how do I make my IAC do what I want it to?

[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, having some trouble around idle. It doesn't want to catch the idle after giving it a rev, I've got it pretty close (see logs attached), however it's still not 'right'. I've attached a second log from a mates car, 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, it drops below target idle speed and 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 (although the numbers look low if comparing to other tunes), I did the Russ K idle config a while back. My cam is a 227/223@115 for reference.

Now, in the logs attached, 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, you'll see in the logs 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.

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. Any help would be greatly appreciated.

Cheers, Ben
vt ss unknown cam.hplmy idle 2.hplcurrent tune.hpt

[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?

[68Camaro]

I like to set my base running airflow a little higher than what is actually required to hold a steady idle to help with this condition. throttle follower and throttle follower decay are important also ^ and I see you have been working with these. I can't look at your tune or log file right now but will try and look later. Add a bit of throttle follower air and slow down the throttle follower decay a bit and see if that helps.

as far as your IAC dropping to 0 when you rev sounds like you may have a grounding problem or a short somewhere. try a different IAC if you have one first and check all grounds.

[Ben.]

Thanks for the assistance 68Camaro. Yeah I was a bit suspicious about the IAC being dodgy, however, I did some more playing today. this is what I found:

When I have all my integral values zeroed out, and just use proportional values to control idle air, if I give it a rev, the IAC steps will stay at around 40 (normally about this at idle), even as I bring the revs up slowly and hold it, or if I just give it a quick stab of the throttle, the IAC values don't seem to fall below this, they stay around 40, then as the revs drop close to target idle values, it will spike out to 85ish steps. Doing it this way, it ALMOST catches it nicely, normally drops just a little under target idle rpm.

If I add any integral values, the IAC will drop to 0 when I start opening the throttle, even just slowly bringing the revs up to just above idle and trying to slowly bring it back down, it will drop way below target idle rpm and then catch and bring it back up.

I've come to the conclusion that the PCM is having trouble deciding if the car is in an idle state or not. even if I bring the revs up to 1500ish, the IAC reduces to zero and it pulls some timing also, as if its trying to get it back down to 800rpm.

As I mentioned above, I scanned another car that ran nicely, in neutral at idle, it was around 60 steps, hit the throttle to give it a quick rev and the IAC goes out to about 120 and slowly comes down to return it to idle. Although that car did have some throttle follower, it was only in small amounts, and when I try adding some in, it doesn't seem to do anything unless I add in huge numbers, and very slow decay rates (which caused other issues for me).

I've attached another log for anyone playing along at home, I tried putting my PID's back to almost stock values (softened the integrals a little), and this is the result (and basically how the car was when I got it)idle.hpl

[68Camaro]

tried to look at your tune file but says it is tunerlocked

[Ben.]

Yeah, that's embarrassing... I'm protecting my intellectual property

Re-attached my current tune. Did some more trial stuff this morning, now that I have VCM scanner working properly again, I think I may have found My issue. I can see that at idle my dynamic airflow is about double my desired airflow. and as throttle is applied, obviously dynamic airflow goes up, and the desired airflow comes down to 2g/s, despite my Base running airflow now being about 6.5g/s. How I fix this though, I have no idea.

As I understand it, dynamic airflow is a calculation the ECM does to work out how much air is going into the engine, based off VE and the other data in the tune. So this morning I reduced my VE (and added some PE in idle regions to richen it up a bit), but I'm getting to the point now where my PE at idle is getting up to 1.3ish and I still cant get my VE down enough to get my dynamic airflow anywhere near desired. Currently dynamic sits at about double the desired.

Thanks again
working tune - trial, reduced VE, increase PE, Stock PIDs.hpt