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Thread: 1986 gt iac ?

  1. #1

    Default 1986 gt iac ?

    Disclaimer: I apologize as I've been blowing up this forum lately and folk are probably tired of seeing my posts by now. I'm just trying to get things buttoned down and working correctly.

    So I've just had the car tuned and the idle was forced to 875 RPM. My MAF and TPS are shiny new. My idle (especially when cold) is about 400 RPM and it will bump up intermittently to 1200 RPM. When it gets warm it does similarly but drops down to around 600 RPM and then finally finds it's happy place. I'm assuming this is the IAC as there really isn't anything else I can think of that would be causing this issue. Have any of you had similar experiences or have any ideas or advice? I really appreciate all of you that have helped me through this whole thing! I'm close but still chasing out a few bugs!

  2. #2

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    Well it's not the IAC

  3. #3
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    Try the base idle reset procedure...

    https://web.archive.org/web/20150505...ic,1031.0.html
    '85 GT

  4. #4

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    Thanks for the advice. I've done this also. I'm going to continue running through possibilities. I think I'll check the 02 sensors and then the air temp sensor in the intake plenum. If that doesn't do the trick I may even replace the ECU. Arg.. runs great.. just won't idle correctly.

  5. #5
    FEP Senior Member danco86's Avatar
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    I very much doubt it's your ecu. You shouldn't have this problem at all after just leaving a shop where you paid good $ for a tune.
    Did the shop do a pre-dyno inspection on the car before running it to burn a chip?
    Has it idled like that right from when they finished?
    Dan

    http://vb.foureyedpride.com/showthre...ng-GT-restomod


    1986 Mustang GT Cobra
    Lotsa stuff, lotsa work. Check my thread above.

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    FEP Super Member erratic50's Avatar
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    I've always just unplugged the air bypass then turned my idle up on the throttlebody then readjusted the throttle position sensor.

    Really the air bypass should only be needed when the AC compressor is engaged and creating engine load. That being said, there are aftermarket parts to help provide adjustment for this too. Something like

    https://lmr.com/item/LRS-9939A/1986-...l-Iac-Adjuster

    Get some throttle body cleaner and make sure everything is clean as a whistle when you start or it's all going to fight you. Especially the back side of the throttle body.

  7. #7

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    Well I finally talked my wife into going for a ride with me and the car just totally fell on it's face. I started feeling surges while cruising and when I came to a stop it was just barely idling. I nursed the throttle trying to keep it running and it was backfiring through the intake which pulled oil and it all went down hill. I BARELY made it into our alley and popping and sputtering she jerked into the garage. I was so upset I had to take a few days away from it. I've purchased a volt meter and timing light and I'm going to go through what I can here today starting with plugging all the vacuum. Wish me luck fellas.

    I've replaced the IAC, MAF, TPS, air charge temp sensor, and the coolant sensor. The throttle body is brand new so I can't imagine it's gunk related. All the vacuum lines are new but I'm going to buy a handful of clamps and make sure. I've got a new distributor I'm going to drop in today as well.

    Danco86- We spent an hour or so working out the cold start parameters before I took the car. The car has had trouble finding it's happy idle since day one. Shifting between 1200 and 500 rpm. The tuner said he'd forced it to 875 rpm but it wasn't listening. I don't know if they checked things before dyno. I assume so. I took the car there on the advice of a family member who said the guy knew his stuff. If I had a rewind button...
    Last edited by Oberu; 04-15-2017 at 11:24 AM.

  8. #8

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    I tore it down and changed the distributor out. I wanted to make sure I knew where top dead was since this aftermarket balancer has about four sets of timing marks on it. I found that the tuner had it out and a few teeth off. He'd moved the wires around on the cap to make up for it. I need a pinata that looks like him... I reset the idle and checked the TPS again. I decided might as well change out the 02 sensors and see if that made any difference. So I spent the day chasing 02 sensors and traveled all over as one store would have one and another store in another town had one. I got home and went to install my shiny new pair of sensors and the connectors were different. After repeating my mantra a few times and re centering my chi I asked the fellow at the counter to pull a 92' version and an 85' version. This is where I'm starting to scratch my head. The 02 sensors I pulled out of the car and brought back to the parts store are from an 85'.. the car is an 86'. I'm starting to feel like I've bitten off more than I can chew with this car now and the Mrs is looking at me with those "I told you so" eyes. Well... parts will arrive tomorrow.

  9. #9

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    OP, PM me, I may be able too help you on this.

