Idle staying at 2,000 RPM

Discussion in 'Fox 5.0 Mustang Tech' started by savegoodautonfg, Apr 18, 2007.


  1. savegoodautonfg

    savegoodautonfg New Member

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    Update:

    Last night my coolant gauge was acting weird. I'm guessing that could have to do with the SIG-RTN wire as well. It was going all the way up and it never does since i put in my new radiator. I dont know what to do lately, i can drive the car no problem, it just these weird things im having problems with. The idle was fine last night.

    jritcher,
    could the problem in the wiring harness only be the SIG-RTN wire right. not the black/white the gray/red wire? The gray/red wire is the one that is in the middle of the engine harness.
  2. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Your car uses the 91 and later wiring scheme, so it is the gray/red wire. The wire changes color,
    one part of it is black/white/ the other is gray/red. The color change may be at the 10 pin connector junction.


    Diagram courtesy of Tmoss & Stang&2birds
    [​IMG]


    The temp gauge problem is either a bad sender or a bad power ground between the engine block and the frame.
    The engine temp gauge sender is a single wire sensor on the driver's side lower intake manifold.
    The temp gauge sender isn't connected to the computer, so it uses a separate ground from the SIG-RTN.
    The engine block is the ground for the temp gauge sender.

    The SIG-RTN and engine power ground are two separate and distinct circuits: don't confuse them.
    They serve different purposes and and go to different things.

    Here is some more information about power grounds:
    The secondary power ground is between the back of the intake manifold and the driver's
    side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor
    clutch and other electrical accessories such as the gauges. Any car that has a 3G or high
    output current alternator needs a 4 gauge ground wire running from the block to the chassis
    ground where the battery pigtail ground connects.
    The 3G has a 130 amp capacity,
    so you wire the power side with 4 gauge wire. It stands to reason that the ground side
    handles just a much current, so it needs to be 4 gauge too.

    The picture shows the common ground point for the battery & alternator

    Picture courtesy timewarped1972
    [​IMG]

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

    Ignition switch wiring
    http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

    Fuel, alternator, A/C and ignition wiring
    http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

    Vacuum diagram 89-93 Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

    HVAC vacuum diagram
    http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

    TFI module differences & pinout
    http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif


    Sponsored Ad

  3. savegoodautonfg

    savegoodautonfg New Member

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    Thanks for the reply, i was thinking though if my secondary ground was'nt there then the alternator and electronic would'nt work your saying right?

    Also the ECT is brand new and it was working fine till last night.

    also about the SIG-RTN what should my next step be to try to figure out the ground issue i think im having. im confused about what you said about soldering and such because im afraid to go touching the engine harness unless i understand completely if you know what i mean.
  4. savegoodautonfg

    savegoodautonfg New Member

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    I fixed the ECT problem, connector was'nt on the sensor, must have came off somehow..
  5. savegoodautonfg

    savegoodautonfg New Member

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    I put my old stock throttle body on and i got rid of that annoying whistle from my bbk throttle body.
    now my car sounds like its missing on idle.

    Jrticher, its definetly i think related to my 10 pin connector problems. when its idling it i play with the white 10 pin connector the idle will fluctuate.

    Does this post that you posted relate to my problem as well?

    http://forums.stangnet.com/showthread.php?t=711190&highlight=TPs ground
  6. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    The signal ground serves multiple functions, and one of them is to enable dumping the computer codes. When it goes bad, the codes won't dump and other sensors lose their ability to send correct signals to the computer.
  7. savegoodautonfg

    savegoodautonfg New Member

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    I can always dump my codes with no problem.

    Jrticher, its definetly i think related to my 10 pin connector problems. when its idling it i play with the white 10 pin connector the idle will fluctuate.

    Any ideas?
  8. savegoodautonfg

    savegoodautonfg New Member

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    Hey could i just run a new ground wire for all of those sensors and just scrap the other ground wire coming out of the 10 pin connector?
  9. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Disassembling the connectors and re-pinning them would be more work than fixing your current problem. Remember that the signal ground MUST go back to pin 46 on the computer to work properly.

