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silly high idle when engine is cold

Discussion in '1994-2004 V6 Mustang Tech' started by bluelionpiper, Oct 5, 2018.

  1. bluelionpiper

    bluelionpiper New Member

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    Hey there,

    My 99 5 speed was doing great for the past year and then I road tripped it to Johnson city TN about 3 hours from me and when I got off the interstate the idle was about 2k. It ran great and throttle response was normal. Had my day and went back home and for that 3 hour drive it did great, other than I couldn't let off the throttle to slow down, almost always had to use brakes. When I got into town, I stopped for some food in a drive though and the coolant temp went over 210, but not by much. The next day she still idles really high but once she warmed up the idle drops to about 1k. That's been pretty consistent for the last week or so.

    I have unplugged the IAC, engine dies
    Checked for vacuum leaks, none that I can find and when I pull and vacuum line off the engine dies
    Haven't checked or messed with the TPS because throttle response has always been good
    Haven't messed with MAF

    Something changes from when the car is dead cold(70F) to when it starts to warm up(150F) that cause things to line up and the idle to drop.

    Could it be EGR? How would I test EGR? Something else?

    Thanks!
     
  2. douglass

    douglass Active Member

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    If you have a way to data log you could see what the engine temp is reading when cold, and when it is hot.
     
  3. Joe Stein

    Joe Stein Member

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    I would look at your coolant temperature sensor. Was your gas mileage decreased on your highway trip? Are you showing any CEL's? Good luck -
     
  4. Sam36

    Sam36 Active Member

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    I fought with this for years. I found out if I reset the computer, that my idle would be "normal" again for about a month. That was after replacing the IAC about once a year (which I later realized it was not the new iac that fixed it, it was the resetting of the computer each time that was doing it...) I did that for years, resetting the computer once a month... Then one day I remembered that I had replaced the TPS with one from autozone many years prior. I bought one straight from Ford. Replaced it, reset the computer.... never had the problem again.