Open Loop Idle Swing
I’ve searched through the forums for an answer, but nothing seems to fit my particular case. Here’s what I’ve got;
1998 Trans Am, 6-Speed
Recently installed a FAST intake and 92mm TB, and an idle problem has started since then.
IAC valve is working, no intake leaks, manifold pressures/vacuum are normal.
Idle is steady when I have it set at 850 (fluctuates slightly when set to 800) and the car drives fine with no surges or dips when I put it in neutral or have the clutch in while moving. (RPMs are a little higher than usual when coasting, which I plan on working with the throttle cracker later.)
In neutral in the driveway, I can increase the rpms to 1500 or so, hold it steady, then let off, and the car returns to idle well enough without much of a dip. Personally, I’d rather see no dip whatsoever.
The issue I’m having is that if I let the clutch out slightly and bring the rpms down to around 700 without the car moving, then I push the clutch back in, the rpms surge to about 1200, then start cycling up and down between 700-1000 unless I apply throttle to stop the cycle. It’s a bigger swing if the rpms are held low for longer and smaller swing if it was only for a moment that the rpms were down. I can drive around this problem, but it’s a street car, and it makes stop and go driving a little uncomfortable, IMO.
It’s in open loop all of the time.
I’ve spent 7 hours messing with idle airflow tables and nothing effected the swing. So I have some questions…
- Does idle airflow learning occur if it’s never in closed loop/Does my PCM ever use learned airflow corrections? If so, is there a way to reset any learned values?
- How big do the changes have to be in the airflow corrections tables to make a noticeable difference?
- The P/N values in airflow corrections are all 0 from the factory, so what is the PCM using when idling in the driveway while in neutral? The statement in EFILIVE isn’t clear to me:
“Vehicles fitted with a manual gear box are assumed to be in gear unless the clutch is depressed.” So, is there a difference between being in neutral vs being in neutral with the clutch pushed in? Seems like a silly question, but...?
- For which conditions should B4512 be adjusted?
- My throttle body is bigger, do I need to change IAC effective area?
I apologize for the long post, but how the heck do I edit my tune to make the idle stop swinging? Thanks in advance!
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