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  1. #1
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    Another 03, 5.3 question

    P0300, multiple misfires.
    I replaced the plugs and wires today, still running tough ( misfire ).
    All of the passenger side plugs looked normal. Drivers side were all carboned up.

    Havent rechecked codes.
    Is the most likely cause, a bad bank 1, upstream o2 sensor?
    Thx

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    #2
    Have you checked the coils, also the rear Injector screens stop up because when you turn the engine off the dirt settles at the lowest point in the fuel rail

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    #3
    have seen one side with a clogged converter cause issue like that. other side fine with a normal flowing converter. it was the drivers side also. remove the upper o2 and see if it runs better open? also plenum gaskets cause vacuum leaks
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    2000 stratos 20 ss (bought new)
    200 hp Rude ficht -re200flssh
    twin 12 ft. talons-blue water leds
    lms520c--hds7-hds9 gen2
    pulled by a 2500HD Denali
    bravo1 fs croxtonized
    proud dad of an ARMY Lieutenant.
    proud uncle of an ARMY Sergeant
    member of "team deplorable" MAGA
    ASE/GM Master tech before tools

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    #5
    I would check those oxygen sensors. I believe these play a role in the timing of fuel delivery to certain cylinders.
    Last edited by JHenry; 03-28-2020 at 02:18 PM.

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    #6
    Thx!

  7. Member Big Ron's Avatar
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    #7
    Have it Decarbed

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    #8
    Quote Originally Posted by JHenry View Post
    I would check those oxygen sensors. I believe these play a role in the timing of fuel delivery to certain cylinders.
    Agree. Had an 03 5.3 that had a disconnected upstream O2 Sensor and drove fine until driving thru a puddle, would start bucking, throwing codes, and going into limp mode. Put a Tech 2 on it to confirm my suspicion. Replaced it and all was well...

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    #9
    Where is Jerry at

  10. Member
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    #10
    New plugs, wires, cleaning throttle body, and a new bank 1 upstream o2 sensor, still misfires.
    Leaking intake gasket replacement is a job, I'd rather not have to do.
    I doubt if the previous o2 sensor was working correctly
    There were small areas of metal missing on the end that goes in the exhaust stream,
    and the electrical connection wasn't firmly seated.

    Im not sure it's the intake causing the misfires, but the next thing to check.

  11. Member masterbass's Avatar
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    #11
    try running the truck and spraying carb clean around the base of the intake to check for leaks! doubtful that's whats causing your particular problem but its worth a shot. Do you have access to a decent scan tool so you can monitor you engine and 02s/fueltrim while the truck is running!?

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    #12
    If it's still missing can you actually feel the misfiring or is it just a code?

  13. Member lpugh's Avatar
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    #13
    Possible but improbable to be an 02 sensor, scan the data stream and scope test the 02 sensors for verification. Disconnecting the 02 sensors to see if the idle improve would give you some idea if they may be reporting out of range. This would force the system to operate in closed loop with richer mixture than normal. Monitor the misfires on individual cylinders to see if there is a pattern to particular cylinders. P0300 means you have random misfire, very rarely an ignition problem, usually a lean run, broken valve spring or sunken valve seat, A sunken valve seat will not all ways show up on a compression test but will show up on a undampened vacuum gauge, Check the fuel trim long and short terms
    Excessive alcohol in the fuel can also cause this
    Also pull the crank sensor and inspect for any indication of rub marks on the end of it.
    Perform the crank sensor relearn procedure (worn timing chain components over time can cause this code)
    This is usually a pretty easy diag for a good tech
    How many miles on this motor?
    Last edited by lpugh; 03-29-2020 at 11:49 PM.
    Thank You Leon Pugh

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    #14
    Quote Originally Posted by mactlman View Post
    have seen one side with a clogged converter cause issue like that. other side fine with a normal flowing converter. it was the drivers side also. remove the upper o2 and see if it runs better open? also plenum gaskets cause vacuum leaks
    +1 have this seen this a couple times also

  15. Member
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    #15
    I can still feel the misfire.
    All 4 plugs on drivers side were carboned up.
    Pass side looked normal.
    About 185k.

  16. Member lpugh's Avatar
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    #16
    Catt could be the issue as mactlman suggested, I am not sure it is possible on this vehicle, but I have seen a few that 02 senors getting connected to oposite sides cause this exact problem.
    I don't think the harness layout would allow for this on yours though. Usually happens after trans R&R, common on some Fords and Rams
    Thank You Leon Pugh

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    #17
    Probably my coils

  18. Member lpugh's Avatar
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    #18
    Quote Originally Posted by rude150 View Post
    Probably my coils
    Nope, not a coil as this would set a cylinder specific code as well as p0300. I have yet to see a bad coil on that year and make that I can recall
    Your money, ignore the advice and do what you want
    Thank You Leon Pugh

  19. Member
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    #19
    Update, just pulled codes at 8:30 pm, and after doing the above mentioned tune up and o2 sensor,
    the codes 0300, 0131, 0171, 0442 popped up.

  20. Member lpugh's Avatar
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    #20
    Need the fuel trim numbers as requested above
    P0300 Random misfire more than one cylinder and usually varies as to which cylinder is miss firing
    P0131 bank one 02 sensor reporting low voltage (may be the result of very lean condition on Bank one as P0171 indicates)
    P0442 small evap leak
    Top of plenum at front there is a purge solenoid, disconnect the electrical connector and the line that goes to it. Start the motor and see if there is vacuum form the solenoid (should be none if solenoid is good)
    This will set a purge solenoid code don't fret over it as this is just to test if the solenoid is leaking
    If the 02 sensor has failed (shifted out of range low voltage low voltage it very well may have set the p0171 and PCM assuming a very lean condition driving the fuel mix very rich on that bank possible causing plugs form black carbon coating
    Really need to analyze fuel trim and compare downstream 02 sensors to upstream before the catts light off with a scan tool
    Have you done a compression test yet
    Have you check manifold vacuum with a undampened gauge
    Not a good idea to throw parts at it without knowing the motor is sound
    Last edited by lpugh; 03-30-2020 at 08:31 PM.
    Thank You Leon Pugh

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