Strange behavior of rpm when returning to idle

[Corday]

One thing that has greatly annoyed me ever since the engine rebuild finished, is the fact that the engine does not "rev down" smooth. If the car sits at idle, and i press the throttle briefly and it goes up to i.e. 2400 rpm, it doesnt go back down to idle as it should. Within the first 3 seconds, it goes down to 1700rpm and then it takes another 4 seconds for it to go from 1700 to 1500. Then it takes two seconds for it to go back down to idle at ~900. In average it takes around 10-12 seconds every time to settle down at correct idle.

I have pondered about this so much it made my head hurt, and i also went through alot of trouble while trying to figure it out... I removed the intake and changed all gaskets, i checked all hoses for vacuum leaks, i inspected the throttle body and cable but couldnt find anything.

But then I discovered something by accident while playing around in EFILive. I logged the IAC motor and was thrilled to find that this ones behavior showed a direct link between its steppings - the TPS - and RPM.

Im new to all this and have just performed my first Calc.VE run, and i thought i'd try for a solution to this as it is very annoying when driving.

Please see the attached log and screenshot

11735
11734

[joecar]

Hi Corday,

Have a look thru these threads:


Idle Tuning
showthread.php?4661-Idle-Tuning-Helpful-Info-Inside (http://forum.efilive.com/showthread.php?4661-Idle-Tuning-Helpful-Info-Inside)
showthread.php?14439-Idle-Tips-amp-Tricks (http://forum.efilive.com/showthread.php?14439-Idle-Tips-amp-Tricks)
showthread.php?5866-Auto-VE-questions (http://forum.efilive.com/showthread.php?5866-Auto-VE-questions)
showthread.php?14153-scan-tool-wont-log-rafig-or-rafpn (http://forum.efilive.com/showthread.php?14153-scan-tool-wont-log-rafig-or-rafpn)
showthread.php?14435-Need-help-Can-Start-Car-but-dies-when-I-give-it-any-throttle-before-going-to-CL (http://forum.efilive.com/showthread.php?14435-Need-help-Can-Start-Car-but-dies-when-I-give-it-any-throttle-before-going-to-CL&p=129519&viewfull=1#post129519)
showthread.php?7011-ETC-cars-and-IAC-parameters (http://forum.efilive.com/showthread.php?7011-ETC-cars-and-IAC-parameters&p=61455&viewfull=1#post61455)
showthread.php?14544-Working-on-my-idle-tune (http://forum.efilive.com/showthread.php?14544-Working-on-my-idle-tune)
showthread.php?14794-Help-with-open-loop-(cold-start)-tuning (http://forum.efilive.com/showthread.php?14794-Help-with-open-loop-%28cold-start%29-tuning&p=133446#post133446)
showthread.php?149-Idle-(Transition)-Tuning (http://forum.efilive.com/showthread.php?149-Idle-%28Transition%29-Tuning)

More Idle Tuning
showthread.php?t=149 (http://forum.efilive.com/showthread.php?t=149)
showthread.php?t=5866 (http://forum.efilive.com/showthread.php?t=5866&highlight=RAFIG)
showthread.php?p=86553 (http://forum.efilive.com/showthread.php?p=86553) post #17
showthread.php?t=2630 (http://forum.efilive.com/showthread.php?t=2630)
showthread.php?t=473 (http://forum.efilive.com/showthread.php?t=473)

Throttle Cracker/Follower
showthread.php?t=3568 (http://forum.efilive.com/showthread.php?t=3568)
showthread.php?t=4081 (http://forum.efilive.com/showthread.php?t=4081)
showthread.php?t=5406 (http://forum.efilive.com/showthread.php?t=5406)
showthread.php?t=5940 (http://forum.efilive.com/showthread.php?t=5940)

[Corday]

Thanks! I have gone through most of those links, and although i have become wiser in some areas, im still clueless about my IAC behavior...

Is there a "term" for what i am describing in the first post? Idle tuning seems to be a vast area of expertise, and i have no idea where to start... I was hoping for a quick fix so i can continue with Calc.VE without being annoyed and embarrased when the car stays at way too high rpm for too long :) People are laughing... Help! :)

[joecar]

Term is "hanging idle".

Yes, idle is a bit more complicated than VE/MAF tuning.

They won't laugh when you can out-accelerate them :cheers:

[macca_779]

Increase your throttle follower decay rates.


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[Sid447]

Assuming your desired airflow (B4307) isn't too high,

Try making some changes in tables {B4315, 4319 & 4320}