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Thread: 4.0 sporatic idle.

  1. #1
    Join Date
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    Default 4.0 sporatic idle.

    When I come of the freeway and slow down my idle drops and raises. It drops to 500 and jumps to 1500 over and over no one can figure it out. Newly Rebuilt 4.0. 3 different distributors. Replaced the timing chain. No codes come up. 2 shops have tried to figure it out with no luck. Any ideas? Thanks

  2. #2
    nickyg's Avatar
    nickyg is offline Level: Uber Jeep Cherokee Trailer Queen
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    Default Re: 4.0 sporatic idle.

    Have you looked into the Idle air control valve or throttle position sensor?
    One more part and I'll have a new Jeep

  3. #3
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    Default Re: 4.0 sporatic idle.

    Yes, both shops checked those exact things thinking it would be an easy fix. Thanks for the reply

  4. #4
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    Default Re: 4.0 sporatic idle.

    Then I would suggest pulling the battery cables and touching them together for a few seconds to reset the computer. Be patient, these ECMs are slow learners
    230 hp 4.3L Chevy
    Built 4L60E with manual lockout
    Atlas 4 speed
    Griffin radiator
    On board air
    Warn 9.5ti on custom bumper
    7.5" RE front and bastard pack rear on custom long arms
    30 gal gas tank
    lockers
    And a bunch of other stuff

  5. #5
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    Default Re: 4.0 sporatic idle.

    Great, I will give that a try.
    Thanks

  6. #6
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    danbury ct
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    Default Re: 4.0 sporatic idle.

    can always spray the throttle body with cleaner to get rid of any build up, and clean the throttle blade real good, any carbon around the blade will cause it to idle funky if its not metering the air correctly, always worked on my XJ to improve idle..

  7. #7
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    Default Re: 4.0 sporatic idle.

    Ok. I cleared the computer so I will
    Drive it this weekend and see what happens. I will also clean the throttle body.
    Thanks.

  8. #8
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    Default Re: 4.0 sporatic idle.

    any results?

  9. #9
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    Oct 2016
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    Default Re: 4.0 sporatic idle.

    It wouldn't start? I left it at work parking all weekend. I'm leaving town for a week so I'll have to get to it when I can. Thanks for checking in I appreciate it

  10. #10
    Join Date
    Oct 2016
    Location
    Southern california
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    Default Re: 4.0 sporatic idle.

    Update. Still having problems with the rebuilt 4.0. We got it to idol and run by modifying the distributor mounting bracket. We ground out one side of the bracket so the distributor could rotate a little more which solves the problem. No idea why that worked? What is wrong inside the motor? Yes the timing chain and gear have been replaced again.
    Now smog CERT Is due and it's running with extremely high Nox. All sensors and cat have been replaced. What the heck is wrong with this motor?
    I just want to get on the trail.
    Even though it's not running right I still went head and finished the install of the front Clayton 3 link.
    Thanks
    Kevin

  11. #11
    Join Date
    Sep 2009
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    Default Re: 4.0 sporatic idle.

    High NOX is usually due to high compression or timing too advanced. Since you have rotated your distributor more than normal, you might want to check your timing. When you put in your dizzy, rotor should be pointed at about 4 o'clock position at top dead center. If yours is not like that, then that may be part of the problem.
    230 hp 4.3L Chevy
    Built 4L60E with manual lockout
    Atlas 4 speed
    Griffin radiator
    On board air
    Warn 9.5ti on custom bumper
    7.5" RE front and bastard pack rear on custom long arms
    30 gal gas tank
    lockers
    And a bunch of other stuff

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