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Thread: 86 Gt 5.0

  1. #1

    Default 86 Gt 5.0

    hey whats up people? i bought a code reader for my car and its giving me the codes 31 and 85. Has anybody dealt with these codes before and if so could you point me in the right direction. i have taken off all smog and ac equipment, i used a EGR delete plate as well but i haven't put in a dummy plug yet, that was about 2years ago so im thinking that is what Code 31 is. The car also just start having the idle bounce around between 750 and 1200, im hoping by fixing these codes that it will fix this problem. Any help would be greatly appreciated

  2. #2

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    Iíll take dirty throttle body for 1000

    What is throttle body cleaner

    itís the gook on the back of the throttle blade that causes it ...

  3. #3

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    Quote Originally Posted by erratic50 View Post
    Iíll take dirty throttle body for 1000

    What is throttle body cleaner

    itís the gook on the back of the throttle blade that causes it ...
    I agree. And/or dirty idle air control valve. Clean both of those up and I bet it evens out.

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  4. #4
    FEP Power Member mcb82gt's Avatar
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    Copied from another recent thread on here:


    From Joel 5.0...

    Summarized/Corrected Base Idle Reset version


    1. Clean TB with carb cleaner and nylon brush (toothbrush size). Engine off, fully open TB blade and spray/brush TB until all gunk and oil residue is cleaned.

    2. Allow to dry, or close TB blade and start engine until it clears all carb cleaner fumes/liquid.

    3. Let engine idle...if it doesn't, increase idle speed via TB stop screw until engine stays idling on its own.

    4. Disconnect IAC valve, if engine dies repeat 3 with IAC disconnected.

    5. Set idle speed to the lowest setting possible between 600-800 rpms with IAC disconnected. It is key to use the lowest possible to prevent idle surge, rolling, etc once the IAC is connected as follows.

    6. Turn engine off, and reconnect IAC

    7. With Ign On Eng Off, check TPS voltage output....if it's between .7vdc - 1.0vdc, it's OK. This voltage check should be done between the Green and Black wires at the TPS side connector as shown in the pic below....(-) terminal of the DVOM on the Black wire and (+) terminal on the Green wire.

    8. Disconnect battery (-) for 3-5 minutes.

    9. Reconnect battery, start engine, allow it to idle for 2-4 minutes to confirm setup,

    10. If idle speed falls too low or stalls, increase idle speed via the TB set screw a little at a time.

    11. Turn engine off for 20 seconds, re-start engine and repeat 9 - 11 if required.

    12. Reconfirm TPS output is within the .7vdc - 1.0vdc range.......you're done! (almost)...

    FYI..no need to reset ECM KAM (reset computer) b/c the TPS minimum value used for idle control, is automatically reset by the EEC-IV every time the ign is cycled on-off for 20 seconds-on, and as explained in the next post

    13. Allow a 10-20 minute "relearning" period under normal driving conditions (drive cycle).....now you're done
    Mike

    Now stang-less.

    88 Cougar 5.0

  5. #5

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    So i have replaced the IAC and the TPS today and the TB is only 2 years old and is super clean on the blades. I just did that check list mcb82gt had mentioned and it still seems to want to boumce around at idle between 6 and 1100 RPM.I changed out the intake a couple of years ago as well and changed out most of the vacuum lines so i know those are good. Any other suggestions?

  6. #6

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    Dried out/leaky caps on the ECM can cause idling issues....

  7. #7
    Moderator wraithracing's Avatar
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    Are you positive you don't have a vacuum leak somewhere? Line came off, didn't seat properly, etc. Check the simple stuff before you start throwing parts at it, is what I have learned over the years. Still have to remember it from time to time, but still good advice.
    ​Trey

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  8. #8

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    Just took off upper intake and checked all vacuum lines and they are good.
    I also got some carb cleaner and sprayed all around and that also
    Didn't find anything else. Any other ideas?

  9. #9

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    31 - EGR Valve Position/Pressure Feedback EGR Circuit Below Minimum Voltage. 0.24 volts (O,CM,R)

    85 - CANP Circuit Failure (O)

    The second one means your vapour capture system is not working but, who cares? The first could be electrical but has to do with your EGR, again, who cares? The car will run fine without either of these systems functional.

  10. #10
    FEP Member
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    I have a car with a vacuum leak behind the dash which causes idle surging. Your car may have a vacuum line through the firewall to the dashboard as part of the a/c and heating system. Spraying carb cleaner under the hood wouldn't get to a leak there.

  11. #11

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    You may want to check the vacuum canister under the pass fender and the check valve for leaks. it gets its vacuum from the black plastic connection on drivers side firewall that also goes to the brake booster, cruise control, and AC. A quick check would be to remove the middle vacuum line on it that goes to AC and cap that port off. The top right corner of the diagram in the link shows it.
    https://www.stangnet.com/mustang-for...-lines.821193/
    1978 Mercury Zephyr boxtop 5.0 EFI T5Z 8.8
    1999 Ford Contour 2.5 V6 5 speed
    2016 Ford Focus ST 2.0 Ecoboost 6 speed

  12. #12

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    I wouldn't ignore that EGR code. It could be a sign of a much different problem.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
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    Everything else stock and fully functional.

