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Rough Idle Only When Warm/Hot

This is a discussion on Rough Idle Only When Warm/Hot within the External Engine forums, part of the LSx Technical Help Section category; I have been having this reoccurring problem for the last thousand miles or so: I have a 93 formula with ...

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    Rough Idle Only When Warm/Hot

    I have been having this reoccurring problem for the last thousand miles or so: I have a 93 formula with 82k miles. Car runs fine for the first 8-10 min (probably until it reaches a closed loop state) then it begins to misfire or hesitate when accelerating and run rough when idling. It's not a bad misfire or horrible idle...just enough to notice. I did my research on this board and tried as many of the suggestions as I could. New spark plugs, new wires (been replaced several times as they keep getting cut on the power steering pulley), new O2 sensors, cleaned air intake, cleaned throttle body, new MSD coil (just replaced this today). Problem still remains. I have not changed the optispark yet. I Really don't want to change that right now.

    However, I do have two possibly related or unrelated problems to add which might help narrow down my next step. First, two days ago while driving home on the freeway, the engine suddenly became unresponsive as the RPM's dropped and as the car stalled out and backfired (the car started right back up and I drove it home). I replaced the coil as soon as I got home that day. Second, since three days ago my voltage regulator on my alt began acting funny. I am under the opinion that the RPM and backfire is related to my rough idle/misfire and the voltage regulator is just coincidence.

    I have no check engine lights by the way.

    I was kind of hoping that maybe my spark plugs got fouled up from running on chopped up spark plug wires, but several people have told me a spark plug issue would happen all the time, it wouldn't be a hot/cold thing. So, I am back to hoping that it is something beside the optispark.......

    Steven

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    Senior Member karpetcm's Avatar
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    If youve checked all of the above as you stated i wouldnt know what else it would be but the opti, i know that you will run rich in the begining when you start up your car so look into seeing if your getting a good amount of fuel once your car is warmed up at your regulator. To me its leading me towards your opti either way, they are notorious for failing and symptoms are not easy to pin point because in the begining stages of them failing its a guessing game and once it gets worse down the road then you know for sure its the opti. The symptoms for a opti failing is not consistant in the begining, atleast it never was for me on my past 3 LT1's i had. Either way GL

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    I just posted a new question asking if anyone in southeast Michigan would be willing to come by and change my opti for me. Post your replies in that thread if you are interested.

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    Senior Member JaycenK's Avatar
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    Have it scanned. Opti's will leave a trail of codes that may not trigger a CEL

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    I scanned my car today, both cold and after 10min warm up....only code was a 61 - A/C Compressor Performance. I am rather surprised to see that there was no code 25, which was IAT high....my IAT sensor was reporting that the incoming air temp at startup was 178*!!!! I replaced it was a brand new and got the exact same value.....most unusual....

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    Does anyone know if the IAT sensor is monitored in the open loop state? The other day I posted that my IAT sensor was reporting a value way too high; I have done more research since then.

    Manually checking the values with an ohm meter reads:
    Approx ambient air temp when car is cold.
    Approx 120* after 12min drive in 90* weather.

    Computer reports:
    178* when car is cold
    Approx 120* after 12min drive in 90* weather.

    I have two IAT sensors that do the exact same thing....can anyone offer an explanation why the computer would misread the IAT sensor in the open loop state but read it just fine when in closed loop?

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