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Thread: Idle Desired and Actual Airflow

  1. #1
    Senior Tuner
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    Nov 2002
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    Central Florida
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    Idle Desired and Actual Airflow

    I have all of my closed loop idle stuff turned off in the attempt
    to root out the surging and cold-start stalling.

    Now I see that cold start is asking for a high (~25 g/sec) airflow
    (Desired) with the Base about 16 (for temperature) but the actual
    is still down about 10-12g/sec.

    I am thinking this must be a discrepancy in the effective area vs
    IAC counts. IAC is "live" and bopping around in the 200 range but
    the air is coming up short (STIT, LTIT off). I do have an epoxied
    TB that is drilled to 3/16" or 1/4" (I forget) in the IAC port.

    Do I have it right, that Desired Airflow should determine an effective
    IAC area, then IAC counts by that table? Is Desired the "real" target
    for airflow at least?

    I think I'm a gonna drill me some epoxy out, since I am allowed to
    be out in the garage tonight. See if that brings actual into line with
    desired.

  2. #2
    Advanced Tuner
    Join Date
    Jul 2007
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    218
    just add timeing in your idle spark advance in park and in drive that should help out alot with the starting and surge. good luck

  3. #3
    the idle desired airflow is the desired or commanded airflow to maintain the desired idle RPM. ie. if you want your LTIT and STIT to be close to 0 then desired airflow is the number you put in your RAF table.

    Effective area is calulated from desired airflow and then converted to IAC steps.

    If your TB has been modifed it is possible that your IAC steps table is incorrect but for the most part this can be corrected in the RAF table anyway (resulting in more or less correction by the STIT/LTIT). if your IAC is not oscillating or doing something weird (like desired steps not matching actual steps) then your surging problem is likely not caused by the IAC. oes it surge when the IAC comes into closed loop or when it is still in openloop (not openloop fuel, openloop IAC).
    I count sheep in hex...

  4. #4
    Senior Tuner
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    Exclamation

    I drilled it last night and ran it today, -HUGE- difference in
    start and idle stability. Excel chart of airflow (MAF) vs IAC
    shows why.

    Basically my IAC air was unable to go over 11 g/sec no
    matter what I commanded. This makes for a lot of "windup"
    if the motor is trying to run in that range (and it is).

    A 3/8" hole is enough to make IAC linear all the way up to
    where I quit looking. The 1/4" hole in the epoxy was a bad
    choke. The "wisdom" years ago when I did my TB porting
    was that 3/16" would do. Wrong. Livable with stock idle air
    needs, not with a cammed up car and a higher idle.

    15 minutes for R&R and drilling, vs the hours I've spent trying
    to figure it out on the screen. Some bits are more significant
    than others. Like drill bits.

  5. #5
    Senior Tuner Frost's Avatar
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    Richmond VA
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    Quote Originally Posted by Chris@HPTuners
    ....oes it surge when the IAC comes into closed loop or when it is still in openloop (not openloop fuel, openloop IAC).

    Hey Chris (sorry to jack JB ) what is the deal with open loop IAC? It sounds like it could be part of the issue I am having with a swap right now... The IAC is good, the PCM outputs are good, and the IAC will just stay wherever it lands after index and not update.... see here please http://www.hptuners.com/forum/showthread.php?t=14587

    Sorry again JB.
    Steve Williams
    TunedbyFrost.com


  6. #6
    could just be that variable is not updated unless the engine is running.
    I count sheep in hex...