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Thread: idle stumble with ac

  1. #1
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    idle stumble with ac

    Can't get this figured out. Doesn"t do it all the time. Sometimes at idle when the ac kicks on it'll stumble and/or die. Usually the stall saver will catch it, but then it just continues to cycle the compressor and not cool at idle. Seems to be closing the iac when it acts up. Tune and log below. Iron 5.7, 218/224 cam. -5 degrees overlap. Ported heads, Gen 4 intake swap, 90mm Throttle body. Both logs are short at idle with it acting up. I've gotten to where I know how to make it happen so if I need to log something else I can re-create the issue. Thanks!


    EDIT, The first log is long. Acts up the last couple of minutes at idle.
    Attached Files Attached Files
    Last edited by truckshop; 06-20-2020 at 02:10 PM.

  2. #2
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    Quote Originally Posted by truckshop View Post
    Can't get this figured out. Doesn"t do it all the time. Sometimes at idle when the ac kicks on it'll stumble and/or die. Usually the stall saver will catch it, but then it just continues to cycle the compressor and not cool at idle. Seems to be closing the iac when it acts up. Tune and log below. Iron 5.7, 218/224 cam. -5 degrees overlap. Ported heads, Gen 4 intake swap, 90mm Throttle body. Both logs are short at idle with it acting up. I've gotten to where I know how to make it happen so if I need to log something else I can re-create the issue. Thanks!


    EDIT, The first log is long. Acts up the last couple of minutes at idle.
    There are some settings related to torque or friction under the torque page that will help.

  3. #3
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    Quote Originally Posted by 240sxpooter View Post
    There are some settings related to torque or friction under the torque page that will help.


    Yes. Found a thread that explains it pretty good. Figured I'd share it here for future finds.

    https://forum.hptuners.com/showthrea...compressor+iat

  4. #4
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    Quote Originally Posted by truckshop View Post
    Yes. Found a thread that explains it pretty good. Figured I'd share it here for future finds.

    https://forum.hptuners.com/showthrea...compressor+iat

    Can anyone shed some light on why its commanding the iac to close when it commands the ac on? Doesn't always do it. mostly it works flawless. Seem to be only after a hot soak. Will act up after its restarted for 20-30 min.

  5. #5
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    Following this thread, sounds like we might have a similar issue.

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    It looks like maybe you are dealing with a few issues that are compounding the effect.
    1. Your STIT and LTIT are both removing airflow. I would try to get these trims closer or raise the target idle just a little.
    2. You might need to mess with your idle PIDs or PID control ranges. You don't want these "winding up"
    3. I noticed comparing your file to another stock file that there are some major changes under the 12881 map and 12882 maps. Both show 38-50 nm of trq on the 0-400 range of the maps. maybe the factory was having some coding issues and this was their workaround. IMO sometimes when you add a cam to the idle tuning it makes things get wacky. Not every OS has the same algorithms and Hp tuners only provide's what they have "defined" so there could be 1,2,100 other idle maps to fix your issue, but you only get the 20 they've defined.
    4. map 2156 did some really weird stuff on my car that made no since , Maybe try increasing that time frame?

    Maybe that helps

  7. #7
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    Quote Originally Posted by 240sxpooter View Post
    It looks like maybe you are dealing with a few issues that are compounding the effect.
    1. Your STIT and LTIT are both removing airflow. I would try to get these trims closer or raise the target idle just a little.
    2. You might need to mess with your idle PIDs or PID control ranges. You don't want these "winding up"
    3. I noticed comparing your file to another stock file that there are some major changes under the 12881 map and 12882 maps. Both show 38-50 nm of trq on the 0-400 range of the maps. maybe the factory was having some coding issues and this was their workaround. IMO sometimes when you add a cam to the idle tuning it makes things get wacky. Not every OS has the same algorithms and Hp tuners only provide's what they have "defined" so there could be 1,2,100 other idle maps to fix your issue, but you only get the 20 they've defined.
    4. map 2156 did some really weird stuff on my car that made no since , Maybe try increasing that time frame?

    Maybe that helps
    12882 table is stock. 12881 I adjusted trying to see if it would make a difference. did not so I put it back stock. I also adjusted 12880 table but had to go opposite of what I thought to make a difference. So that confused me. I'll see what 2156 does. I've been reading about tuning the iac since I am using a 90mm throttle body. That seems to be helping. I guess I didn't realize how far off it would be. I've also noticed that my stit changes with tuning the iac. As does my fuel trims on my ve table. So it looks like i'll be doing it all some more.

  8. #8
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    Another few things to consider is that if you are running MBT timing at idle then your idle advance undershoot cannot correct for a low idle dip (so you might want to decrease your timing some and increase your airflow) . I've also made it on my idle and main spark maps in the 400-600 range where it will advance the timing to up the idle speed even if the idle advance PID is not working.

  9. #9
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    Quote Originally Posted by 240sxpooter View Post
    Another few things to consider is that if you are running MBT timing at idle then your idle advance undershoot cannot correct for a low idle dip (so you might want to decrease your timing some and increase your airflow) . I've also made it on my idle and main spark maps in the 400-600 range where it will advance the timing to up the idle speed even if the idle advance PID is not working.
    Will look into this. Thanks! It?s acting much better since tuning the iac. Don?t guess I realized how far off it was.

  10. #10
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    Getting better. Still wants to act up after a 30 or so minute hot start.