  10. #10
    FEP Senior Member danco86's Avatar
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    That's the worst, man. I can just imagine your disappointment.
    I don't want to rub salt in the wound, but the tuner you used is indeed brutal. He should have never attempted to tune it if he couldn't get it to run right.
    A pre dyno inspection is essential before tuning. The place I took it to wouldn't do it until it passed their inspection. They went through everything, plugs, compression, smoke test for vac leaks, the works. After the dyno runs and chip install, they made me leave it overnight so they could cold start test it in the morning and check again for driveability. It burns me that they figured that was the best they could do for you, but still take full price for the work.
    That should not have happened. A tune is supposed to remedy the issues, not create more.
    Grrrrrr
    Dan

    http://vb.foureyedpride.com/showthre...ng-GT-restomod


    1986 Mustang GT Cobra
    Lotsa stuff, lotsa work. Check my thread above.

    Daily drivers
    2002 F-350 7.3 Powerstroke crew 4X4
    2012 F-150 Ecoboost Screw FX4

  11. #11
    FEP Senior Member danco86's Avatar
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    The way you describe your issues, It does sound like a MAF or wiring from MAF to ecu issue.
    My car ran like how you describe one day. Idle everywhere, surging, bucking, stalling, wouldn't start at all.
    I chased the problem for a day until I looked at the connector on the MAF unit, gave it a push, and "click" it wasn't plugged in all the way. The car ran perfectly after. I couldn't believe it was something so simple.

    I would pull the chip, plug the ecu back in, and see how it runs without it, for comparison.

    Did your issues start after the MAF conversion?
    What size and brand MAF do you have? Which ecu are you using? & how did you wire it?
    Last edited by danco86; 04-15-2017 at 08:22 PM.
    Dan

    http://vb.foureyedpride.com/showthre...ng-GT-restomod


    1986 Mustang GT Cobra
    Lotsa stuff, lotsa work. Check my thread above.

    Daily drivers
    2002 F-350 7.3 Powerstroke crew 4X4
    2012 F-150 Ecoboost Screw FX4

  12. #12

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    So I ordered new 02 sensors and they came in this evening. I used some special cleaner on the MAF even though it was a shiny new one set up for 24 lb'ers. I disconnected the battery and I put the new 02 sensors in. I went through resetting the idle one more time to be certain and plugged the battery back in. I crossed my fingers and cranked it over. Seems to have solved my problems. I drove it around all evening and haven't had a single problem yet! I'm not sure what exactly fixed the issue but I'll take it. Thanks for all the help fellas!

  13. #13
    FEP Super Member erratic50's Avatar
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    Ok - now the problem is becoming clear. You have a recalibrate/tuned MAF -- which is another word for a broken MAF -- on your car and you are trying to tune it.

    All of the respected tuners I've interacted with have flat refused to even try to tune with a recalibrated MAF in the picture. It's just too hard to make them run right.

    Get yourself a standard calibration 70 MM MAF that's known good. Perhaps the 1983 Cobra part number. Get yourself a MAF ECU for a 1993 Cobra from your local big box auto parts store. Their "stock" ECU tune is designed to work properly with a standard tune MAF because the fuel map is set for 24 lb/hr injectors to begin with.

    I'll explain my advise this way - imagine how thin you have to make the computer think the air is when it's firing a 24 lb/hr injector to make it deliver the correct amount of fuel while the ECU thinks it's firing a 19.

    You lose massive amounts of power due to removal of spark advance in this scenario. Extremely undesireable.

    Once the right parts are on it, Now revisit your idle set screw and TPS settings.

    If your car is an AOD you won't be able to drive it but this will prove out what the problem is.

    Hit up dirtydirty racing if you need a 'make it run properly' tune if you have an automatic. Or (cringe) Bama.

  14. #14
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    Quote Originally Posted by erratic50 View Post
    Ok - now the problem is becoming clear. You have a recalibrate/tuned MAF -- which is another word for a broken MAF -- on your car and you are trying to tune it.

    All of the respected tuners I've interacted with have flat refused to even try to tune with a recalibrated MAF in the picture. It's just too hard to make them run right.
    Any respected tuner can overcome this by simply entering into the tune the correct MAF transfer function for the meter being used. In this case we're talking about going from 19's to 24's so there won't be any issue with this. I'll leave it here and not muddy the water talking about other applications.

    Quote Originally Posted by erratic50 View Post
    Get yourself a standard calibration 70 MM MAF that's known good. Perhaps the 1983 Cobra part number. Get yourself a MAF ECU for a 1993 Cobra from your local big box auto parts store. Their "stock" ECU tune is designed to work properly with a standard tune MAF because the fuel map is set for 24 lb/hr injectors to begin with.
    Bad idea at this point as his tune is already done. His tuner likely did what I mentioned above.

    Quote Originally Posted by erratic50 View Post
    I'll explain my advise this way - imagine how thin you have to make the computer think the air is when it's firing a 24 lb/hr injector to make it deliver the correct amount of fuel while the ECU thinks it's firing a 19.

    You lose massive amounts of power due to removal of spark advance in this scenario. Extremely undesireable.
    And none of that is a problem either for a tuner. The correct MAF transfer function fixes that and one has total control over any number of timing and fuel tables in the EEC. With no chip it still is usually ok to use a recal'd meter. Is it perfect? No. Does it work? Of course it does. Just as it has been for the last 25 years or however long it's been since it was done the first time. Will it be optimum for every application? No, but for a 300 hp or so HCI car, no sweat.

    Quote Originally Posted by erratic50 View Post
    Once the right parts are on it, Now revisit your idle set screw and TPS settings.
    The right parts are on it. It's the parts the car was tuned for. He just did revisit it and it's likely that since he went through it a second time he corrected whatever was off with the base idle setting.

