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Thread: to clear up the idle tuning questions about IAC

  1. #21
    Tuning Addict WS6FirebirdTA00's Avatar
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    im just looking for more info on why we shouldnt touch it, esp when it works so well
    Sulski Performance Tuning
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    2018 Sierra SLT 5.3L A8 - Airaid intake tube, GM Borla catback, L86 Intake/Ported TB

  2. #22
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    The idle stop screw is so that you have the butterfly
    closed position held by the shaft, not by it jammed
    against the casting; that sticks, and wears. You want
    the blade just a skosh short of metal-metal contact in
    the throat.

    Regardless whether the port filling adds anything, it's a done
    deal (and my eye says it has to improve the airflow dynamics
    without the discontinuities). All I know is, the effective area
    table value makes sense for as-cast, is nothing like my real
    hardware, and I prefer the tables to be individually realistic.
    That way I don't have to keep track of what lies I told, to
    who, and whether they talk to each other.

    Some day I'll have a reason to remove the TB and when I do
    I'll drill it bigger on the IAC port. For now it's stable enough
    (for me) just putting the base airflows w/ ECT up to nearer the
    closed loop observed numbers. Which means, to me, that it's
    a loop stability problem that minimizing errors reduces, but
    minimizing the work the loop has to do is not the same as
    stabilizing it. I would like to hear more about the integral,
    derivative tables and what people know about changing the
    loop dynamic behavior to make it more robust to non-stock
    lags in the system. Not a PID controls man myself. But I
    know what I want.

    I don't cotton to dogmatic pronouncements, when they
    ignore (or worse, deny) physical realities. There's no
    prophet in it. Heh.

  3. #23
    Tuning Addict WS6FirebirdTA00's Avatar
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    jimmy, i had the same issue with mine and i went to drill the hole larger and just messed it up so i knocked out the epoxy. i had it figured out the diameter hole i needed to not create a restricition but i cant remember. on a very cold start after i did heads and cam the iac motor only wanted to open up like 200 steps, with it at 310 it was better but still needed more air. but messing with the effective area was the ONLY thing to do for the car to run and run right besides knocking out the filling.
    Sulski Performance Tuning
    2000 WS6 M6 - LS6 (long block, refreshed top end), 10.8:1 CR, 90 mm ported FAST, Exo-Skel, 227/232 cam, QTP HVMC, EWP, GMMG, 9" w/4.11s
    2018 Sierra SLT 5.3L A8 - Airaid intake tube, GM Borla catback, L86 Intake/Ported TB

  4. #24
    Senior Tuner S2H's Avatar
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    Quote Originally Posted by ws6firebirdta00
    jimmy, i had the same issue with mine and i went to drill the hole larger and just messed it up so i knocked out the epoxy. i had it figured out the diameter hole i needed to not create a restricition but i cant remember. on a very cold start after i did heads and cam the iac motor only wanted to open up like 200 steps, with it at 310 it was better but still needed more air. but messing with the effective area was the ONLY thing to do for the car to run and run right besides knocking out the filling.
    the issue was probably that it was onlt commanding to open the 199 steps represented by the MAX Desired area table...stock is 84...which on my table correlates to 199 steps....
    try increasing that table and you will see a larger MAX IAC step
    all you did by moving the table was allow it to command more spark control....and according to ken and chris....you mess up other stuff relating to that IAC table...it still needed the air..it just wasnt as apparent with the spark doing its job....
    -Scott -

  5. #25
    Tuning Addict WS6FirebirdTA00's Avatar
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    i have logs and the spark moved just the same as it did with the stock table in there. yes the max desired area has been changed for a long time now so i know that is not the issue. it has absolutely nothing to do with spark because the spark was unchanged.
    Sulski Performance Tuning
    2000 WS6 M6 - LS6 (long block, refreshed top end), 10.8:1 CR, 90 mm ported FAST, Exo-Skel, 227/232 cam, QTP HVMC, EWP, GMMG, 9" w/4.11s
    2018 Sierra SLT 5.3L A8 - Airaid intake tube, GM Borla catback, L86 Intake/Ported TB

  6. #26
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    Sometimes things get lost in a 5 minute conversation.
    While I was explaining to Soundengineer what he should tune on his particular idle problem, I did not intended it to mean NEVER change the steps vs. effective area table, NEVER change the TPS stop or any other method of adding extra air to the engine. Now obviously every engine combination is going to require slightly different tuning needs, and one size does not fit all here.

