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"Rough" Idle 2003 GC L6

Discussion in 'Cherokee' started by SpartanRouge, Jul 3, 2018.

  1. Jul 3, 2018 at 11:40 AM
    #1
    SpartanRouge

    SpartanRouge [OP] Member

    Joined:
    Oct 6, 2017
    Member:
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    Messages:
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    Gender:
    Male
    First Name:
    Aaron
    Vehicle:
    2003 Grand Cherokee Larado I6 4.0 2WD
    For those of you that have read some of my posts you might know that a few months ago i bought this GC with a bad motor and put a re-maned one in it and replaced a bunch of key parts that were easy to do with the engine out. It runs great now! Except i have a strange idle. It feels like it is a bit rough and seems mild when cold and seems to get a little worse when its warmed up. You can feel it whether you are in PRN or D. The idle sits at about 475-600 in park or drive, which is perfect. It is not setting any misfire codes and doesn't feel nearly as bad as a misfire either, it kinda feels like a small vacuum leak but i smoke tested it and came up with nothing besides the throttle plate shaft was leaking a good amount but i swapped it with a different truck and it does the same thing (leaking wise and idle wise). I even replaced the IAC with a brand new high quality one and it didn't change a thing. The only things i did not get new when i did the engine swap was the coil rail (all 6 come as one part so its really expensive) and all the injectors (NEW plugs). Could either of those be an issue with a rough idle with no hard misfire? Also, as soon as you increase the RMP even just a little with the gas pedal, the vibration goes away completely. I also checked motor and trans mounts, they are perfect, can still feel it with motor and trans jacked up at same time. Another thing i notice is if its at idle and you tap the gas pedal really quickly, it acts like it wants to die for a fraction of a second before coming up (never actually stalls though, its more like a mild stumble) and since i work at an auto shop, i've tested that quick tap test on other GC with the same motor and none of them do that, the rmp comes up right away. Please help!!! considering i am a mechanic and this is my truck its driving me crazy not being able to figure it out, ive exhausted every idea ive had besides replacing the coils and all the injectors. which i dont want to do if they are not the problem. could it be an issue with the motor itself, considering it was re-maned? any info will be greatly appreciated!!!!

    One other thing i will say is that with our Maxisys Elite Scan tool, there is an option to test the IAC which forces the idle to race at whatever you set it at (800-2000, in increments of 100), i believe when you do that, the computer is supposed to hold the idle at what you set it as but for me, when i click it, the idle jumps for a split second then goes back to normal. that doesnt seem right to me but like i said i even put a new IAC in it and did the same thing. I havent been able to try it on a customer's GC to see if theirs reacts the same way though.
     
  2. Jul 3, 2018 at 12:58 PM
    #2
    Prerunner1982

    Prerunner1982 Well-Known Member

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    Jon
    Vehicle:
    93 XJ 2 door
    You might check the Throttle Position Sensor (TPS) with an analog volt/ohm meter. It could have a bad spot that could cause the delayed response/stumble.
    As far as the idle itself, all 3 of my 4.0 liters (XJs) have had a bit of a rough idle with an occasional hiccup. Not really a miss, but more of a stutter. Doesn't effect the driveability or anything.
    A common upgrade on the 4.0 that I have heard helps with idle is swapping to 4 hole injectors in place of the factory 1 hole. Raffi is popular on the XJ Facebook groups for injectors, he also has 12 hole injectors for sale. I have never purchased from him so I can't speak personally but a lot of people on FB recommend his stuff. https://www.ksuspensionfab.com/store/c3/Jeep_Cherokee_XJ/MJ.html
     
  3. Jul 3, 2018 at 1:10 PM
    #3
    SpartanRouge

    SpartanRouge [OP] Member

    Joined:
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    First Name:
    Aaron
    Vehicle:
    2003 Grand Cherokee Larado I6 4.0 2WD
    I got a new TPS right after i did the engine swap because the connector on it was broken and wasnt making good contact.
    i did not know they made different kinds of injectors like that. Besides the idle change what are other effects of it? more power? worse gas mileage?.. more holes sounds like it will use more gas :p Im already paying an arm and leg for gas in this thing.. only getting an average of about 11.5mpg and gas aint cheap in south florida! $3/gal round here :p
     
  4. Jul 3, 2018 at 1:59 PM
    #4
    Prerunner1982

    Prerunner1982 Well-Known Member

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    Here is his WJ Grand Cherokee section: https://www.ksuspensionfab.com/store/c6/Grand_Cherokee_WJ.html (didn't realize he had one).

    It should be about the same fuel it just gives it more holes to spray through to create better atomization as opposed to one stream through a single hole. Have seen a number of review mention better throttle response.

    I haven't swapped injectors and it's not in my immediate plans, I get 18.5-20mpg depending on which XJ I am driving.
     
  5. Jul 4, 2018 at 8:14 AM
    #5
    Awrench

    Awrench Well-Known Member

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    you are using a scanner to check codes?
    will your scanner allow viewing live data?
    ECT, O2sensor voltages, Fuel Trim long and short, MAP.

    Any issues -dash lights - something not working?
    Plugs - what brand? Thermostat Temp?
    NO aftermarket BS - magnets, filters, programmers,

    High quality parts = jeep OEM !


    try using the old school "https://www.youtube.com/watch?v=mYa8rcHeN3A" -
     
  6. Jul 30, 2018 at 6:09 PM
    #6
    SpartanRouge

    SpartanRouge [OP] Member

    Joined:
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    Male
    First Name:
    Aaron
    Vehicle:
    2003 Grand Cherokee Larado I6 4.0 2WD
    haha wow!!! its amazing that you used THIS video, because the car in that video is exactly the same as my other car, 96 saturn SL2. i would recognize that engine anywhere
    Yes i am using a scan tool and can view live data, haven't checked out the fuel trims though, i really should look at them, just haven't had time and since my wife drives it, im not usually allowed to have it :p .. but there are no codes in any modules in the entire truck, no lights on the dash (except the seat belt light when I'm being naughty).
    I have tried to do a vacuum leak test (not this way ^) with a smoke tester, which is a little more reliable than the "oldschool" way. the ONLY leak that i find is coming from the throttle plate Shaft, which I'm told is completely normal. I even switched it with a different truck and it does the same thing.
    I believe when i find the extra cash and the time, i am going to buy and install Prerunner1982's suggestion and go with the upgraded fuel injectors seeing as that is one of the only 2 things i did not get new when i changed the engine (the other being the coil pack). They are actually fairly cheap considering they are fuel injectors AND the website they are made by Bosch. only $170 for the set of 6!!!
     
  7. Jul 31, 2018 at 8:15 AM
    #7
    Awrench

    Awrench Well-Known Member

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    I am not sure on the injectors - no reason to suspect them YET- always a can of 'seafoam' in fuel. there are no misfires- did you look at misfire counts? -the fuel trims will be a suggestion also o2 voltages, ECT, MAP, you have and can use a scanner SO before spending lets just look.

    If there is a schraider valve on fuel rail - check fuel pressure and then with KOEO observer thr pressure drop -- drops Xpsi in 5 minutes...?
     
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