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Thread: Studder/backfire and rought Idle...Help!

  1. #1

    Icon312 Studder/backfire and rought Idle...Help!

    My buddy just bought a 98' TJ 4.0L with 61k on the clock. He is having these problems: When Jeep is cold it idles very rough and sometimes surges like it is about to stall...it almost sounds like my 89' Mustang cammin'...lol also when driving sometimes at low rpm the Jeep will sputter and backfire and come close to stalling, it is almost like hitting rev-limiter. Both problems go away once the Jeep warms up. I already took the T-body off and cleaned it, also changed the plugs and gave the Jeep a full tune-up. Any ideas?? O2's fuel filter? Any help would be greatly appreciated.

    Whoops, rough idle...not rought.

  2. #2
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    Is it throwing any codes? Have you checked the fuel psi? It should be ~49psi.

    It may be the IAC solenoid or TPS. As for changing the fuel filter, you have to drop the tank to get to them. One is on the outside of the tank at the pump outlet and the other is inside the tank on the pump inlet.
    97 TJ Sport

  3. #3

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    I'm having the same issue with my '98: Once it's started after sitting for several hours/overnight, it misses and backfires/pops back through the intake if I give it any throttle, and sometimes tries to stall (almost acts like a dead accelerator pump on a carb'd vehicle). Once it runs for a minute or two the popping/backfiring goes away (it does have a slightly rough idle, though). No codes, nothing showing as out of spec using a Snap-On scanner. Removed the throttle body and cleaned every orifice, cleaned every electrical connection I could find, and I've replaced the cap, rotor, plugs, wires, TPS, CPS, and the coil...and haven't made a bit of progress.

    So, if you figure it out, let me know. I'm beyond frustrated with this thing, and will probably just take it to the dealer when the funds allow.
    1998 TJ, 4.0, 5-speed
    D44/HP D30, 4.88s and Detroits, Tera 4:1 and 2-Low, 35's
    Will work for Jeep parts

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  4. #4
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    Quote Originally Posted by TrailX2C View Post
    My buddy just bought a 98' TJ 4.0L with 61k on the clock. He is having these problems: When Jeep is cold it idles very rough and sometimes surges like it is about to stall...it almost sounds like my 89' Mustang cammin'...lol also when driving sometimes at low rpm the Jeep will sputter and backfire and come close to stalling, it is almost like hitting rev-limiter. Both problems go away once the Jeep warms up. I already took the T-body off and cleaned it, also changed the plugs and gave the Jeep a full tune-up. Any ideas?? O2's fuel filter? Any help would be greatly appreciated.

    Whoops, rough idle...not rought.
    spark plugs
    GUNS DONT KILL PEOPLE, BUT THEY SURE HELP

    TJ Sahara
    5 speed
    4" R/C lift
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    Grande 35 kit



  5. #5

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    No codes are showing and changed the spark plugs the other day along with the cap, rotor, and wires. I think next we will try the IAC and fuel filters. Would a crack in the exhaust manifold cause these types of problems even with no codes? Any other suggestions? Thanks for the help.

  6. #6
    Registered rcherb's Avatar
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    Clockspring maybe, its under the steering wheel in the column. You can try unplugging the CC from it. Sounds similar to my previous problem and it was my clockspring.

  7. #7

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    upstream o2 sensor. a lot of early tjs have that EXACT same problem. myself included. try cleaning it before you replace it. its the one BEFORE the cat.

  8. #8
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    A manifold with cracking could possibly cause it due to the crack(s) causing the O2 to falsely read the AF ratio............and when the manifold warms up, the crack(s) closes up and the O2 starts reading right and all is well again. Just an idea, don't take it as 100%..
    97 TJ Sport

  9. #9

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    Sweet, we'll try a few things and let you know if anything works out. Thanks for the help!

  10. #10

    Icon312 The Fix

    The fix: a $25 can of BG 44k added to the fuel tank. Guess the valves were pretty gunked up.

  11. #11

    Icon312

    Ok, problem came back 10-fold. It actually ended up being the 1st O2 sensor before the cat. Now the Jeep runs perfect. Just thought I'd let everyo ne know.

  12. #12

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    Thanks for the update - guess I'll make that my last try before I take it to the shop, and keep my fingers crossed.
    1998 TJ, 4.0, 5-speed
    D44/HP D30, 4.88s and Detroits, Tera 4:1 and 2-Low, 35's
    Will work for Jeep parts

    My Pics

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