F-body-fan

Looking for some clarification on the IAC after reading so many topics over the years.

1st, Is the IAC numeric position that is provided from the ECM, the actual position of the pintle, or is it what the ECU would like it to be?

2nd, is there really an IAC reset, or is that just a popular misnomer?
I understand the process of closing the pintle in diagnostic mode, to set the minimum air in the throttle body. But after the iac is plugged back in, is anything actually relearned in the ECM, or reset with it? I don't really see how there would be a reset but maybe when its plugged back in the ECM starts at zero & its learns that new position?

Vader

There is NO position feedback from the IAC to the ECM. The "counts" you are reading from the ECM data are the steps applied to the IAC to get it to a presumed position, but if the IAC is filled with varnish and sludge, it may not be complying with ECM instructions.

The "reset" is what you think it is. It's an adjustment of the throttle minimum air position to allow the ECM to control the IAC for a desired idle without having to exceed its maximum number of ± steps to get it there. The ECM RAM will store the learned value of average steps required for starting IAC position.

F-body-fan

What initiates the learned value of average steps? OR is it constantly re-learning?

I guess what I am asking is, seeing the stepper motor has no feedback, The ECM would have no clue where the pintle is located. Does the ECM at some point close it completely & then start counting back a preset amount of steps? otherwise I am confused how does it figures out where zero actually is? or maybe these steps are not really much of anything other than a starting point.

Vader

It's actually not that sophisticated. Depending on coolant and air temperature, the start routine will add some steps to the IAC whether it needs it or not. Once running, the target idle is calculated with input from CTS, IAT, run time, and a few minor variables. The ECM will simply step the IAC open or closed depending upon the target idle and RPM input from the distributor reference pulses. The integrator can hold any skew in the steps required at a given condition during a run cycle, and perhaps affect BLM tables in the '730 ECMs, but I don't think that happens in the '165s except on a hot restart.

The most important factor is that the IAC must respond to the input from the ECM without hesitation, or the dither (error band) of the IAC could get really loose with variable idle RPM resulting.

As you stated earlier, there actually is no "setting or resetting" the IAC since it is just a dumb actuator slaved to the ECM, The real setting is the throttle plate closed position, and that should be in conjunction with a thoroughly cleaned and lubed TB and IAC.

F-body-fan

This make perfect sense now & clears up my confusion. I always thought that the counts at idle should be a specific number & I am realizing its more of a number range once the ECM has figured out its approximate position. There are a couple things that have added to my confusion over the years, up until your explanation.

I don't recall where the thread is posted, but there was a topic I was reading when I was working on my 87 a year or 2 ago. In that thread you were explaining the minimum air setting procedure & somebody jumped in & posted something along the lines of, "set the idle so the counts are at 20 & call it good" there was no contesting of his comment, so I just figured it was another way to do it. What I have learned today is that information would not be very accurate & guesswork at best.

This also explains why I have seen crazy high counts after doing my min air adjustments. sometimes I have seen it well over 100 counts, But now this makes sense. it took 100 counts to get it to idle from the pintle slammed shut & ECU thought the pintle was in start up position. I have also seen counts in low single digits too. Now I am realizing these numbers we can read from the data stream are nothing more than a means for us to monitor the iacs activity during other processes. How many steps from where its at, not so much the exact numerical data its starting out at.

Thanks for the clarification with this. it just got a little easier.

F-body-fan

I recorded some IAC activity that I found interesting & has helped. I recorded the signal at the idle air control wires so I could see what its actually doing, or should I say being told what to do At start up of the warmed up engine, the iac takes 100-110 initial steps in from 144 within the first 7 +/- seconds, then as the idle settles in around 1000 rpms, the ECM will keep applying steps at a slower rate following its desired idle at the time based on engine temp, etc. What I didn't realize completely until now is once the engine is shut off the ECM closes the iac with (I counted) 224 steps to completely seat the pintle, then brings it back 144 steps so its ready for the next restart. This happens within 3-4 seconds.

This makes perfect sense why the minimum idle setting is so critical. The first start up is a combination of 144 steps on the iac & wherever the the blades are set.
The combination is going to determine how easy it starts & how well it initially runs the first few seconds afterwards as the computer takes control of everything.
if it doesn't start smooth its going to be trying all kinds of adjusting to make it right resulting in a rough startup.

Here is the activity. These are the steps I counted in my case, but still gives a good visual of whats going on when working as it should.

The second image is a zoomed in image of just the last seconds after the engine is shut off, that sets everything back up for the next restart.