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Thread: Turn off all IAC functions.

  1. #1
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    Turn off all IAC functions.

    Pulling my hair out...

    Is there a way to disable all IAC functions and then introduce them back one at a time.

    I have changed just about everything i can and just not my tuck insist on going back to 850rpm for no reason. It was at 700 for an entire day and now, back to 850. I changed nothing and I am in closed loop.

    no programming will manually change my IAC counts, But idling at 850, I drop into gear, goes down to 700 and i can watch the IAC count up trying to reach 850.

    I do not understand all of the decay/timer scalars with %s of this and that. Should i toss my 1228747 $4F ECM and switch to a 1227747 or something different.

    Side note, I have looked a a few late 80 suburbans to buy and they all idle high. Makes me think it is a EMC defect?

    Thanks,
    Chris

  2. #2
    Fuel Injected! donf's Avatar
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    You might check for vacuum leaks. Even when new the tbi trucks had a bad reputation for blowing out base gaskets and sticking IAC's. There is no programming defect, that keeps a stock tbi vehicle from idling. Take the time to diagnose a problem correctly. I see it over and over, people say "they have checked everything" but you ask if they have checked for vacuum leaks with a smoke machine, and the answer is no. There is an idle troubleshooting diagnostic tree in the factory manual that is handy.

  3. #3
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    I gave up on C3 ecm's years ago because I couldn't get the idle strategy down. It worked fine with a stock engine but when you started making changes to the powerplant the IAC didn't always work correctly.

    There are a number of factors involved in making them idle at the speed you want, at the time you want. Working VSS is critical to success. Without a valid VSS signal the ecm thinks low rpm caused by such exotic events as shifting early is actually low idle speed and it will continue to open IAC. It's also critical to making the throttle follower work correctly. Throttle follower is a function that opens IAC when throttle is opened and closes IAC when throttle is closed.

    Here is a list of everything in the IAC logic for the 7747. The 8747 will also use many of these values. Figuring out what makes the idle speed wrong, or when it goes wrong, may help figure out where to make changes in the calibration.


    http://www.gearhead-efi.com/Fuel-Inj...pdated-12-4-00

  4. #4
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    Thanks for the replies,

    Don,,, with the IAC zero'd out and unplugged the truck will idle perfect, I set it at 525 RPM. No variation, no surge, smooth a silk.

    1project, I read that post and went through everything over and over. After i posted my rat earlier today,, I have to run an hour away to load a trailer. It idled high the entire way. Turn off the truck, went inside, came out, idle at 700. There is some parameter somewhere that is default to the idle logic.

    There are 15 open/closed loop idle scalars in TP. idle vs TPS, max AFR, MPH, coolant,cold time, warm timer ETC. What the heck is "closed loop- idle integrator value" and integrator window value??

    My guess is one small entry is on the cusp of its value and some times it works and sometime not?

    I will emulate tomorow some and see if i can affect a change. I thought i had it solved with the table, IAC- position vs coolant but it did not solve it.

    All the other tuning works, timing, fueling, tunrning off the egr. park to run delay etc..

    Chris

  5. #5
    Fuel Injected! donf's Avatar
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    Quote Originally Posted by chris989 View Post
    Thanks for the replies,

    Don,,, with the IAC zero'd out and unplugged the truck will idle perfect, I set it at 525 RPM. No variation, no surge, smooth a silk.

    My guess is one small entry is on the cusp of its value and some times it works and sometime not?

    Chris
    From your description, the minimum air rate is probably correct. I would look at the IAC counts when its idling well and when its not. I use to work as a diagnostic mechanic at the Chevy dealership when these vehicles were new. There were upgraded proms but never ever was the computer reprogrammed to fix what you describe. Slight revisions but nothing drastic as turning them all off. I would look at grounds and IAC counts next, but there are a lot of inputs that can affect idle control. I really don't know why people have an aversion to following the diagnosis charts in the manual. You may end up with a steady idle by playing with the settings on the chip, but really it's a bandaid to whatever is really causing the problem.
    Last edited by donf; 12-29-2018 at 07:51 AM.

  6. #6
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    What would be awesome is having a device that i could change the RPM from inside the truck. All i really need is cold start and maybe air con increase in RPM. THink of a tow truck/roll back. In the 80s did they use a separate solenoid to increase the RPM when loading a car. Or did they electronically adjust the IAC.

    Dump trucks, ambulances tow trucks, there are a host of vehicle that required a way to increase the idle outside the OEM computer. I am thinking of zeroing out my IAC, setting my idle where i want it and have a simple solenoid to increase it when i choose.

