Weird issue with start up idle - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 19 Old 06-08-2019, 07:12 PM Thread Starter
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Weird issue with start up idle

So I started having a weird issue with my car last week.

Specs: 347 block, Twisted Wedge 165 Heads. FTI Cam. Lightning 90mm MAF. BBK 60mm TB. T4M0 computer with SCT Chip professionally tuned by Alternative Auto in Michigan. Same exact setup since 2011.

All of a sudden, I go to start it up last week, and it hunt idle and dies. If I gave it gas, it would run but I had to hold on to the pedal for about 1min before it was warm enough.

I have a Wideband O2 sensor. when it dies in idle, it measures 11:1 ratio. As soon as it starts picking up and stays steady, the WB O2 will read 13:1, and eventually 14.7:1 once warmed up (over 150f coolant temp).

Then the car ran fine, as will start fine as long as it's warm.

So today, I unplug the battery, take apart the airbox, clean the little sensors on the MAF with ABS safe brake cleaner, and the IAC sensor (looks like a little capacitor).

Put it all back together, reconnected battery, and bam, still hunts for Idle, but now I only needed to hit the gas for 20 second to keep it up before it was stable.

Once warm, it runs fine, and pulls hard fine.

Anyone have any idea what I should be concentrating on? I'm going to check next to see if I need new plugs.

And how the heck would you test a 1995 PCM? Is there a device that you can plug the car computer into to test it? I don't mean the OBD-I port either. Is it worth running codes at all? I don't have an OBD-I scanner, I'd need to buy one.


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post #2 of 19 Old 06-09-2019, 10:16 PM
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Could be anything really. I would start by looking at your IAC and fuel pressure. A bad fuel pressure regulator with a leaking diaphragm could cause the car to run rich.


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post #3 of 19 Old 06-10-2019, 09:26 AM
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I would check the chip connection

chip to j3 port can be a little loose or corrosion on the traces.....

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #4 of 19 Old 06-11-2019, 03:17 AM Thread Starter
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Cool, thanks guys. IAC might be a good place to start. I also have another SCT Chip with the exact same tune, and another T4M0 (assuming in good condition but I've never tested it). I guess that might be another place to start.

That's one thing I despise about these cars: A symptom could be one of 20 different causes. Started it up yesterday, this time it needed about 10 second on the gas before it stabilized. Same thing, drops to 11:1 but this time climbed out much faster.

Fuel pressure measure around 39 - 40 at idle, but my gauge is electric, so not sure how accurate I should consider it. So IAC is about 10 years old, but I drive this car maybe 2000km a year, with some years getting less than 500km. Could IAC still go bad?

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post #5 of 19 Old 06-11-2019, 08:55 AM
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these cars are very easy to diagnose, with the correct tools

just takes more time without

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post #6 of 19 Old 06-11-2019, 02:39 PM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
these cars are very easy to diagnose, with the correct tools

just takes more time without
What are good tools to use to make it easier to diagnose?

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post #7 of 19 Old 06-11-2019, 03:54 PM
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real time data logging with a tuning device

2 channel o-scope

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post #8 of 19 Old 06-11-2019, 07:26 PM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
real time data logging with a tuning device

2 channel o-scope
I have a Quarterhorse that I don't use (due to the professional tune requiring SCT chip). I suppose in this case I could datalog sensor stats at idle. I also have an O-Scope that I hardly ever use (mostly for laptop repairs, and hardly ever truly needed, unless you need to get precise readings of ASIC waveform signals, or VRM's.

Sadly I don't have enough knowledge or experience dealing with the electronic side of things on our cars. I have a wiring harness manual stored away somewhere, which includes pinout diagrams. Would be nice to have a schematic of the T4M0 computer though.

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post #9 of 19 Old 06-11-2019, 07:32 PM
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you have the tools, and choose not to use them

you could see commanded fuel, fuel trims, o2 reporting etc

scope........

look at

maf
injector wave, there is a lot of insight with that'
secondary ignition wave form, even more info

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post #10 of 19 Old 06-14-2019, 01:13 AM
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40 psi at idle seems a bit high, do you have an AFPR?


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post #11 of 19 Old 06-14-2019, 02:40 PM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
you have the tools, and choose not to use them

you could see commanded fuel, fuel trims, o2 reporting etc

scope........

look at

maf
injector wave, there is a lot of insight with that'
secondary ignition wave form, even more info
I haven't had time to look into it, since the car works fine once it gets its 15 - 20 seconds of holding RPM's with the gas pedal. And I will be out of the country for the next week, but when I come back, I am pulling out the Scope and the Q-Horse and start troubleshooting.

