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PO300 despite Jeep running fine

Discussion in 'Performance & Tuning' started by Tarashuk, Nov 9, 2019 at 2:01 PM.

  1. Nov 9, 2019 at 2:01 PM
    #1
    Tarashuk

    Tarashuk [OP] New Member

    Joined:
    Saturday
    Member:
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    Male
    First Name:
    Matt
    Vehicle:
    2005 Wrangler LJ Unlimited
    I have a 2005 Wrangler LJ Unlimited and the check engine light comes on solid and sometimes flashes. The only code I am getting is the PO300 random multiple misfire. The Jeep starts, idles and runs fine, the vehicle access fine - no obvious sounds of hesitation. I can clear the code and a week or so later it comes back.

    Any advice? Has this happened to anyone?

    I can tell you that the plugs have been changed along with O2 sensor
     
  2. Nov 9, 2019 at 2:22 PM
    #2
    Awrench

    Awrench Well-Known Member

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    May 12, 2018
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    was there any problem BEFORE changing parts?
    you did use correct plugs.?
    why did you change the o2 sensor? You did use a OEM or a NTK brand?

    Ideally a scanner looking at data - ECT, MAP. TPS, O2 voltages and fuel trims...as well as checking for ALL codes.
     
  3. Nov 9, 2019 at 6:12 PM
    #3
    Tarashuk

    Tarashuk [OP] New Member

    Joined:
    Saturday
    Member:
    #4067
    Messages:
    3
    Gender:
    Male
    First Name:
    Matt
    Vehicle:
    2005 Wrangler LJ Unlimited
    I bought it 3 weeks ago and the owner gave me all the receipts of the work he did to it. This is what he did:
    Replaced:
    Coil pack 2 yrs ago
    CAM an crankshaft sensors 2yrs ago
    Bosch spark plugs 2 yrs ago
    O2 sensor - for a P0038 code 2 yrs ago

    The previous owner said that the plugs, CAM crankshaft sensor and coil pack was replaced for the PO300 error. He said that the light has been coming on and off for the past 2 years but the vehicle has been running with no hesitation and after replacing the above parts the engine light keeps coming on. Of course when I took it for a test drive the engine light was off.
     
  4. Nov 10, 2019 at 7:48 AM
    #4
    Awrench

    Awrench Well-Known Member

    Joined:
    May 12, 2018
    Member:
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    Messages:
    888
    Gender:
    Male
    Vehicle:
    3 jeeps
    LOts going on here.

    You can drive the jeep and it seams to drive well.
    SO lets make a plan.
    When the CEL comes on be sure to use a SCANNER* to check codes, this is FREE at many auto part stores. The scanner will display ALL codes. Key on & off trick is lacking.

    You want 6 NGK (3459) ZFR5N spark plugs - Why? Bosch sucks in jeeps.
    See if they have loaner tools - A fuel pressure tester**. And a compression tester. We should do the fuel test first AND if no guidance on issue then the compression test.


    *If you have a scanner will it see all codes and display live data? let us know that would be fast.

    **Look at your fuel rail @ #2 injector there should be a schraider valve for checking fuel pressure - confirm this as some jeeps were built without the test port.
     
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