Idle Air Control

It sounds like the IAC is not doing too hot. Have you cleaned it? Have you done any funky tuning with your IAC function that could have affected it (i.e. made it run at 100% DC)?

As I recall, the IAC uses PW modulation. Ergo, it'd take watching the square wave function on a scope to get a real good idea of what it's doing (any datalogging type stuff not-withstanding).

Good luck bud.

Like JT said, the isc could be stopped up or something.

Here is my take on a stable idle

If everything mechanical is OK ........

1) I'd bet you got the tb blade stop screw too far out from past attempts to
raise the idle speed.

2) You may have your bleed screw too far in or out as well.

Those two things kinda work together

In the pcm you got ....

3) tb airflow

4) isc neutral idle

5) idle neutral rpm

that work together as well

Then you got the relationship of the mechanical adjustments (1&2) working with
the pcm values (3&4&5) which need to be optimized for the best idle possible.

Item 1 from above .........
is not for raising the idle speed
it is for preventing the tb blade from sticking on the tb bore at closure

I'm not saying it won't increase idle speed
I am saying you might effect other things when you use it for that purpose

The idle speed is mainly controlled by the pcm

If one has access to the pcm ...........

Doing 1&2 without doing 3,4,&5 is not gonna be effective
and
Doing 3,4,&5 without doing 1&2 is not gonna be effective

Other things can come into play for a stable idle but .......
It is my hope you can see the main things that from a relationship
between mechanical and pcm with this little bit of info here

The whole issue is the pcm is told how much airflow to expect at idle
rpm. The pcm also knows airflow values a bit below and above idle
rpm so it can bring in the isc for help with a stable idle as the ac is
cycled in and out as an example of load consuming accessories which
can lug down your idle rpm thus causing an unstable idle.

When we change mechanical things such as 1&2 from above .....

We may have done enough to throw other things out of whack
and
We may have not

Now ... Lets talk about a Stang with mods like blower or h/c/i

We have changed idle airflow values ... Big Time
which means
The values in the pcm are no longer gonna work

If we then hose around with mechanical stuff like 1&2 ...

There is a good chance we have unintentionally added to the
airflow issue error all the more

As an example to show how things can change with mods

I had to add about 70% to my tb airflow pcm value
and
About 10 to 15% to my isc neutral idle pcm value points

Then I had to go over the mechanical adjustments to make
everything play nice with each other for a stable idle.

Just some things I've observed by hosing around with my little GT

Grady

Guero said:

But if i had my t/b screw out to much... wouldn't I idle high when i unplugged the IAC??? When I disconnected it idles like a 800-1000

Click to expand...

I am gonna ask Mr. JT to take over here as I have seen him post very good stuff
about doing things with the isc I have just never dealt with.

I've only done it where the isc is totally involved in the whole process and is known to be in good working order. I've never dealt with an isc that is not functioning correctly so I got no experience ... don't ya see

I look for JT's or any other peeps experience here and hope to learn as well

Grady

Guero,

Do clean your IAC. If your brake cleaner is the type that doesnt leave a residue, I would: first use WD40 to clean the IAC. Then douche it with the brake cleaner (I like electrical contact cleaner, but residue free brake cleaner is just about the same). The WD is a solvent that breaks coke up. Then rinse with more WD and flush with the 'clean' cleaner (brake clean/electrical contact cleaner).

How does your car idle when cold? Are the RPM's higher than whatever RPM it's sticking at now? Do you have a means of seeing IAC duty cycle (a code scanner [not a reader but a scanner that views PIDs])? Any insight gleamed from little stuff like that helps.

I really have the feeling that once you clean it, the issue will change. If so, you can feel good about the IAC being stupid. If you clean it and the issue doesnt change, then I'd scratch my bean for a minute.

Grady's great info was lost on me. I only have two screws I can turn.
That said, here's where my thoughts are coming from:

I wouldn't hose with the throttle stop screw too much because right now when your IAC is at 0% DC (unplugged), your idle is where you want it. This is not how the smart folks like Grady set their idle, but it is how I do mine (just to say it can work, especially for those of us who can't tune). You're already above the commanded idle so the IAC shouldnt be doing anything when it's plugged in (hot, no-load idle).

One more thought: Is there any chance your IAC bleed screw (that POS under the rubber plug) backed way out? I think Grady asked about it earlier, and it's something worth asking again because that would be a 5 second fix.

Wow, lots of rambling from me there (been the same all day. Ugh).

Good luck bud.

Guero, I'm kinda stuck here. NOrmally we'd think it was an issue with the IAC sticking itself. However, when you disconnect the electrical, the issue is fixed. That makes me wonder if there is a PCM signal issue. Without seeing duty cycle, I'm not sure what I'd try next.

I know what you're going to do but it scares me.

A toggle switch on the IAC motor input wire.

FWIW, you totally should be spitting idle-limit codes with this occurring.