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  1. #1
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    Default Idle adjust 4.0 Impossible?

    Got a friend who knows a lot about Jeeps as far as History & the different variations and seemed pretty sharp when it came to my 4.0 . My engine idles around 800 at startup but after it warms up about 3 minutes or so it will idle a steady 550 or so .....which is TOO LOW cuz mines a stick, & we know what happens when your driving & have to let off quikly & the engine "almost" wants to die,but doesnt quite die. But almost does.
    So I mentioned one day about setting my idle up a bit to around 1000 so when it warms up it should be around 800. To which he says "ya cant adjust the idle on 4.0's theyre set how theyre set from the factory" .......I havent tried yet, but just find it a biot hard to believe. Just wondered what anyone elses opinions might be.

  2. #2
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    Default Re: Idle adjust 4.0 Impossible?

    Quote Originally Posted by 88AMCOMANCHE View Post
    Got a friend who knows a lot about Jeeps as far as History & the different variations and seemed pretty sharp when it came to my 4.0 . My engine idles around 800 at startup but after it warms up about 3 minutes or so it will idle a steady 550 or so .....which is TOO LOW cuz mines a stick, & we know what happens when your driving & have to let off quikly & the engine "almost" wants to die,but doesnt quite die. But almost does.
    So I mentioned one day about setting my idle up a bit to around 1000 so when it warms up it should be around 800. To which he says "ya cant adjust the idle on 4.0's theyre set how theyre set from the factory" .......I havent tried yet, but just find it a biot hard to believe. Just wondered what anyone elses opinions might be.
    This is correct, the 4.0 has an IPS or Idle Position Sensor, it is not adjustable. I took mine out and it was Full of Carbon. A few hours later after drying idle was good. The sensor is right next to the TPS on the throttle body and is an easy clea/replace if needed.
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  3. #3
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    Default Re: Idle adjust 4.0 Impossible?

    Theres no way to adjust the idle on that... theres gotta be a vac leak of some kind or like Outlaw says ISP could be dirty...
    Last edited by Punisher; 02-20-2011 at 01:13 PM.
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  4. #4
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    Default Re: Idle adjust 4.0 Impossible?

    I will put that on my list of things to do ....the ISP* ....that wil be my starting point when I get to it~

  5. #5
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    Default Re: Idle adjust 4.0 Impossible?

    Well thats not completely true , there is an air bleed than can be manually adjuted but for all intents and purposes the IAC motor (idle air control) is normally the issue take it off clean the pintel on it and the passage in the TB both are carbon fouled. Put it back in and the jeep will rev to about 3k an then settle back down to the pre set idle speed. If you still have an Idle issue check the Throttle Pos sensor it may be bad or out of range. It can be adjusted check your manual for specs. If the issue persists I would replace the IAC motor. I had the same issue on mine and others and following these procedures they all got running right. Good Luck
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  6. #6
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    Default Re: Idle adjust 4.0 Impossible?

    Only one thing to add. The thing that determines the idle speed is the ECM, which controls the IAC. Theoretically it could be adjusted, but it would have to be reprogramed to do so and I don't know of anyone who can.
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  7. #7
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    Default Re: Idle adjust 4.0 Impossible?

    OK this is how I adjusted my idle... I am not saying it is an official way.
    I too had problem of idling going down under 500rpm when warm, I also have a manual.

    I bent the tab....see when the throttle cable attach on the TB, it is a tab that rest on some kind of pin. well I took a pair of pliers when engine was warm, and bent it in a little and after idle settled, kept adjusting it until I was happy.
    Now it is idling fine at 750rpm....

    I did that after I was sure that there was no exhaust leak (manifold bolt was not tight on the rear bolt) that there was no vacuum leak (usually high idle is the symptom for that) and after cleaning intake and adjusting TPS.

    I know my IAC is acting up and some times it idles at 2 or 3000 rpm.... need to be changed but it is working for now...


    So to sumarize... look into that little tab bending.....
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