    And yes,, I now i should just make it work properly.

    Chris

  7. #7
    Fuel Injected! donf's Avatar
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    There was a tool that does exactly what you describe for diagnostic use. I still have one somewhere out in the shop. Sticking IAC valves were so common back then there was a little tool you plugged in to move the IAC in or out to see what effect it had. A lot of times you would find a bad spot in the IAC intermittently. Intermittent TPS and CTS signals can also cause crazy idle problems.

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    Before computer controls, solenoids and mechanical devices were used to increase idle speed. But in some cases even with computer controls they used solenoids to adjust throttle angle. Ambulances, tow trucks, utility trucks are typical examples of vehicles that had aftermarket controls to increase idle speed. This is because GM didn't want to support the aftermarket guys in an inexpensve and easy to use way, and the aftermarket guys weren't interested in changing how they did things.

    There is some parameter somewhere that is default to the idle logic.
    Hmm... There may be a single value causing this. But there may a combination of values that are at fault. IAC logic in a C3 is not a simple subject.

    What the heck is "closed loop- idle integrator value" and integrator window value??
    It's part of something called PID control and it's the way the ecm is programmed to respond when the actual RPM does not match desired RPM. Think about driving... if you notice your speed is too high or low on the highway you naturally change the throttle angle. What if the only change you could make was "all on" or "all off?" It wouldn't be easy to maintain a steady speed. The PCM is programmed so you can adjust the size of the change to IAC settings so it does a better job of maintaining a steady engine speed.

    After looking through your other posts I'm agreeing that you may have another issue to diagnose. If the engine is not running well you probably want to diagnose that first.

    There's a good chance we can get this figured out. It might also be helpful to list how your vehicle is set up. I see that you mentioned a Lunti cam but which one? Are you using a stock TBI intake and heads? Factory exhaust? It might also help if we can get the logs and your .bin posted. I may be able to help with the log you tried posting in the other thread.

  9. #9
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    I have a 89 v2500 burb,
    5.7, New GM crate "stock piston/heads"
    Lunati 10180700
    .437"/.455"
    250/256.
    Headman full length headers,
    Edlebrock intake.
    4l80- w/small block convertor
    456 gear

    Later today I will log a file and upload.

    Is the C3 ecm problematic? Is there a better unit i can replace it with?

  10. #10
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    Is the C3 ecm problematic?
    It's not problematic. It's more like you're using Windows 98 on a Pentium 90. Lots of people use that ecm and make it work. I tuned them for years and made them work. But there are newer computers that can do the job better.


    Is there a better unit i can replace it with?
    16197427 from 94-95 trucks and vans.

  11. #11
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    dirty, rescaled or damaged MAF sensor if it is MAF sensor car. Vacuum leak (MAP and/or MAF) would cause RPM rise but IAC count would go to full closed position


    best regards

  12. #12
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    It is the IAC steps that are causing the high idle. I can watch them i n real time.

  13. #13
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    IAC steps are not the cause of high idle, they are only tool to achive desired idle speed that ECU commands.

    What is the count of steps when car is at 700RPM, and what count is at 850RPM.

    Can You share Your tune file ? Was the tune already done for all mods like cam and after while started acting funny or are You in process of tuning and can't find solution ?

    best regards

  14. #14
    Fuel Injected! donf's Avatar
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    Quote Originally Posted by chris989 View Post
    I have a 89 v2500 burb,
    5.7, New GM crate "stock piston/heads"
    Lunati 10180700
    .437"/.455"
    250/256.
    Headman full length headers,
    Edlebrock intake.
    4l80- w/small block convertor
    456 gear

    Later today I will log a file and upload.

    Is the C3 ecm problematic? Is there a better unit i can replace it with?
    From the way your post read up to this, it sounded like you were looking at stock suburbans with high idle speed. Yes with a cam you may have to make some minor changes.
    Last edited by donf; 12-30-2018 at 03:18 AM.

  15. #15
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    The cam is 207/213 at .050. I have 18" of vac. It idles smooth at any desired RPM all day long with the IAC unplugged. 40% of the time it will idle right even with the IAC active.

    Periodically some parameter causes the IAC to march up in counts until 800+ RPM "in gear" is reached. Sometimes it will Idle at 1250 cold. There is something,, somewhere that is missing by a small amount.

    I copied my bin file from my own OEM chip. maybe i will try a different file, or possible a completely different year/model if that might work.

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