Engine 1988 Foxbody idle issues

What is your idle at?

8LD is a Speed Density computer and a A9L is a Mass Air computer. They aren't interchangeable without other changes. Which one are you trying to use RIGHT NOW?

Need to know what year car this is and again have you checked for codes? Even if the CEL is not illuminating there can be codes.

Can you post a pic of the engine compartment so we can see the layout, where the mass air sensor is, etc?

fernandosgarage55 said:

I will be running a A9L.

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I didn't ask what you will be running, I asked what you are running NOW, because you say you are having idle issues right now. Lets start with understanding there.

And for the third time what is your current idle speed? When you say it is good and when you say it is too high?

fernandosgarage55 said:

1988 mustang gt 5.0 convertible, T5 trans is what the car came. The only thing I changed was dash fuse panel harness from 1989 5.0. When unplugging IAC falls to 0, car shuts off.

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My understanding is that the car should be able to idle with the IAC unplugged by setting the idle speed with the mechanical throttle stop screw. The IAC really only exists to compensate for extra load at idle from accessories like the alternator or A/C.

A few more questions so bear with us.

Was the car running just fine and then all of a sudden this started happening?
Is the car an original mass air car or was this added? Just asking because most 88's are speed density except California cars and I see you are in California.
What size is the BBK mass air meter and what size injectors is it set up for and what size injectors are you running?
Can you elaborate more on the "fuse panel harness from an 89 5.0"? Do you mean you swapped out the whole dash harness? Depending on your answer here there may be more questions.
Mustang5L5 and I have both asked if you have checked for codes so have you and if so what did you get? At a minimum you should get a Code 11 then a Code 10 and then a Code 11 if everything is good to go. If you do not get any codes whatsoever this leads to the ECU having a problem.