out, and its driving me nuts I have gotten it better but it still is happening. It seems to happen mostly when I'm coasting down to a stop under about 30mph. Once i lift off the throttle and it is returning to idle the afr seems to bounce around.

    I think the VE table is good but its odd as sometimes it will be reading a lean afr then another place in the log using the same VE cell its too rich.

    Another question is what is a good timing value for idle? it seems in gear it is always going to 18 degrees, which is the value of the High Octane Spark table, but the base spark in gear table i have set to 28 (which is stock values).

    Lastly, I have noticed that when i lift off the throttle sometimes it is still reading 1-2% throttle opening. Why is this? is the IAC position affect what your TPS % shows? Do i need to go adjust the throttle blade so it goes all the way closed? or is it maybe a flaky TPS sensor? cause it isn't consistent. there is slack in the cable so it should close all the way. Wondering if some how this could be causing my issue?

    Attached is a screen shot of the log where it was doing it, along with my log and tune. Any and all ideas are welcome, I have been chasing this problem for a while since i got the motor back together. Specs are a 2002 camaro with single 78mm turbo. Engine is ls1 with 4" stroker, 9.7:1 cp pistons, stage 2 BTR turbo cam.
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  2. #2
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    Try doing a TPS relearn first. Unplug the TPS, turn the key on and leave it on for about ten seconds. Turn the key off, plug the TPS back in, and the next time you log it should read 0-0.4% when you're foot is off the gas. Start there and see what happens.

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  3. #3
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    is reading .4% throttle typical? seems like it reads that at the beginning of the log, then by the end it was showing almost 2%

    Also what would be the correct procedure to adjust the throttle blade stop? I see it is adjustable, not sure if I should try and set it?

  4. #4
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    Sometimes they read up to 0.4%, happens to me regularly. More than that is too high. You probably aren't at your base spark in gear timing because your TPS is reading too high. A relearn should help, quick and free to try.

    To adjust your blade, you want your IAC steps to be about 30-70 at idle when warm. It'll vary depending on ambient air temp and humidity. Adjust your throttle blade so your IAC steps are within that range, if needed, before doing the TPS relearn. You may need to perform RAFIG and/or RAFPN tuning as well, I believe there's a sticky covering those.

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  5. #5
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    Hello,
    I've found if your tps voltage is above .6 you will need to drill a larger hole in the throttle body since the tps is non adjustable and will cause all kinds of idle problems. I usually strive for 15 to 30 steps at idle with big cams in neutral warm(Sometimes this might require as much as a .250" hole). With big cams that have low vacuum at idle you then need to alter the timing tables(closed throttle tables as well as off idle tables) with a much lower timing setting so the idle doesn't hang due to being "happier" at off idle since vacuum has increased during that instance. I'm not sure I am explaining it well but I fix other tuners idle issues all the time.
    -Carl
    p.s. don't forget the min mass air flow table vs rpm so it recovers transient.

  6. #6
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    Quote Originally Posted by gpr View Post
    Another question is what is a good timing value for idle? it seems in gear it is always going to 18 degrees, which is the value of the High Octane Spark table, but the base spark in gear table i have set to 28 (which is stock values).
    There's your main problem. Your spark is locking at 18 degrees and you're not getting any idle spark corrections... so it's entirely up to your idle air corrections to try to keep your idle stable, that's if your idle air corrections are even kicking in (which I can't tell from your screenshot).

    I can't look at your tune file right now but maybe someone else can have a look and try to figure out why you're not transitioning to idle properly.

  7. #7
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    Thank you for all the tips! i think I figured out what is going on, but not the cause. Attached is a screen shot, log and tune. To me it looks like when i let off the throttle for some reason once in a while the throttle is hanging (but it doesn't do it every time as you can see in the log) above the Idle tps threshold, thus when it does this it surges and isn't working and spark is stuck at 18 which is the value in high octane spark table. As soon as I stop for some reason the TPS % returns to .4% and the idle instantly smooths out.

    now i'm not sure if it smooths out cause the tps is reading .4% or because it is below the vehicle speed threshold to use idle tables again....

    I noticed my tps voltage is showing .7 volts when closed, I believe this needs to be .6 so I will adjust the throttle stop down. The IAC steps at idle is also over 100. I think once when i was coasting down in the log it dropped down to 70, but at idle it goes back to 120-130 usually.

    So where do i go from here? I'm thinking I'm going to adjust the throttle stop down till i see .6volts as suggested. then drill a bigger hole in the blade. I also want to play with the throttle body to see if the blade is hanging up for some reason or why it isn't closing all the way once in a while. I have a spare throttle body I might also get out and try to see if it does the same thing.

    Any other suggestions? Do you think the throttle blade hangs open because of vacuum from the larger engine I am now running?
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  8. #8
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    Quote Originally Posted by gpr View Post
    So where do i go from here? I'm thinking I'm going to adjust the throttle stop down till i see .6volts as suggested. then drill a bigger hole in the blade. I also want to play with the throttle body to see if the blade is hanging up for some reason or why it isn't closing all the way once in a while. I have a spare throttle body I might also get out and try to see if it does the same thing.
    Yep, do all that. You really need tps to be 0 or 0.4 for idle... and try to get IAC counts down to about 50 at idle.

  9. #9
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    Your timing is also too high and your air going in the engine too low...
    "All that is needed for the triumph of evil is that good men do nothing..."

  10. #10
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    Ok its been a week but I finally got some time to mess with it. I adjusted the throttle stop out and it is now reading only .5v when close. I also drilled a .25" hole in the throttle body. This seemed to make it run lean so I added 10% fuel to the VE and VE while cranking tables to help it start quicker etc.. as a quick test. Anyway I noticed that the LTFT are showing negative by a lot (probably cause I changed the ve table) but for some reason it is running lean at idle, around 15.5:1. Which i don't know why it would run that lean seemed like before it was always idling around 14.5:1, so not sure why it would pull fuel to make it run leaner.

    Also the IAC steps are still around 140 even after the engine is warmed up and idling.

    Attached is the log and a screen shot of the log.

    Highlander, could you explain a bit more. How is the air going to the engine too low and timing too high? what should they be?

    Also I noticed now the map sensor is reading about 8.3 psi now instead of 8.7 psi what it was before I drilled the larger hole.
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