  13. #13
    Last edited by 4idfox; 06-06-2020 at 05:53 PM.

  14. #14

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    Quote Originally Posted by 4idfox View Post
    Yeah, actually that just reminded me, I have one of those on my car!! A couple years ago I had some challenges getting the idle to stabilize so I got one of those iac adjusters and it really let me dial it in and forget about it. In fact, I totally did forget about it... Until just now!! So clearly, it worked for me. Haven't even thought about the idle or surges in a couple years.



    Sent from my E6533 using Tapatalk

  15. #15

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    The 86 wiring harness has a diode that should be removed when used with a new iac. The new iacís have the diode built in. They do not play nice together in some cars.
    Last edited by 4idfox; 06-07-2020 at 09:45 AM.

  16. #16
    FEP Super Member xctasy's Avatar
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    Quote Originally Posted by 4idfox View Post
    The 86 wiring harness has a diode that should be removed when used with a new iac. The new iac’s have the diode built in. They do not play nice together in some cars.
    xactly.



    Quote Originally Posted by QisJob1 View Post
    So my 85 CFI was converted to SEFI using an 86 GT harness years ago. Slowly the dreaded hanging and surging idle has gotten worse with all the upgrades even after Mass Air conversion.

    I tried everything....vacuum lines all replaced, gaskets all replaced, charcoal cannister. Reset idle, checked codes....on and on and on. And beer...lots of beers to offset hair pulling aggravation.

    Then today my Ford tech longtime friend and I were in the garage and I told him I just didnt get it and I just didnt get it. He said, hey ya know what...that diode in the IAC wiring harness could be the problem. He said there was a TSB years ago for that, the harness diode fights the IAC's because all the updated/new IAC's have the diode built in. The TSB says to swap/reverse the wiring in the connector to the IAC. We never did that in all the years it was present....guess what, firing up smooth as a baby's bottom!!!

    Most if not all the IAC's now have the IAC built in....if you have the diode either remove it or switch the wiring into the connector to deactivate the harness diode and see what happens. Oh man I think Im just gonna go out to the garage and crack open a few and watch it idle for an hour!!! Hope it helps at least one other on here.....
    https://www.allfordmustangs.com/foru...old-start.html

    Heres a picture showing a 86-88 harness with the diode inline with the iac wiring.If its not there,the previous owner could have removed it and installed a remanufactured iac valve,which has the diode inside the valve.If you dont see a diode present,it was probably removed already.
    If its still present,it might/might not be a possible cause for the idle issue.
    I haven't dealt with the issue personally,but this is some info I bookmarked.


    ** A rough idle created when the air bypass valve (idle air control) is replaced may be caused by a revision made to the valve's wiring harness. Vehicles built before the 1989 model year had a diode in the air bypass valve's wiring harness. The new replacement valves have a "D" marking on the plastic portion of the solenoid, above the connector cap.They have a diode in the air bypass valve. On the air bypass valve used on vehicles built before the 1989 model year, the positive and negative leads are not important to the valve's operation. However, the polarity on the new replacement air bypass valve is important because, if the wiring harness is reversed, the valve will not work.
    ACTION: Reverse the wires in the air bypass valve connector and retest the valve.**
    Attached Thumbnails

  17. #17
    FEP Super Member xctasy's Avatar
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    Other photos from same post 4idfox quoted. Thanks.


    Quote Originally Posted by erockk View Post
    it wired into the connector coming off of the idle air controller.


    mine was in the wiring harness (not my engine but the best picture i could find)

    Quote Originally Posted by dtmilsap View Post
    After removing cover this what I found. installed new idle air control about one month ago. Should I cut red and reconnect them and just remove yellow.

    Sent from my SAMSUNG-SM-G900A using Tapatalk

  18. #18

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    Okay so I did disconnect the a/c from the vacuum tree on the firewall and capped it but it didn't change.
    So now I was thinking of changing the wires on iac connections, do I just swap them on the harness? I put up 2 pics of it

  19. #19

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    Have you disconnected the plug to the IAC and had it running? You will have to turn up the idle with the idle screw.
    It will take the IAC out of the picture.
    1978 Mercury Zephyr boxtop 5.0 EFI T5Z 8.8
    1999 Ford Contour 2.5 V6 5 speed
    2016 Ford Focus ST 2.0 Ecoboost 6 speed

  20. #20

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    I did disconnect IAC, I had to tweak the throttle scree a bit to keep it going
    But after that it still had the idle bouncing

  21. #21

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    I did just run another KOER test but with it running this time and gave me 3 codes 42,92 and 31.
    42 and 92 are for it running rich right? The car is putting off and stickier exhaust smell then usual
    I am running no smog and off road x pipe. I'm really appreciating the help so far guys��

  22. #22

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    Yep, both banks rich with this.....

  23. #23

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    There is some really good info in this link, I have used it several times.
    https://www.stangnet.com/mustang-for...cklist.698148/
    1978 Mercury Zephyr boxtop 5.0 EFI T5Z 8.8
    1999 Ford Contour 2.5 V6 5 speed
    2016 Ford Focus ST 2.0 Ecoboost 6 speed

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