Engine 1988 Foxbody idle issues

[Mustang5L5]

The only thing I changed was dash fuse panel harness from 1989 5.0.

'88 Cali MAF car with an 8LD originally? Now an A9L? Still running the 5-pin MAF?

There are some wiring diffwrenced between '88 MAF and '89 MAF cars. Trying to rack my head and think of if any can cause your issue.

As far as you know the T5 top plug NGS is still hooked up?

I would dump codes to see if anything is present and then perform a base idle reset procedure. Throttle stop screw needs to be opened up so that with car running the engine will idle (albeit low) with the IAC unplugged.

Not necessarily the full solution to your problem but it's checking off a few boxes in terms of things that it is not.

 

[fernandosgarage55]

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 problems

I bought the car, but it wasn’t running did a lot of work to it suspension paint, harness was butchered by the fuses panel so I decided to replace it completely an engine harness off of 89.

 

[AeroCoupe]

You should have lead with the info above as it would help us get to a solution a lot quicker. So you replaced the entire dash harness and engine harness from an '89 V8 car, correct?

@Mustang5L5 - the answer to this will tell us a lot here.

 

[fernandosgarage55]

You should have lead with the info above as it would help us get to a solution a lot quicker. So you replaced the entire dash harness and engine harness from an '89 V8 car, correct?

@Mustang5L5 - the answer to this will tell us a lot here.

Yes, complete.

 

[AeroCoupe]

Did you add a check engine light being the dash harness would have that capability? Either way you need to check for codes as one can be present and not trip the CEL if you added one and if one is present and you do not have a CEL you won't know until you check for codes.

Electrical Thread 'How To Pull Codes From EEC-IV In 86- 95 5.0 Mustangs'

Jan 27, 2016

@Noobz347 - The code dump procedure isn't in the sticky - you may want to add it...



Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for...

• jrichker
• Replies: 10
• Forum: Fox 5.0 Mustang Tech

• 

At a minimum you should get a Code 11, Code 10, and then a Code 11 if everything is functioning correctly according to the ECU. If you get no codes or just end up not being able to get the ECU to go into the diagnostic mode then the ECU most likely needs to be repaired. Check for codes first and we will go from there.

 

[fernandosgarage55]

Did you add a check engine light being the dash harness would have that capability? Either way you need to check for codes as one can be present and not trip the CEL if you added one and if one is present and you do not have a CEL you won't know until you check for codes.

Electrical Thread 'How To Pull Codes From EEC-IV In 86- 95 5.0 Mustangs'

Jan 27, 2016

@Noobz347 - The code dump procedure isn't in the sticky - you may want to add it...



Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for...

• jrichker
• Replies: 10
• Forum: Fox 5.0 Mustang Tech

• 

At a minimum you should get a Code 11, Code 10, and then a Code 11 if everything is functioning correctly according to the ECU. If you get no codes or just end up not being able to get the ECU to go into the diagnostic mode then the ECU most likely needs to be repaired. Check for codes first and we will go from there.

I will try this out first. Thank you.

 

[AeroCoupe]

So with these cars you need to post up every detail you can in your first post and have already checked for codes. If you get codes post them so we know what the ECU is trying to overcome. If you don’t do this then it doesn’t help us help you. You might as well buy one of these and start throwing money at it.