How do you normally get readings? Do you have to peel into the harness to expose the wires so you can put your probes there? And where do you attach the grounding leads? To the grounds on the chassis?

Quote:
Originally Posted by darkbarghest83 View Post
40 psi at idle seems a bit high, do you have an AFPR?
Yes I have a AFPR, I believe it's a Kirban. What should Idle FP be? I know while driving, it sticks around 40psi while driving, and when really hot, will dip to 37psi when letting off the gas.

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post #12 of 19 Old 06-14-2019, 07:01 PM
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ground to chassis is fine

use T pins or piercing tips

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post #13 of 19 Old 06-18-2019, 04:44 PM
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Quote:
Originally Posted by Future303 View Post



Yes I have a AFPR, I believe it's a Kirban. What should Idle FP be? I know while driving, it sticks around 40psi while driving, and when really hot, will dip to 37psi when letting off the gas.
stock fuel pressure is around 30psi at idle (vacuum present) and around 40psi at WOT (no vacuum).

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post #14 of 19 Old 06-28-2019, 11:11 PM
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And how the heck would you test a 1995 PCM? Is there a device that you can plug the car computer into to test it? I don't mean the OBD-I port either. Is it worth running codes at all? I don't have an OBD-I scanner, I'd need to buy one.
A scanner is not needed to read or clear codes stored in the ecu (on 94-95 models) A paperclip
or jumper wire is all you need.You won't be able to view live data,etc,like you can do with a scanner,but retrieval and clearing of any codes is possible.Touch one end of the jumper to the Signal Return terminal (@ the diagnostic port) and the other end of the jumper to the single wire STI terminal,turn the koeo and the test will begin.Codes will be flashed through the mil.
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post #15 of 19 Old 06-29-2019, 06:56 PM
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A scanner is not needed to read or clear codes stored in the ecu (on 94-95 models) A paperclip
or jumper wire is all you need.You won't be able to view live data,etc,like you can do with a scanner,but retrieval and clearing of any codes is possible.Touch one end of the jumper to the Signal Return terminal (@ the diagnostic port) and the other end of the jumper to the single wire STI terminal,turn the koeo and the test will begin.Codes will be flashed through the mil.
He was not referring to pulling diagnostic codes, he was referring to testing an EEC to tell if itís faulty or not. (Fried traces, bad resistors, chips, etc.) that would take take some serious equipment that only a manufacturer would have. All we can do is open the case and visually look for defects.

Not being able to retrieve diagnostic codes, no CEL with key on, flooding/black smoke, and constant clicking of relays are some symptoms of a faulty EEC.

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post #16 of 19 Old 06-29-2019, 08:41 PM
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this is a 1 hour max diag with common tools

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post #17 of 19 Old 06-30-2019, 06:02 PM Thread Starter
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Update.

I drove the car for the first time in 2 weeks again yesterday. Still hunting for idle a little bit, but nowhere near before AND no dying. Once it's warm, works perfectly, pulls hard, no other issues, normal temps, normal fuel/air ratios.


Soooooooooooooooooo I think it's the IAC. I'm going to swap it tomorrow and let you know if it made a difference. I pulled out my QH out of storage, and a laptop, and I got the Scope out of storage as well in case I need to troubleshoot more.

The reason I went to have this car professionally tuned is because I'm sick of having to do all the work of troubleshooting. I wanted the car to be turn-key from now on, no more mods, no more crap. I wanted it to be like my Echo. I hop in, turn the key, and it works.

So yeah sorry if I am a bit annoyed at having to do some grunt work here. But thank everyone one of your for your support

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post #18 of 19 Old 06-30-2019, 06:09 PM Thread Starter
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He was not referring to pulling diagnostic codes, he was referring to testing an EEC to tell if itís faulty or not. (Fried traces, bad resistors, chips, etc.) that would take take some serious equipment that only a manufacturer would have. All we can do is open the case and visually look for defects.

Not being able to retrieve diagnostic codes, no CEL with key on, flooding/black smoke, and constant clicking of relays are some symptoms of a faulty EEC.
Thanks man. Yes, this is what I meant.

I have another T4M0 here as a backup, with a 2nd SCT chip on it with the same tune, but I have never tested it. And based on what you are saying, only way to test it is to stick it into the car and see if there's any symptoms/if it fires up properly, and drives properly.

It doesn't help that I am an ultra scared paranoid freak about this stuff.

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post #19 of 19 Old 06-30-2019, 08:42 PM
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dollars to donuts, IAC is fine

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