High Idle, Hanging Rpms (i Promise I Searched)

Discussion in 'SN95 4.6L Mustang Tech' started by GT99BLACK, Jul 27, 2013.

  1. Hey guys,

    Just to start off I searched a bunch of forums and used this wmburns post as a start:


    I came out of work the other day and my car had a high idle, shifting gears the RPMs would hang. There was also a new sound like a loud hum/low horn so I figured vacuum leak/IAC. I have replaced the old IAC and the hum went away and I have also cleaned the MAF sensor with the appropriate cleaner. I checked fuses 2.2 and 2.8, nothing wrong.

    Today I started the car and pulled the IAC plug. My idle went exactly where it usually is - 700 and the hanging RPM issue went away. As soon as I plug the IAC in, the idle spikes and I have the hanging RPM issue. I tried letting the car sit idling for 15 minutes and nothing changes.

    Has anyone had this problem? In wmburns' post he goes into detail of what the next steps are if the idle doesn't change but mine went right to normal when I disconnect the IAC. Please let me know if you can help.

  2. I was having some issues with my idle last month, my revs would hang and there would be a strange humming noise like you described. First off where did you get your new IAC? I bought one from Advance Auto that was a Borg Warner replacement and it made the idle a little better but still gave me issues. I went ahead and purchased a Motorcraft replacement from my dealership, after installing it my problems went away. So I recommend getting a Motorcraft IAC. Also have you checked your hoses for a vacuum leak?

    Hope you can get it figured out.
    GT99BLACK likes this.
  3. I have checked for vacuum leak. I have one hose that is dry but doesn't seem to have a leak, it's the one that goes from the back of the throttle body to the passenger side fender. I bought the IAC at advanced auto so it sounds like I may have to go get the motorcraft. Did you have the same plug/unplug issue with the Borg Warner? Thanks for the help.
  4. The FACT that disconnecting the IAC results in a 700 RPM idle PROVES there is a vacuum leak or the throttle isn't closing all the way.

    Think about it. Without the IAC providing bypass air, where is the air coming from that lets the motor idle? The IAC disconnect idle must be lower than standard in order for the IAC to "ADD" bypass air. Understanding the concept of bypass air is one of the keys to troubleshooting IAC/idle issues.

    Check the EVAP purge line for leaks. An easy way to check is to disconnect and plug the line. If the idle improves, then you will know where the leak is coming from.

    Looking for possible easy fixes? Another common leak source is the PCV system. When was the last time the PCV value was changed? What about the PCV grommet? Does it fit snug and air tight?

    Here's some more information on how to trouble shoot IAC/Idle issues.

    GT99BLACK and trinity_gt like this.
  5. I changed the pcv in the past year but the hose from the back of the throttle body to the evac system is dry rotting. I was trying to find the evac box to replace the entire hose but I was looking under the fender liner BEHIND the wheel and didn't see the square evac box. Is it closer to the headlight? I'm going to plug that hose later today and see what happens. Thank you for the help!
  6. Just plugged the evap line at the TB and idle went to normal. Now I just need to find where that hose ends up and buy a new hose to route from the tb to the evap canister. Thanks and please let me know if there's a diagram/pic of where the canister is located. I keep reading behind wheel well liner on front passenger wheel but I am having trouble finding it.
  7. Will be looking for this during the week/this coming weekend.

    Attached Files:

  8. What is the model year of your Mustang?

    If interested in getting a copy of the Ford service manual and wiring diagrams for yourself I maybe able to help. PM if interested.

    For the 98 and older MY, the EVAP canister is behind the right hand fender well.

    For the 99+ MY, the charcoal canister is behind the left hand wheel near the gas tank. The Vapor management valve (VMV) is located inside the right hand fender well.
  9. It is a 99 GT. I think ill be looking for the VMV in the fender then. At first glance behind the liner I only saw metal plumbing from what I thought was the AC evaporator, no rubber hoses.
  10. Found where the hose went, 12 o'clock inside fender liner. Waiting to get motorcraft IAC and I should be back in business.
  11. Quick update. Got a new hose and motorcraft IAC/new gasket. Started the car and still had high idle and hanging rpm. Let it run for 15 minutes and drove around the block and car still has the issue. It must be something further down the evap since I definitely isolated it there. Any common issues? Will be looking into this on the weekend.
  12. Intake manifold gasket leak?
    JPK60 likes this.
  13. Have you disconnected the battery since replacing the parts? This forces the computer to relearn the idle.
  14. Yes, I had battery disconnected when I installed the IAC but that took about 5 minutes so I may leave battery disconnected for a few hours today and restart tonight.
  15. Final update everyone. So the Borg Warner IAC didn't work so I thought vacuum leak like everyone said. I fixed the hose that was dry rotted but figured I would spend more $ on a motorcraft IAC from rockauto just in case. I installed the motorcraft IAC but was getting fed up because the idle was a bit better but rpms were still hanging and there was no more vacuum leak. Said F it and got an IAC straight from Ford. The same part # but twice the price and also heavier out of the box for some reason (newer batch?). Installed the part from Ford and idle went to normal and rpms stopped hanging. I guess third times a charm and you get what you pay for. Thanks so much for the help everyone.
  16. Glad to hear it's fixed.

    A couple of quick questions. Which type of IAC has been used (the one with the black vent or no vent)?

    With the new IAC, what happens to the idle when the IAC is disconnected? I suspect it will drop much lower this time. Perhaps the vacuum leak was in the IAC itself (the black vent is supposed to close when the motor starts).

    These problems are always more difficult when the new replacement part itself is also bad.
  17. Glad to hear its resolved bud, I paid an arm and a leg for the OEM idle air control valve from my local stealership. It was something like $186.89.

  18. It's the IAC w black vent. When I disconnect now the car immediately stalls/dies. Thanks again for all the help.
  19. Thanks again for the advice. Yeah $175 for mine.