  11. #11
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    Think of the IAC behavior right before the AC compressor kicks on as 'filling a tank' for later, immediate use. You're filling that tank slowly with a thin hose, but once you have it full, it's all available there when you need it. This is what the IAC is doing when an AC compressor clutch engagement is pending - it's filling that tank, which is the intake manifold in this case. Air moves slowly relatively to how fast you can change spark advance, which is key here...they are both components of torque production. Air is the 'slow' part of the torque, spark is the 'fast' part. However in order to keep the intake manifold full of air and prevent generating excess torque and causing unstable idle, the ECM pulls spark (ideally) exactly proportional to the amount of (potential) torque that was added by the air introduced to the manifold by the IAC. You cans see this in your data: IAC opens, MAP increases, spark drops, and hopefully RPM doesn't change.

    Now if it isn't obvious, we did this for one reason...so have a large torque reserve that is capable of handling the 20-50Nm load that the AC compressor will be putting on the crankshaft. If we tried doing this with air alone, the compressor would drag your RPM around because air is slow to respond, which is exactly what you're experiencing. But now with that spark retarded temporarily, if the AC compressor tries to lug the engine, we just have to wait until the next power stroke (happening 10 times a second for a V8 idling at 600rpm). It's much faster and thus allows a more stable idle RPM during torque transients.

    So to your question, why does the IAC close when the compressor turns on... As the compressor loads up the crankshaft, we want to deplete this reserve and get back to normal idle while the AC torque load feedforward terms ramp in and take over steady state load of the compressor. This involves choking back the IAC (or the throttle body in newer vehicles) and bringing spark back up to its new steady state level. Once you've done this, the torque transition is complete. It's important to remember though that this isn't just for AC compressors. Any calibration worth its salt will have the idle torque reserve carefully dialed in so as not to have too little reserve and be unstable, but not too much and wasting energy. It is also used for transmission shifts of all kinds, electrical load transients, etc...anything that can affect torque at the crankshaft can be mitigated by using a torque reserve.

    This brings me to your original problem of idle oscillation and eventual engine stall. To put it simply, your current idle spark advance is far too high and is driving the idle out of control. In your case, 27 degrees of idle spark is generating a lot of torque with little air, thereby minimizing your idle torque reserve. Because of that high spark value, it is trying to push RPM above its target and driving LTITs and STITs negative as the IAC generates the torque reserve in the manifold. So when it comes time to deplete that torque reserve to handle the AC torque transient, the IAC is slamming completely closed, choking your engine of air and causing oscillation and stalls. By the time the software catches this impending stall event, sometimes it's too late to save it because the air can't fill the manifold fast enough.

    tl;dr ... If you have idle oscillation issues, it's a good bet to pull out some spark and add a little air.

  12. #12
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    Quote Originally Posted by smokeshow View Post
    Think of the IAC behavior right before the AC compressor kicks on as 'filling a tank' for later, immediate use. You're filling that tank slowly with a thin hose, but once you have it full, it's all available there when you need it. This is what the IAC is doing when an AC compressor clutch engagement is pending - it's filling that tank, which is the intake manifold in this case. Air moves slowly relatively to how fast you can change spark advance, which is key here...they are both components of torque production. Air is the 'slow' part of the torque, spark is the 'fast' part. However in order to keep the intake manifold full of air and prevent generating excess torque and causing unstable idle, the ECM pulls spark (ideally) exactly proportional to the amount of (potential) torque that was added by the air introduced to the manifold by the IAC. You cans see this in your data: IAC opens, MAP increases, spark drops, and hopefully RPM doesn't change.

    Now if it isn't obvious, we did this for one reason...so have a large torque reserve that is capable of handling the 20-50Nm load that the AC compressor will be putting on the crankshaft. If we tried doing this with air alone, the compressor would drag your RPM around because air is slow to respond, which is exactly what you're experiencing. But now with that spark retarded temporarily, if the AC compressor tries to lug the engine, we just have to wait until the next power stroke (happening 10 times a second for a V8 idling at 600rpm). It's much faster and thus allows a more stable idle RPM during torque transients.

    So to your question, why does the IAC close when the compressor turns on... As the compressor loads up the crankshaft, we want to deplete this reserve and get back to normal idle while the AC torque load feedforward terms ramp in and take over steady state load of the compressor. This involves choking back the IAC (or the throttle body in newer vehicles) and bringing spark back up to its new steady state level. Once you've done this, the torque transition is complete. It's important to remember though that this isn't just for AC compressors. Any calibration worth its salt will have the idle torque reserve carefully dialed in so as not to have too little reserve and be unstable, but not too much and wasting energy. It is also used for transmission shifts of all kinds, electrical load transients, etc...anything that can affect torque at the crankshaft can be mitigated by using a torque reserve.

    This brings me to your original problem of idle oscillation and eventual engine stall. To put it simply, your current idle spark advance is far too high and is driving the idle out of control. In your case, 27 degrees of idle spark is generating a lot of torque with little air, thereby minimizing your idle torque reserve. Because of that high spark value, it is trying to push RPM above its target and driving LTITs and STITs negative as the IAC generates the torque reserve in the manifold. So when it comes time to deplete that torque reserve to handle the AC torque transient, the IAC is slamming completely closed, choking your engine of air and causing oscillation and stalls. By the time the software catches this impending stall event, sometimes it's too late to save it because the air can't fill the manifold fast enough.

    tl;dr ... If you have idle oscillation issues, it's a good bet to pull out some spark and add a little air.


    Thanks! This makes sense. I have taken away some timing and added air as per an earlier post. I need to post the latest tune. Removing timing along with tuning the iac for the newer throttle body helped tremendously. Still fighting the stall saver taking over at idle sometimes and disengaging the compressor. Thanks for the explanation!