Help with PID controls for IAC.

[Ben.]

Short version: I want my IAC to stay open with throttle application, then slowly taper back down to idle when lifting off.

Long version: Hi guys, I use HPtuners but have found this forum to generally be more helpful than the HPT forum, so I thought I'd join up and ask here!

I'm having some trouble around idle. It doesn't want to catch the idle after giving it a rev, I've got it pretty close, however it's still not 'right'. I've scanned a mates car for live data, 1 bar MAF enhanced, car is AUTO, unknown cam specs, however from listening to it at idle, it sounds as though it has more overlap than mine. His tune still retains closed loop fueling at idle, almost stock PID's for idle control (just slightly softened Integral values) and fairly aggressive under and overspeed timing. (9* each way). It runs and drives great.

My tune is 3bar SD, has fairly hacked up PID's, open loop fueling at idle, and idle spark tables cut right back to stop idle swings. It idles rock steady at 800, but if I give it a rev, on return to idle it drops below target idle speed and then picks back up. It also has a bit of oscillation when I'm slowly letting the clutch out with small throttle openings. I'm confident my Base Running Airflow is very close, I did the Russ K idle config a while back. My cam is a 227/223@115 for reference.

Now, in the logs of mine and my mates car, I notice my mates IAC position goes up with small throttle applications, and SLOWLY comes back down to idle nicely, mine does weird things with light throttle like going to 0, then spiking back up, then back to 0. With the tune I have now, if I give it a rev, the IAC goes to near 0, then spikes up to about 80 just as its returning to idle, before settling back into its normal idle range values (38ish steps).

What is causing our two IAC's to act so differently? I've tried all manner of PID configurations, from all stock, to slightly softening each individually, to softening all of them, tried increasing them etc, tried adding in throttle followers, tried running massive amounts of Base Running air flow to try and make it catch really high, then use the PIDs to slowly bring it back down but nothing seems to work. Anything I do results in it dropping to 400ish RPM, then springing back up. Any help would be greatly appreciated.

Cheers, Ben

[joecar]

Hi Ben,

Firstly, given 2 different cam specs, each engine (assuming same intake manifolds and same throttle bodies) flows a different amount of air (at idle and anywhere else).

Secondly, the idle and throttle parameters/tables determine when and how (damping factor) the return to idle occurs... this in turn determines how the IAC behaves (in order to bring engine speed down) and how much IAC and spark timing interact...


I would suggest going back to stock, getting each VE table correct, and then tuning for idle.


( also, make sure ignition timing has enough range to control idle )

[Ben.]

Thanks for the reply joe, I appreciate you putting in the time considering I can't provide you with a log or tune you can read. My VE table is dialed in and I have timing where I want it. I did this by tuning for best vacuum and smoothest idle the old fashioned way. I have a pretty sound understanding of basic PID principles, just not how the PCM implements them. I have tried going back to stock, it all goes pear shaped very quickly.

To simplify my question as much as possible: As it stands now, with throttle application, my IAC steps falls to 0, is there any way to keep this at around 40 (where I idle) or greater? Without adding in throttle follower, as this causes other issues.

[Ben.]

No one with anything to ad?

Is there anything apart from throttle follower I can do to keep MY IAC open? is there a minimum IAC value to set anywhere that maybe I'm missing?

[ScarabEpic22]

Hi Ben,
Not sure if these will be directly applicable, but I bet with a little work you can make them work with HPT. I don't deal that much with LS1B/P59 PCMs (more of a Gen IV guy that all have DBW) so cant help a ton with the IAC steps. RAFIG PDF Thread Idle Tuning Helpful Info Inside

[joecar]

Hi Ben, sorry, I missed your last post.



Yes, I agree with Erik, read this thread: Idle-Tuning-Helpful-Info-Inside

[joecar]

If your IAC shows zero counts, try to open the throttle blade a little using the stop screw (you will have to reset the TPS).

[Ben.]

Nah, it actually idles at about 40 counts, but drops to zero with throttle application, so if I'm taking off from the lights, I give it light throttle, the revs quickly come up to about 1500-1700, then the IAC closes and it drops down to about 1100 revs and the IAC intermitently spikes back up to 40 or so, then down to 0. It makes it really weird to drive. However, it only does this if I have some integral effect acting on the IAC. If I zero out all my integral tables, and just use the proportional, the IAC seems to stay at around 40 counts (where it idles) all the time, even with throttle application.

I've come to the conclusion now that the PCM is having trouble deciding if the car is in an idle state or not. With small amounts of throttle, it seems to reduce IAC and spark timing in an attempt to get it back down to 800rpm (idle).

Thanks for the input guys.

[joecar]

Hmmm, interesting... does it think it has too much air when this happens...?

[Ben.]

The light bulb above my head just came on after reading that joecar. This is quite embarassing for me at this point as I remember reading somewhere on here about dynair vs desired or actual or something similar (for the life of me, I can't find it now). Coupled to this that I only got my scanner working properly yesterday, I can now finally see that my 'dynamic air flow' is a fair way off desired airflow, and I suspect this is a big part of my problem.

How close should my desired vs dynamic air values be?