    There are lots of other things that affect idle quality and should be adjusted in a certain order to keep you and the VCM from fighting with each other. If you break down what the VCM is trying to do with the idle RPM you will quickly understand how to tune it. After all we are actually troubleshooting here, and not just tuning. You need to determine WHY the idle is unstable rather than just making blind corrections and hope they work. An old adage I like to use it "Quit twisting and start testing".


    Lets look at what you have changed, and what it affected. You just added a large cam and your race motor will no longer be able to maintain the 50-60 rpm idle error of a stock motor no matter what you change. The idle RPM is going to bounce around 100 RPM or so even under the best conditions.
    The VCM has two ways of combating this problem. A short term measure for dealing with idle rpm errors is with spark advance, and long term measures are the adaptive airflow corrections. If you look at the idle under/overspeed spark tables you are now working in a RPM error range that can vary the spark advance as much as 14 degrees. Not great way to start things off. Zero out those tables for now and tune them last. Also make sure you are not jumping off a spark "ledge" on your Idle Spark Advance tables.

    Your new cam has opened up an opportunity for a new airflow source for the engine. The exhaust... In order to make the intake air more attractive than the exhaust stream, you are going to have to increase the IAC/ETC position. ETC vehicles will increase the TB position and can be seen as a %TPS change. The VCM also knows the difference between TB position and ACC pedal rotation.

    On non ETC vehicles the TB bump stop is used to control minimum airflow and TPS position. As GM never intended this to be the main source of idle air, you can make some minor adjustments before you run into %TPS problems. As a lot of people will read this, so make certain your TB type does not have timed vacuum ports near the throttle blade. Adjusting the blade past these positions will be the cause of many hours of grief. Your method adding more idle air can include anything from turning the TB bump stop, increasing the IAC orifice, drilling the TB blade or pulling a vacuum hose to create a leak It's your motor do what you want.

    I looked around at some of the IAC/TB parts I have and it seems the trucks got a slightly larger pintle/orifice combo than the cars. It may be a good time to start finding what part number/size/etc wound up on various models over the years, and what can be swapped/modified for those interested in changing the IAC size.

    The first thing the VCM is going to do is look at the Target Idle Speed tables. It picks the RPM based on the temperature. Then it looks at the Base Running Airflow tables to determine the initial IAC/ETC position needed based on engine temperature to get to that RPM. It will quickly move the IAC/ETC to that position and if the engine rpm rises or drops outside of its dead band error range, the VCM says "whoops we have a problem". The first thing it will try to stabilize the RPM is the Idle Spark tables. As you have taken those out of the process, it will now try and compensate with the IAC or ETC using the STIT and LTIT.

    The airflow learning process takes minutes, so be prepared and patient. Don't forget the VCM will also use any previous learned airflow values, so your next correction to the VCM will take a few minutes to be accounted for. We are looking for the minimum STIT and LTIT correction numbers when all is said and done. For you automatic guys, use the gear selector to put the vehicle in/out of gear without actually letting the vehicle go into gear. You will see what the VCM is commanding for ETC/IAC position and see the actual STIT and LTIT learning rate. If you see the IAC or ETC close when you are putting it into gear, you can be assured the engine will stall when it does go into gear.

    With the Base Running Airflow tables adjusted, look at your new IAC position. If that thing is close to its maximum opening position, the VCM will have a hard time trying to maintain closed loop idle RPM control. This is where we previously talked about getting more air into the engine come into play. So take your pick on how you plan on achieving this. The bottom line is to keep the IAC somewhere in the lower/middle range so it has plenty of room to move on either side to account for cold/hot engines/days. Go back and check the RAF tables after you get the IAC position down to a "working" position. ETC guys will just see a higher %TPS which should not alarm you.


    Now that you have the base running airflow and IAC position set properly, you can start working on how fast IAC needs to move to make corrections. It should be noted here that most people will have to go no further than the Base Running Airflow tuning and added IAC idle air to achieve good idle quality. I rarely if ever need to change these, and when I do it because I am trying to make the idle learning "seamless".

