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Thread: TBI tuning

  1. #1
    Electronic Ignition!
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    TBI tuning

    Hello,
    not sure if anyone here can give some helpful insight.

    I have a 95 4.3 tbi with manual trans. I spent years trying to figure out itís idle hunt. Unfortunately I didnít figure it out until after I installed megasquirt ms2 and found this forum. It was running pretty well but now I seem to have a problem with a hesitation when opening the throttle. I checked for spark, VE and spark tables have not changed. I am not 100 percent on what the IAC should be doing. The megasquirt forum has not helped that much.

    Any help is appreciated.

    will

  2. #2
    Super Moderator dave w's Avatar
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    I'm wanting to make sure I understand your request, you are asking for help for a MegaSquirt system?

    dave w

  3. #3
    Electronic Ignition!
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    Quote Originally Posted by dave w View Post
    I'm wanting to make sure I understand your request, you are asking for help for a MegaSquirt system?

    dave w
    No my request is not for info about a megasquirt system. If you have useful megasquirt info, I will gladly take it as I believe in always learning. What I am asking for is advice about tbi system and where to look for hesitation problem. For example: Is the IAC supposed to open during accel enrichment?

  4. #4
    Super Moderator dave w's Avatar
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    Thank you for the clarification.

    Next question, what is the computer part number?

    dave w

  5. #5
    Electronic Ignition!
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    Quote Originally Posted by dave w View Post
    Thank you for the clarification.

    Next question, what is the computer part number?

    dave w

    I fear my clarification was not good enough.
    I originally ran this 95 4.3 tbi with a computer from automatic gm minivan (the number of which escapes me right now) and a harness from painless. I may still have the computer lying around.
    The motor developed an idle hunt and hestitation on throttle initiation. After years of being stuck I found megasquirt 2 board 3.57 and shortly after I found the wisdom of this forum and the good people suggested it was my IAC stepper motor.
    since I had already sacrificed the harness I stuck with the ms2. It ran good for a while. After doing some work to the drive train for a while it now has this hesitation on the throttle again. I do have the ability to make adjustments in real time, data log, share my tunes, and have learned a lot about efi. I tend to think there is an issue with the engine or a component as the tune has not changed. Hence why I was wondering if the IAC was supposed to move when fully warm and the throttle begins to open.

    sorry for any confusion.

    will

  6. #6
    Super Moderator dave w's Avatar
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    Factory test for the IAC below. Other factory IAC tests not applicable for MS.

    Disconnect the negative battery cable.

    Test resistance between terminals A and B, then test between C and D.

    If the resistance is not 40-80 ohms, replace the IAC.

    Connect the negative battery cable.

  7. #7
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    In OEM configuration the IAC will move when the throttle is opened. In GM software this is called the "Throttle Follower."

  8. #8
    Carb and Points! FoundSoul's Avatar
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    Quote Originally Posted by ironwill11 View Post
    Hello,
    not sure if anyone here can give some helpful insight.

    I have a 95 4.3 tbi with manual trans. I spent years trying to figure out itís idle hunt. Unfortunately I didnít figure it out until after I installed megasquirt ms2 and found this forum. It was running pretty well but now I seem to have a problem with a hesitation when opening the throttle. I checked for spark, VE and spark tables have not changed. I am not 100 percent on what the IAC should be doing. The megasquirt forum has not helped that much.

    Any help is appreciated.

    will

    Couple things that may be helpful-- there are a lot of books on the subject that you could dig in to, I wrote one with one of our engineers Matt Cramer, it can be found on Amazon and is called Performance Fuel Injection Systems. Greg Banish's books are great as well. Our tech support team can be reached at support@diyautotune.com also and they could likely help if you're using one of our ECUs, which is sounds like you are.

    I also started writing a big series of articles that'll be book-sized by the time I'm complete. You can check that out here if you want: https://www.diyautotune.com/support/efi-tuners-guide/

    Hope some of this is helpful man!

  9. #9
    Fuel Injected! ralmo94's Avatar
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    Just off the top of my head, are you running an egr, and if so, could it be sticking open causing a hesitation?
    One thing about Ms and other aftermarket boards is, as far as I know, they usually don't have diagnostic codes,
    While obd1 might not always be helpful, at least it gives you what's out of range.
    No need to re-invent the wheel. But we can make it better

  10. #10
    Carb and Points! FoundSoul's Avatar
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    Chances are, it's the Acceleration Enrichments that need to be dialed in. This is kindof the equivalent of an accelerator pump on a carb, but infinitely tunable. When you stab the throttle, a RUSH of air enters the throttle body/intake. If you don't compensate for that rush of air with some additional fuel, it'll be lean for a second, and it'll hesitate. Same if it's overly rich if too much AE has been added. This has to be tuned at slow movements off idle, and at faster movements there as well. Then similarly it's fine tuned while under load at cruising rpms and above. Tuned properly, you'll have incredibly crisp throttle response with no hesitation.

    Here's a pretty good article on our website that gets into it-- would love to hear if this is helpful.

    https://www.diyautotune.com/support/...el-enrichment/
    Last edited by FoundSoul; 09-24-2022 at 06:12 PM.
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