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  1. #1

    Default Wanted dead or alive: Hanging/Surging Idle..SOLVED!!

    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.....
    Last edited by QisJob1; 06-12-2012 at 09:19 PM.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  2. #2

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    thanks for sharing, that is very positive. that is awesome to hear a nice success story like that. I bet you are pumped. and it pays to have knowledgable friends ! wish i could find some LOL !!
    1979 Ford Fairmont Futura "The F Bomb"

  3. #3
    FEP Super Member Gemini1999's Avatar
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    Default

    A question...

    IAC?

    Translate please..
    Bryan

    1983 Mustang GLX Convertible
    1984 Mustang Turbo GT Convertible

  4. #4

    Default

    Quote Originally Posted by Gemini1999 View Post
    A question...

    IAC?

    Translate please..
    Idle
    Air
    Control valve. Yes, your turbo has one, LOL. It's the large cylinder stuck on the side of the throttle body with the plug on the passenger side.

  5. #5

    Default

    Is that the lil box deal in the wiring to the iac valve. Mines been hanging since I got it.
    86 mercury Capri 5.0 5 speed.

  6. #6

    Default

    Yes, the gray rubber rectangular one in the wiring that comes from the fuel injection wiring harness to the IAC connector.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  7. #7

    Default

    Quote Originally Posted by sircrunch View Post
    thanks for sharing, that is very positive. that is awesome to hear a nice success story like that. I bet you are pumped. and it pays to have knowledgable friends ! wish i could find some LOL !!
    Glad to help...lots of good knowledge around here. Im very fortunate, when I moved into my house little did I know I moved next door to a Ford Tech and another one kitty corner. Both have mustangs!

    Pumped is an understatement!!! And the really nice thing, everything is new now so I can just drive the car and enjoy it.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  8. #8
    FEP Power Member Ourobos's Avatar
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    So removing it is just as well as reversing the wires? I don't understand reversing the wires, as one is power and one is signal return (ground). I wouldn't think you could reverse those..
    1986 CHP SSP Coupe

  9. #9

    Default

    If you remove the harness diode that takes care of it too....Im no electrical expert but if you were to swap wires in the connectors the connection would never be able to complete its signal to the IAC. It would eliminate having to cut the wiring.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  10. #10
    FEP Power Member Ourobos's Avatar
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    So it IAC wouldn't function?
    1986 CHP SSP Coupe

  11. #11

    Default

    Do you happen to have a picture of the wiring with the diode or where it is? I'm not sure if mine has already been deleted or i just can't find it.. thanks
    00 GT Terminnated
    86 GT Vert 9L
    00 SVT contour

    Past cars
    92 coupe 2.3t conversion, 85 SVO 1C, 87 turbo coupe 1C, 88 turbo coupe 1C,98 svt contour black

  12. #12
    FEP Senior Member 4EYED85's Avatar
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    Does this work on a 85 3.8 cfi as well? I found a yellow rectangle box in the efi harness.

  13. #13
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    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.....
    Any way you can post a pic and show us more specifically what on the IAC needs to be addressed. I am familar with the throttle body and IAC components in terms of their locations and connections but have absolutely no freaking idea what you are talking about with regard to the diode or the wiring reversal that you are referring to! Thanks.

  14. #14
    FEP Power Member Ourobos's Avatar
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    It's the injector harness wiring that goes to the IAC.. If it has the diode, there will be three wires running to the IAC, although only two (red and white) go into the plug. Simply snip the diode out of the hanress and leave it's wire capped off, and tie the two red wires you now have together. I just did it, and it actually solved my idle problem too. I'd been searching for a few days. As mentioned, the newer (90+) injector harnesses don't have it in there.
    1986 CHP SSP Coupe

  15. #15
    FEP Senior Member 4EYED85's Avatar
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    How about a 85 3.8 cfi? The little yellow piece does say diode on it.

  16. #16
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    I only see a red and a white wire to the IAC on my 86 GT. I'm assuming mine doesn't have the diode thingy? Where would the 3rd wire referred to be? Is the gray rectangular thing on the red wire the diode? Do you simply cut the gray thing out and then splice the wires together? Someone please elaborate!
    Last edited by HAD85B4; 06-13-2012 at 06:51 PM.

  17. #17
    FEP Senior Member svo6's Avatar
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    Remove the gray thingy in the red wire!
    408 C.I.-Sleeper

  18. #18
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    My 86 GT also has 2 wires going to the IAC. If you look under the tubing covering, there is a white wire which "Y's", 1 branch goes to the IAC, the other branch goes to the back of the plastic connector. The red wire goes to the front of the plastic connector and also comes back out of the front of the plastic connector and then goes to the IAC. No 3 wires, anyone know what the plastic connector is?

  19. #19
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    I still dont understand what to do about the diode thing

  20. #20

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    Sorry wasnt near computer all day....just snip the wires that go into the diode and out. Then connect them using whatever crimp method you want. Only do this if you have the early harness with the diode. Just like the others have said, the later fox body EFI harnesses dont have it hence why the diode is in the IAC itself.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  21. #21
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    What is the diode? The gray rectangular piece in the picture above connected to the red wire? Do you cut the two red wires on the front of the gray thing facing the IAC and then splice those together? There is also a white wire going into the back of the gray thing. What do you do about this white wire and the gray thing? Does it just hang?
    Last edited by HAD85B4; 06-13-2012 at 09:40 PM.

  22. #22

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    The diode is the gray rectangular rubber feeling piece with the wires going in and then out and then into the connector that clips into the IAC. I did not cut anything, what we did was take a pick...remove the electrical pins that go into the connector that clips into the IAC. Switched the wires which makes them not work and lets the diode in the IAC do the job. I did not want to cut the harness but it wont hurt it, the way I did it was quicker and easier but just removing the diode and connecting the wires is the same. Dont leave anything hang, whatever goes into the diode is connected to what comes out.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  23. #23

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    So just cut it out, and reconnect??

  24. #24
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    Anyone has access to the TSB's? # 87257, December '87, covers intermittent/high idle. Curious if it addresses this.

  25. #25
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    "Switched the wires which makes them not work"

    Isn't this the same as completely bypassing them or disconnecting them? They do not need to go to the IAC?

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