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Thread: High speed missfire

  1. #1
    Join Date
    Mar 2006
    Posts
    27

    Exclamation High speed missfire

    I just ran pocono this weekend.Just installed a fresh motor,sohc.Fired right up in the shop,let it run in for a while. I figured everything was good. All I got was two laps then the motor started to missfire and loose power. This happened at about 5,000rpm. Poped the hood found a vaccum line off at the throttle body. I put it back on and it fired up and ran fine. Went back put for the race and it did the same thing again. I talked to my mechanic and he said that it could be the speed sensor. Possibly making the car thinking it was going a million miles an hour and that the speed limiter should cut in. So i changed that and the pcm, Car did the same thing again. Anyone have an idea.

  2. #2
    Join Date
    Feb 2001
    Location
    Purcellville, VA USA
    Posts
    902

    Default

    Codes?

    Some Ideas

    Ignition wires
    Engine Harness
    Injector
    Spark Plug
    Valve Spring
    coil pack
    VSS
    Cam Sensor
    Crank Sensor


    I have seen all of the above cause this on Neons. I listed them is order of probability

    you can emaill me at [email protected]

    Cheers
    Chris "The Cat Killer" Childs
    Angry Sheep Motorsports
    810 417 7777
    angrysheepmotorsports.com

    IT,SM,SS,Touring, and Super Touring

  3. #3
    Join Date
    Mar 2006
    Posts
    27

    Default

    Thanks Chris I'll try them and see what happens.

  4. #4
    Join Date
    Nov 2001
    Location
    Syracuse, N.Y., USA
    Posts
    124

    Default

    Quote Originally Posted by DaveBrooks View Post
    I just ran pocono this weekend.Just installed a fresh motor,sohc.Fired right up in the shop,let it run in for a while. I figured everything was good. All I got was two laps then the motor started to missfire and loose power. This happened at about 5,000rpm. Poped the hood found a vaccum line off at the throttle body. I put it back on and it fired up and ran fine. Went back put for the race and it did the same thing again. I talked to my mechanic and he said that it could be the speed sensor. Possibly making the car thinking it was going a million miles an hour and that the speed limiter should cut in. So i changed that and the pcm, Car did the same thing again. Anyone have an idea.
    You can also check the TPS with a ohm meter - it's a potentiometer. I've had issues with these at WOT in the past..... Though as the previous post has stated, the wiring harness is a known weak point - Since it's the most expensive - I'd try to rule out the others first.

  5. #5
    Join Date
    May 2008
    Location
    CT/NY/NJ
    Posts
    1,157

    Default

    does the idle change if you unplug the MAP/intake air temp sonsor?

    if not, it(or the harness to it) may be the issue... should idle high with it disconnected.

    will it rev past 5000? if so this may not be it...

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