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Thread: Surging idle when coasting

  1. #21
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    Quote Originally Posted by gpr View Post
    Also I noticed in the log that when I open the throttle the IAC also opens, is this normal?
    Yes.

  2. #22
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    Take note of what Statesman pointed out, he knows what he's doing.

    There are a few things I learned that should help you out here. Like Statesman alluded to, your airflow may not be correct. You can correct it by performing the RAFIG process. If you have an auto trans, do RAFPN as well. You can read about how to do that here: https://forum.efilive.com/showthread...dmon-idle-info

    Also as pointed out, your desired airflow and actual airflow need to line up, which is why you would modify or shift your IAC effective area table. What I found helped dial in that table was logging IACDES_B and DYNAIR. I setup a custom PID that was equal to IACDES_B minus DYNAIR. You want the difference between these two logged pids to be zero when your engine is at operating temperature, above 80C. Below that they may not line up, but focus on when your engine is at operating temperature first. If IACDES_B - DYNAIR is positive, shift B4403 up. This means shift the table so if 1 IAC step was 18.0 sq. mm, now 1 IAC step is 20.0 sq. mm. Again, do this at operating temperature. Once you get this done, perform RAFIG (and RAFPN if you have an auto trans) and your airflow should be spot on.

    Once B4403 and then B4307 are corrected, look at your IAC steps. Shoot for 30-70 (or less according to some people, I'm an amateur to so take this with a grain of salt) IAC steps when you're idling at operating temp. This will vary based on humidity and air temp. If you're in that ball park, you're good, but make sure your TPS is reading 0 or 0.4%, no more. Do a TPS relearn if needed, or close your throttle blade a bit. This was discussed earlier in this thread.

    Get airflow set, check your TPS and IAC and make any adjustments as needed, then repeat the above process. Tuning is iterative, so expect to run through things at least a couple times until they are dialed in.

  3. #23
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    Why do you want the IAC steps so low? What is wrong with having the IAC open more?

    Also how do you know if it is getting too much air or not enough at idle? Is everyone basing that off of the spark timing? Thus its getting too much air and trying to idle high, so it retards the spark past what your base spark of (in my case 18). Therefore you know its getting too much air?

    I did the RAFPN process tonight, however something seems wrong. The RAFPN was at a constant 2.0 the entire time. Is this like a maximum number? or did I do something wrong?

    Attached is my log file of the RAFPN test.

    The odd thing I noticed tonight is it started up and idled just fine. I didn't have to hold open the throttle when cold, but the IAC steps were also higher. I wonder what the difference is? The car hasn't moved and the tune hasn't changed....
    Attached Files Attached Files

  4. #24
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    Here is a screen shot towards the end. You can see RAFPN never changes the entire time....
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  5. #25
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    You want the IAC steps fairly low so you can use the entire IAC function, The IAC is just an electronic vacuum leak, so you use your blade opening to do the majority of the idle function(keep your tps in range .5-.6v with big cams you usually have to drill a bigger hole in the throttle body to keep tps voltage in range or modify the bolt holes in the tps) then let the computer handle the rest. When you have the IAC near full open(around 150 steps usually) you are out of control.

  6. #26
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    Quote Originally Posted by In-Tech View Post
    You want the IAC steps fairly low so you can use the entire IAC function, The IAC is just an electronic vacuum leak, so you use your blade opening to do the majority of the idle function(keep your tps in range .5-.6v with big cams you usually have to drill a bigger hole in the throttle body to keep tps voltage in range or modify the bolt holes in the tps) then let the computer handle the rest. When you have the IAC near full open(around 150 steps usually) you are out of control.
    Isn't the max IAC steps around 300? As the last log I provided when the key was turned on and engine cranking IAC hit about 280 steps. I would think you would want your IAC in the middle at around 150 steps. Then it can go in either direction to open or close to control the idle speed?

    Anyway, anybody have any ideas on why the RAFPN values are constant during the entire log?

  7. #27
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    Quote Originally Posted by gpr View Post
    Isn't the max IAC steps around 300? As the last log I provided when the key was turned on and engine cranking IAC hit about 280 steps. I would think you would want your IAC in the middle at around 150 steps. Then it can go in either direction to open or close to control the idle speed?
    Yes, ~300 steps. You are at the side of your house with a garden hose. You turn the valve open a 1/4 turn. You have flow out the spigot. Turn the valve another 1/4 turn to 1/2 open on the valve. You have more flow. Turn the valve another 1/4 turn to 3/4 open. You have more flow. Make that last 1/4 turn to the valve wide open....do you have any appreciable more flow? Nope. I have found at 16' above sea level, NE FL, one has about 170 use-able steps. In order to reduce the demand on the lAC opening (increasing) the throttle stop set screw to give the necessary bypass airflow. Adjustments to idle set screw should only be performed with the vehicle fully warmed up. Throttle Body set screw / TPS relearn must be performed as well.

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    Back in September, you said this....

    Quote Originally Posted by gpr View Post
    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.
    Have you done this yet?

  9. #29
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    Quote Originally Posted by statesman View Post
    Back in September, you said this....



    Have you done this yet?
    yes, drilled a .25" hole and adjusted throttle stop down to .5v when close. Also did a tps relearn.

    I tried the RAFPN process to try and adjust the air flow at idle but for some reason it didn't work and showed a constant the entire time.

    I adjusted the IAC step table down a few rows like suggested but IAC steps are still around 140ish.

  10. #30
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    Sorry, forgot to ask, stock TB? The key part of your last comment, "adjusted throttle stop down to .5v" This does not make sense to me. When you move the screw the voltage IS going to change. TPS reset, or relearn or whatever, IS the process where the pcm gets it's NEW electro-mechanical "ZERO."

    Try this, physically remove the TPS sensor from the TB. With KOEO, observe the TPS voltage while cycling the TPS with your finger 0-100% Your TPS should have a spring inside that helps pull back the wiper to zero position. An old or worn out spring or the plastic not fitting snug enough on the shaft of the TB will produce the phenom of an inconsistent zero. If zero position displays ~+/- .6v don't worry about it.

    Bear in mind, the first time pwr/grnd & KOEO the pcm will "record/remember" the physical/mechanical position of the TB shaft as it will produce the voltage output of the TPS sensor displayed on your scan tool.

    Sometimes the TPS reset procedure can be a PITA. That is forcing a failure of the TPS.

    Big picture, increasing the position of the set screw will (or drilling the TB, not my favorite in 2019) will provide more bypass air to achieve your target idle. The pcm will not "need," the IAC counts. The IAC counts will go down on their own.

    2019
    Holley LS Throttle Bodies 112-581

    A little late to the party but, Holley had some really great ideas with these TBs. Notice the adj bypass air screw on the pass side? You can turn that sucker as much as you want, no TPS reset. On the driver's side, the TPS position is adjustable, so you can get your electrical zero nailed down as well. The adj TPS is really handy on big cam / heads. These TBs are not cheap but, they make things a lot more fun. There are two styles. I would recommend the tapered lip for a street car. The hogged out to the max non lip, I would use on a track only car where you want / need max airflow and don't care about street manners.

    2000 Silverado Full Size 4x4: Forged 6.2, H/C F1R Procharger
    98A4 Z28: 02 PCM H/C Forged 347, 9" Moser 3.73
    V1 V2 99+up RR COS #5 OLSD Dual Stg N20
    www.eifalchemy.com
    www.greatamericancarwar.com

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