    Everything about the idle is based on Idle Rpm Error. You can create a custom PID using the desired idle rpm and actual engine rpm to come up with a number that while is not the %delta error it is "close enough". By changing the Enable RPM delta error (%rpm slope) and Update RPM Error max values, you can set new bounds for the idle learning based on what your motor can actually achieve. While you can play with the idle PID airflow values, its not recommended nor really needed. I personally like to decrease the PI airflow values in the RPM error areas where I know the engine RPM will be unstable. The key here is to make small changes and verify the results. Trying to call fast values on a slow moving IAC motor will just wind up with overshoot.

    Now that you have somewhat relative airflow values being called for from RPM errors, the VCM needs to convert those into mechanical movements. You can make small changes to the Desired IAC Steps vs. Desired Idle Effective area if you want the IAC to have a different transfer curve. Again most people won't need to play with this, and if you do you are on your own. This table really should match the actual hardware.


    Lastly you should tune the Idle Spark Advance tables. These are designed to handle short lived idle RPM errors that the VCM does not know about, as spark advance works faster than an IAC motor. Keep your over/under spark between 1-5 degrees in the RPM error areas you are interested in. Obviously adding 7 degrees of timing where you know the motor is going to jump around 50-100 rpm is going to cause headaches.


    This is not intended to be an end all to be all for idle tuning, but rather a few pointers on where to start and what to look at. Everybody has their preferred methods, and this is mine.
    What you will find is no two engines are alike, and your idle tuning is going to take a while. You will undoubtedly find yourself making small corrections with seasonal temperature changes until you hit all of the Vs. Coolant Temp ranges.



    Happy tuning and keep it safe.
    -Ken


    *A quick edit here. My preferred method of adding more air is to drill the TB blade. This method actually exists on certain GM throttle bodies.
    Last edited by Casey; 01-21-2006 at 06:03 PM.

  7. #27
    Tuning Addict WS6FirebirdTA00's Avatar
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    thanks for all that Ken
    Sulski Performance Tuning
    2000 WS6 M6 - LS6 (long block, refreshed top end), 10.8:1 CR, 90 mm ported FAST, Exo-Skel, 227/232 cam, QTP HVMC, EWP, GMMG, 9" w/4.11s
    2018 Sierra SLT 5.3L A8 - Airaid intake tube, GM Borla catback, L86 Intake/Ported TB

  8. #28
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    ok heres how i understand this, tell me if its correct.

    what your doing is trying to increase the accuracy of the IAC motor, because as the steps increase the air flow will follow a non-linear exponential graph. therefore by reducing the amount needed to flow through the IAC passage. IAC steps can remain low and be more accurate. You lowered the demand on the IAC stepper motor by increasing the TB blade hole.

  9. #29
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    Quote Originally Posted by Spenser309
    ok heres how i understand this, tell me if its correct.

    what your doing is trying to increase the accuracy of the IAC motor, because as the steps increase the air flow will follow a non-linear exponential graph. therefore by reducing the amount needed to flow through the IAC passage. IAC steps can remain low and be more accurate. You lowered the demand on the IAC stepper motor by increasing the TB blade hole.

    No, you are just making sure the IAC has enough room to move on either side of its physical min/max restrictions. If not the VCM will have a hard time with cold starts and the stall saver can't do its job.

    Just keep it simple. Tune the RAF settings and make sure the IAC under normal ops is not sitting close to it's max limit. If it is the hole gives the IAC a helping hand..

  10. #30
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    ohh ok, i was under the impression that a step was an increase in diameter of the opening for idle air. thereby non-linear. this made sense because keeping idle would involve switching between 2 or so steps causing more air and then less air to enter and if the steps were larger than the idle would hunt more. I guess i need to take apart the tb and look.

  11. #31
    Does this apply to the V6 as well? HPT hasn't arrived yet, but I would like to be ready. With my new cam Park idle is 800 to 900 and in gear is 600 to 725. When it drops to the 600 range, it will cause the headlights to dim for a second.
    I sure hope I understand more of the abreviations and what is being described here, after I have the tuner in hand.

  12. #32
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    a related issue--i cant quote all the models and years affected,but i know my 2000 suburban was one of them--due to throttle blades sticking at idle, gm issued a tsb to plug the idle hole in the blade and then turn in the stopscrew until a specified voltage is read at the tps. this superceded the earlier tsb that replaced the throttle bodies.