Engine 1988 Foxbody idle issues

[fernandosgarage55]

Hello everyone, I’m looking for some help or advice with my Foxbody. I’m having an issue with the idle—it’s idling too high when the clutch is disengaged, and it drops back down when I press the clutch in. The car doesn’t have any major modifications: E-cam, GT40 intake, BBK mass airflow, and a 70mm Accufab throttle body.

Any insights or suggestions would be greatly appreciated. Thanks in advance!

 

[KRUISR]

Have you checked for vacuum leaks? Have you done a base idle reset after the changes (or recently if it has been a while)?

 

[fernandosgarage55]

Have you checked for vacuum leaks? Have you done a base idle reset after the changes (or recently if it has been a while)?

No vacuum leaks. A few months ago idle reset was done.

 

[KRUISR]

So what is idle now - normally. What ECU are you running? Did you or someone else do the MAF conversion?

 

[keel]

I would see if your neutral sensing switch (on the transmission) or neutral gear switch (under the dash) are working properly.

 

[Mcmahst]

What is the vacuum reading at the normal, not high idle?

 

[General karthief]

Codes?

 

[fernandosgarage55]

So what is idle now - normally. What ECU are you running? Did you or someone else do the MAF conversion?

No, Idle still issue, ECU 8LD, that’s how I bought the car. I also tried A9L still same issue.

 

[KRUISR]

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?

 

[AeroCoupe]

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.

 

[fernandosgarage55]

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?

I will be running a A9L.

 

[Mcmahst]

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

 

[fernandosgarage55]

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

 

[KRUISR]

I will be running a A9L.

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]

I apologize for the confusion, now im using A9L, when it starts it sits at 4000, when I press on the clutch it lowers to 800.

 

[Mustang5L5]

WHat is the car? What year/model so we know what wiring we are dealing with? WHat transmission was it originally equipped with? These are important because there are wiring nuances between the cars that will affect how it runs

4000RPM to 800 RPM is a big jump. There is a neutral sensing circuit that runs in parallel with the clutch pedal and the switch on the top of the T5 case. It does have some effect on idle strategy and can cause a hanging and normal idle depending on the state. However, a 3K RPM difference is huge.

Step #1, dump codes. See what you get. I wonder if you get a code 67 when you try to run the KOEO codes.

When it's idling at 4K RPM, unplug the IAC. What RPM does it fall to then?

 

[fernandosgarage55]

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.

 

[keel]

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.

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.

 

[AeroCoupe]

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.

 

[Mustang5L5]

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.

That is correct. Base idle reset procedure needs to be performed here. Car should idle on it's own with the IAC unplugged by setting the throttle stop screw.