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