Is this ever necessary? If so, under what circumstances?

 

The IAC is used in a number of ways.

For starting, Throttle follower, A/C load correction, P/S load correction, and idle speed.

If your car has a cold start rpm flare up you can play with the IAC limit and park route, start stuff to reduce that.

On some masks you can command a low idle rpm, and then run a 0 IIAC count at idle for a slightly more regular idle.

The Trottle Follower opens the IAC some anytime you go off idle, and often is used to slow the engines return to idle speed. This is the one that some folks play with, but it can be exasperating.

If you go for playing with the Throttle follower don't be surprise if you have to chase the AE around some. Since it's kind of an air bleed. Reducing the T/F basically adds a touch of AE.

 

Grumpy, Thanks.

What I'm finding is that while driving around, sometimes the engine doesn't want to idle down while coming to a stop and the auto trans tends to downshift somewhat firmly. This is particularly noticeable after getting off the freeway. After I stop, if I shift into neutral, the RPM shoots up to about 12-1300 rpm for a second and then comes back down to my normal 650 rpm idle speed. Other times it behaves normally. What's more is that there seems to be no correlation between ambient conditions and this behavior. Will modifying the IAC steps get this under control? Timing and fuel adjustments have had no effect.

 

That's the throttle follower routines.
I was just thinking while mask 42 specific there might be a paper on some of the routines at DIY-EFI.org, or GMECM

 

I've seen a car exhibit those same simptoms when all that was needed was cleaning of the throttle body and iac airways...It's not alway computer science.

 

Thanks. I'll check it out.

 

Yeah, not idling down quickly as you come to a stop sounds like a sticking IAC for sure. The surge when you put it in neutral could also be from the IAC sticking in the higher loaded position.

The fact that these only happen part of the time would really lead me to believe it's a sticky part as opposed to something in the tune.

It amazes me what a POS these IAC's are. When i got my car it had a new IAC on it. Within a year and a half i was already completely useless. I've dissassembled and cleaned it a few times, which helps for a bit, but it doesn't last long. Going through the records i got from the original owner, looks like he was getting IAC's replaced at the dealer every so often too so it's not just me. And i hear a lot of people trying to chase 'ghost' problems (stalling, surging, hesitation, etc) that all mirror what i know a bad IAC to do.

 

And maybe limiting the range of motion that it runs thru all the time will lead to less wear and tear on it.
After all the pintle just rides on that little soft plastic bushing in there.
The fuel detergents also may have a large role in it all, seems like some intakes develope alot of gloop in them, and having that in the IAC passage way isn't going to do the IAC any good.
Maybe also limiting the IAC throttle follower would generate a higher velocity of air thru the passage way, and min the gloop formatiions.

 

One thing I did recently was to disconnect the PCV from the throttle body and run a simple breather cap. I was noticing some oil developing in there so I should probably go back and clean the throttle body out. Maybe some of that junk got into the IAC motor and is causing problems. Along with lowering the IAC steps in the calibration it should probably do the trick.

I did a search on IAC steps and from what I gathered, the thing to do is set the throttle blade idle position such that the IAC steps are less than 10. I only want the IAC coming on when it's necessary.