    If the problem is in the engine injector harness, a trip to the junkyard for a replacement may be in order. Otherwise, plan on removing the old harness, peeling all the tape off and repairing the bad wiring.
  10. savegoodautonfg

    savegoodautonfg New Member

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    SO can i run a new ground wire to all the sensors and run it back to pin 46 on the computer then?
  11. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    You can run a peplacement wire, but there are better solutions. Whatever you do will require removing the upper intake manifold to do it.

    Replacing the engine injector harness would be a better solution. The junkyard should be able to supply an 86-93 5.0 Mustang engine injector harness for a reasonable price. The FMS Racing part number is M-12071-E302.

    If you decided to run your own wire from the sensors to the computer, here is the link to the pins and crimp tools. http://rjminjectiontech.com/?p=11. They have all the harness repair parts like connector shells, pins, crimp tools, etc.

    Soldering pigtails onto existing sensor connectors is just asking for more problems.
  12. savegoodautonfg

    savegoodautonfg New Member

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    Two things jritcher,

    1. how hard is it to put in the new engine harness?

    2. if i do it myself and i use a regular ground wire like 14 or 16 gauge, what would be the best way to go about doing it? and what do i need off that website? i'm really confused.
  13. fastimes69

    fastimes69 Founding Member

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    Hey I just finished working on a friends car whom was doing the same thing as yours. His is an 87 GT speed density. I changed out the injector harness with an extra one I had laying around,and set the TPS voltage to 1.0 volt. The car ran fine after this.His was also hanging at 2000 and now after 30 minutes of driving it idles fun and rpms come back down to 800-900 rpm at a stop. You may want to look into replacing your harness. On his old one I noticed some wires from the IAC connector were burnt up from laying on the heater pipe so I guess they were grounding out,and also some wires from the EGR were stripped so those were grounding out on the pipe as well.Good Luck...
  14. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Removing and replacing the upper intake is the hardest part of the job. That takes about 30 minutes,
    maybe a little more the first time you do it.

    Once the upper manifold is off, the engine fuel injector harness unplugs from the fuel injectors,
    other items and sensors.

    Installing a new ground wire means that the TPS, ACT, ECT & EGR sensor connectors will need
    to be disassembled. The black/white wire gets removed, a new wire and pin installed. That new
    wire will run back to the SIG-RTN pin on the 10 pin connector or a common junction point
    that runs to pin 46 on the computer. That means you need new wire, connector pin, and a
    crimp tool to crimp them on.

    You will find that the sensor connectors come apart so that you can remove the pins. A very
    small screwdriver releases the lock in the front of the center insert, while another small
    screwdriver inserted in the back pushes it out. Once the center insert is out of the connector
    shell, the pins come out easily. Use 18 gauge automotive wire to replace any wiring. Standard
    harware store wiring or household wiring will not hold up to the high temperatures under the hood.
  15. savegoodautonfg

    savegoodautonfg New Member

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    Ok it seems like its easier to just get a replacement harness. this harness doesnt need to go thru the firewall to the computer or anything? does it come with the salt and pepper shakers or anything or no? i dont know exactly what it consists of?

    anyone have one for sale?
  16. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    The engine side salt & pepper 10 pin connectors comes with the replacement fuel injector harness. You don't do anything to the the body side connectors but reuse them.
  17. savegoodautonfg

    savegoodautonfg New Member

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    thanks so much.. anyone have one for sale?
  18. savegoodautonfg

    savegoodautonfg New Member

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  19. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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  20. savegoodautonfg

    savegoodautonfg New Member

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    Got one for $60.00 off ebay.. greatttt shape and great feedback for the seller.

    Does it matter what fuel injector plugs go on what fuel injector and such?

    quick explanation on how to remember what goes where and such?

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