Close



Results 1 to 6 of 6
  1. #1
    FEP Senior Member FuturaGuy's Avatar
    Join Date
    Jan 2005
    Location
    Windsor, CT
    Posts
    835

    Default Won't idle at operating temperature

    My 4.6 Cobra Futura idles fine when it's cold but won't idle and stalls after it reaches operating temperature. The stall problem seems to have started last summer after I took the Vehicle Speed Sensor (VSS) from the transmission to count the teeth on the gear. (Speedometer reads low.) The OBD2 is giving a code that the VSS isn't sending a signal to the computer. I didn't remove the speedometer cable when I took out the VSS, so the speedo cable probably pulled the VSS electrical connector in tension and broke a wire.

    How does a VSS work - is it a pulse generator whose frequency varies with speed or is it a variable voltage generator?

    I'm waiting for a break in the weather so I can take out the transmission electrical harness and check the wiring. The Shelby-American All-Ford meet at Mustangs Unlimited is only five weeks away.

  2. #2
    FEP Super Member PaceFever79's Avatar
    Join Date
    Feb 2009
    Location
    Western PA
    Posts
    9,618

    Default

    Have you checked your IAC valve?

  3. #3

    Default

    Fox body vss are voltage generators which are roughly calculated out to be about 8000ppm vs the oss in 99 and later tramsmission which use 32000ppm for a higher resolution reading using pulse width modulation. On fox body cars (pre obd2) the vss is shared with the computer, cruise comtrol and speedometer (electronic speedos only) using the same signal. On oss cars it is split between cruise abs traction control transmission and finally the ecu. It will control fans (switched off at 45mph or so) how the trans shifts and what the brakes do, as well as when to idle up a manual trans car.

    Everyone makes a big deal about the analog (vss) vs digital (oss), but they seem to be compatible. I ran a 2003 trans in my 87 tbird which worked flawlessly, minus reading 4x too high. Everything I read on it said that it would burn up the computer or speedo and I didn't have an issue other then too high of a speedo reading for about 80k miles.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  4. #4
    FEP Senior Member FuturaGuy's Avatar
    Join Date
    Jan 2005
    Location
    Windsor, CT
    Posts
    835

    Default

    I replaced the IAC valve - didn't help any.

    HAYSTACK - thanks for your input. I'm using an '87-'93 Fox VSS with a '98 Cobra wiring system and computer, which came from the donor car, and it did work before I pulled the VSS. What you said agrees with the wiring diagram, with the VSS sending a signal to the computer. After I fix the wire harness, I'll let everyone know how it turned out.

    I won't fix it today - the weather guessers are predicting a Nor'easter coming up the coast and are not sure how much snow we're going to get.

  5. #5
    Venomous Moderator Hissing Cobra's Avatar
    Join Date
    May 2002
    Location
    Wareham, Massachusetts
    Posts
    9,720

    Default

    I hope you can fix it Rock. I hate when things go haywire and it's not obvious what is wrong. I'll be keeping my fingers crossed for you!
    Pete Slaney

    1979 Mustang Cobra

    347/T-5/4.30's
    420 rwhp/380 rwt (New Motor)
    11.49 @ 121.86

    306/T-5/4.30's (Old Motor)
    307 rwhp/278 rwt
    12.38 @ 111.38

  6. #6
    FEP Senior Member FuturaGuy's Avatar
    Join Date
    Jan 2005
    Location
    Windsor, CT
    Posts
    835

    Default

    Found the problem - the Vehicle Speed Sensor was disconnected. Last summer I took out the VSS to count the number of teeth on the gear and when I put it back in either I didn't put the connector back or didn't put it on securely and it came off.

    The car is running fine and idles nicely at 800 RPM. Hope to see everyone next week at the Shelby-American All-Ford meet at Mustangs Unlimited.
    Last edited by FuturaGuy; 04-23-2016 at 07:44 AM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •