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Thread: Surging idle when coasting

  1. #11
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    Look at your timing when coasting vs the timing when you are idling... your coasting timing should be slightly lower than the idle timing and the air going through it should be higher than idle. The purpose is to create enough torque reserve so idle routines can easily bring it back to idle.

    With all idle routines shut off (no iac adjustments and no timing adjustments) you want your car to naturally idle 200 rpms higher than your commanded, and then let your idle routines bring it down. The more unstable the idle is, (more reversion caused by overlap) the more torque reserve you need to achieve proper deceleration/coming back to idle and idle stability. This is contrary to what most people recommend which they want you to adjust your timing so that you get the most vacuum, thus removing ALL torque reserve from the routine and thus extremely unstable because now no timing changes make any difference in the torque output of the motor that is idling.
    "All that is needed for the triumph of evil is that good men do nothing..."

  2. #12
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    Also.. check your intake are temp sensor.. Why is it -38? this causes huge problems with VE calculations and other idle adjustments. Car needs to be 100% before touching the computer. Your trims are way too negative, which means the vehicle is overfueled and the STFTs are catching up. During those issues you will have a lot of instability. FIX THOSE FIRST.
    "All that is needed for the triumph of evil is that good men do nothing..."

  3. #13
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    Quote Originally Posted by gpr View Post
    Also the IAC steps are still around 140 even after the engine is warmed up and idling.

    Attached is the log and a screen shot of the log.
    I'm on a really tight internet downloads budget right now (metered internet in Australia) so I can't download the latest version of Efilive... which means I can't look at your tune file. The screenshot of your log shows me that your idle airflow is now way too high... When you alter your airflow (cracking the blade or drilling a hole), you need to adjust the IAC Effective area table. I would recommend you shift everything in that table down about 4 or 5 cells and see how close that gets you. The really high idle airflow is what's messing up your idle trims... it's complicated, but trust me on that one. Once you get your airflow sorted out, your trims will fix themselves up and you'll probably have to adjust your VE table again.

  4. #14
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    Quote Originally Posted by Highlander View Post
    Also.. check your intake are temp sensor.. Why is it -38? this causes huge problems with VE calculations and other idle adjustments. Car needs to be 100% before touching the computer. Your trims are way too negative, which means the vehicle is overfueled and the STFTs are catching up. During those issues you will have a lot of instability. FIX THOSE FIRST.
    At the beginning of the log I forgot to plug the IAT sensor back in, thus why it didn't want to start and idle with out me holding the throttle open. After I saw IAT at -38 I went and plugged it in and it started and idled better. (had the intake tube off to drill throttle blade)

    If it is getting too much air, why doesn't it close the IAC? I thought that was the point of drilling out the hole so it would close the IAC down and be around 50 steps instead of 130? To correct the IAC effective area table, how should I correct this now since I drilled out the throttle blade?

    I think the ltft are that far off from me adding 10% to the VE table in the idle rpms. I did this to see if it would idle richer, which it didn't. I will revert the VE table back to what it was.

    Also I thought that it was pulling timing at idle to back off the rpms to idle at the correct RPM i'm commanding. Should I change the Idle Desired Airflow table to command less airflow at idle? Would this help close the IAC?

  5. #15
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    Quote Originally Posted by Highlander View Post
    Look at your timing when coasting vs the timing when you are idling... your coasting timing should be slightly lower than the idle timing and the air going through it should be higher than idle. The purpose is to create enough torque reserve so idle routines can easily bring it back to idle.

    With all idle routines shut off (no iac adjustments and no timing adjustments) you want your car to naturally idle 200 rpms higher than your commanded, and then let your idle routines bring it down. The more unstable the idle is, (more reversion caused by overlap) the more torque reserve you need to achieve proper deceleration/coming back to idle and idle stability. This is contrary to what most people recommend which they want you to adjust your timing so that you get the most vacuum, thus removing ALL torque reserve from the routine and thus extremely unstable because now no timing changes make any difference in the torque output of the motor that is idling.

    I'm a little confused by this. How is it even possible to get your spark timing when coasting less then when your at idle? Because when I'm going down the highway and let off timing usually goes to 40 which is what is commanded in my High octane spark table, then as I slow down and the engine rpms come down it drops back to 18 which is what is also commanded in High Octane Spark.

    Or are you talking about if I am in park and rev the engine and let it come back down on its own? As the rpms come down the spark should be less then when it is trying to maintain idle? Because in the last log I posted and on the screen shot shows where I cracked the throttle. You can see the engine rev and as it comes back down it goes to the default 18 degrees and stays there for about 5 seconds before it starts changing. It looks like where the timing is at 18 degrees the rpms are too high. This leads me to believe i am commanding to much air flow or it is getting too much from the larger throttle blade hole.

    Also why does it take so long before it starts to correct the idle by changing the timing? The vehicle isn't moving and the throttle % is .4% like it should be.

  6. #16
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    Quote Originally Posted by gpr View Post
    How is it even possible to get your spark timing when coasting less then when your at idle?
    Is this a serious question?

    Quote Originally Posted by gpr View Post
    Because when I'm going down the highway and let off timing usually goes to 40 which is what is commanded in my High octane spark table,
    Oh look, you've already answered your own question.

  7. #17
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    Get IAT fixed as Highlander pointed out.

  8. #18
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    Alright its been a while. Life has been crazy, so not much tuning time and its that time of the year to park the car away. However, I wanted to get the idle dialed in before I forget about it next spring.

    Anyway I modified the IAC Effective Area table as mentioned by shifting the values 4 rows down. Here is a log of the tune with this modification.

    It is a lot leaner at idle, but before I start messing with correcting the VE I wanted to get everything else nailed down.

    The IAC steps once warm only comes down to 115. and the air flow grams/cylinder ~.3. Is this too much airflow still? I don't know what it should be on a 383ci engine with stage 2 turbo cam.

    It didn't want to idle when cold unless I held the throttle open slightly. Anytime I let it close it would about die. Also it didn't idle as high as it should of when cold, even though the AFR was still pretty rich. (it was around 12:1 afr, but remember this is running E85). HOwever when warm it seemed to idle decent and didn't hunt for idle much when i would give it a little throttle.

    So where do I need to go from here. Should i change the IAC effective area table a little more so the IAC steps come down? Also what would be causing it to idle lower then desired idle speeds?
    Attached Files Attached Files

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    Also I noticed in the log that when I open the throttle the IAC also opens, is this normal?

  10. #20
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    Quote Originally Posted by gpr View Post
    So where do I need to go from here. Should i change the IAC effective area table a little more so the IAC steps come down?
    You shouldn't be adjusting your IAC effective area table for the purpose of reducing your IAC counts (but that does happen). You adjust the IAC effective area so that you get your desired airflow and actual airflow match. If your airflows don't match, then you need to adjust that table until they do.


    Quote Originally Posted by gpr View Post
    Also what would be causing it to idle lower then desired idle speeds?
    Most likely not enough commanded airflow.

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