    Quote Originally Posted by erratic50 View Post
    If your car is an AOD you won't be able to drive it but this will prove out what the problem is.

    Hit up dirtydirty racing if you need a 'make it run properly' tune if you have an automatic. Or (cringe) Bama.
    WTH? Let's just say it has the correct auto for the production year... An AOD. If so, there is absolutely nothing electronic that makes this trans shift. There is nothing a tuner can do with a chip that would change a single thing with how THIS trans will operate.

    Let's be sure to keep FEP a place where folks can get good tech. There's plenty of bad info out there and I'd hate to think that someone might get confused by misinformation. Especially something as "proved out" as what has been brought up here.

    erratic50, if you'd like to learn more about anything I've mentioned here I'd be happy to go into further detail via PM or in another thread if you start one asking some relevant questions.
    Last edited by qikgts; 04-17-2017 at 12:57 AM.
    '85 GT

  15. #15
    FEP Super Member erratic50's Avatar
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    I'm simply here trying to help. I'll be the adult here and look past the insulting aspect of what was said in the response above

    To each their own. We clearly disagree on the approach to fixing the problem. That's fine. Not either of our first rodeos.

    Fact is the tuner that did this sent a car out of their shop with issues and who knows what they did.

    Could a person throw in a quarter horse and used binary Editor plus the data logger and a wide and to figure some stuff out - sure. But that's tune development and that's what the tuner was paid for and failed to deliver at a satisfactory level.

    Tuned MAF - a few tuners sometimes disagree on the merits of what I said. I see that you do too - fine. My experience is this - the ones that do as I've suggested consistently have a way better running car in --* less time * ! Part of it also has to do with the quality of the meter.

    I'm suggesting swapping a MAF and an ECU - two simple parts a novice can swap in 15 minutes. If it cures it then a ton of time, headache, and heartache is avoided. If it doesn't, no big deal - sell the parts off or keep them as an emergency backup. Personally I've never lost a dollar on parts like this because I know where to buy them cheaply and how to sell them for more than I paid for them.

    Stock stick computers are not recommended with an AOD as drivability issues are more frequently encountered - that's why I suggested avoiding it.

    No need to start a thread or PM with any "questions". My recommendations stand on their own based upon their merits. One of the things that makes FEP great is the level of detail and the manner that most of us can respectfully interact. Most.

    I've been in the car hobby my whole life and have had my car for 25 years and on this one I see a firm disagreement. I call them like I see them and after all the years in the hobby what I have said is completely based upon my experience. Continued posts will not change either of our experiences or make either "more right".

    My honest opinion has been stated, insulted, explained, etc. I have no stake in any of this if I'm listened to or not. By all means - carry on. I am bowing out of it.

    In closing I hope this gets solved ASAP!
    Last edited by erratic50; 04-17-2017 at 04:07 AM.

  16. #16

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    I thank all of you and I can see what you are saying. I've got an A9L ECU in the car now and I feel like the problem has be solved. Now if I only knew exactly what the problem "was" lol. The car runs well and idles when it's supposed to and even fire nicely on chippy mornings like today. I appreciate all of your combined advice on this!

  17. #17
    FEP Senior Member danco86's Avatar
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    Awesome, man!
    Better take wifey out for a cruise again.
    Dan

    http://vb.foureyedpride.com/showthre...ng-GT-restomod


    1986 Mustang GT Cobra
    Lotsa stuff, lotsa work. Check my thread above.

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    2002 F-350 7.3 Powerstroke crew 4X4
    2012 F-150 Ecoboost Screw FX4

  18. #18
    FEP Super Member erratic50's Avatar
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    Rock on- glad to hear it's running OK with the A9L and recalibrated MAF.

    I am not sure which would run best with your 24's - an A9L with recalibrated MAF or a Cobra computer and normal MAF. I'm certain someone has dynoed this and documented their results.

    A9L have among the most aggressive MAF timing tables out of box which is great for power production. Some guys with 93 Cobras have switched over to an A9L retuned with a chip for 24's and saw a nice boost in power by doing so.

    I also run an A9L in my 86. It does not seem to be quite as aggressive as the VM1 speed density computer I removed when I did my MAF conversion but with a little more base timing my car seems to run far better at part throttle and can match what it did with SD. Mine was leaning out up top as the 19 lb/hr injectors were not keeping up at stock fuel pressure - bumped it up a few PSI at the rail.

    In the long run you will likely see more performance with a "standard" MAF and a simple update to the binary in the ECU to tell it the injectors are 24 lb- this is my educated guess.

    A full tune from a reputable dyno shop will yield gains for sure once everything about the combination is dialed in.

    I don't think it applies in but I wanted to note that I did come across some idle quality discussions and burned up ECU threads in cases where an AOD car was switched to 5-speed. Apparently there are some differences in the O2 sensor wiring in some years that come into play. It sounded like it was in model year 90+ and another symptom was the idle hanging high then winding back down. Doubt it applies as that would be an A9P conversation most likely....

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