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P0300 misfire code...help:(

This is a discussion on P0300 misfire code...help:( within the General Help forums, part of the LSx Technical Help Section category; Got a P0300 code, rough idle and ses light, changed plugs, wires and 02 Sensors, didn't help. Mechanic has the ...

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    pewter
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    P0300 misfire code...help:(

    Got a P0300 code, rough idle and ses light, changed plugs, wires and 02
    Sensors, didn't help. Mechanic has the car 2 days and says it's the #3 & 4 cylinders misfiring but can't find what's causing it. He checked injectors, fuel pressure, coils, egr, vacuum leaks, maf - all functioning properly.... Any suggestions on what else could cause this problem. Thanks. BTW car has 13,000 miles, just a few bolt-ons.
    2002 SS, A4. Mods: SLP Blackwing lid, FRA, Throttle body bypass, Smooth bellow, 160 Stat, HPP3, SLP SFC, Tourque arm, STB.

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    Spaz is My Mentor SMWS6TA's Avatar
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    Quote Originally Posted by Lotapwr View Post
    Got a P0300 code, rough idle and ses light, changed plugs, wires and 02
    Sensors, didn't help. Mechanic has the car 2 days and says it's the #3 & 4 cylinders misfiring but can't find what's causing it. He checked injectors, fuel pressure, coils, egr, vacuum leaks, maf - all functioning properly.... Any suggestions on what else could cause this problem. Thanks. BTW car has 13,000 miles, just a few bolt-ons.
    Read more at: OBD-II Trouble Code: P0300 Random/Multiple Cylinder Misfire Detected


    A code P0300 may mean that one or more of the following has happened: (I bold the ones you said you've already done)
    Faulty spark plugs or wires
    Faulty coil (pack)
    Faulty oxygen sensor(s)
    Faulty fuel injector(s)
    Burned exhaust valve
    Faulty catalytic converter(s)
    Stuck/blocked EGR valve / passages
    Faulty camshaft position sensor
    Defective computer
    MAF
    IAC - Idle Air Control
    TPS - Throttle Position Sensor

    The 2 in red are the last to check for after everything else has been exhausted.


    Take one coil pack off of #3 or 4 cylinder and move it to #1. Clear the codes if able or disconnect the battery both terminals for 20-30 mins so that the codes are cleared from the PCM memory. After codes are clear, start the car see if CEL comes back on and read the codes. What you are looking for is to see if the code has jumped to the #1 cylinder. If so then the coil pack is the problem, if it doesn't move, try it the same method with the wires. Still no change, pull plug. Make sure it isn't cracked. Cracked pulls from over tightening will cause misfires.

    Still no change, check the follow and do the suggestions listed:

    MAF -
    Buy some MAF cleaner, you can remove it or leave it on the car but you need to be able to access the front of the maf, next spray it down pretty good. let it dry on it's own, DO NOT TOUCH THE WIRE.

    IAC -
    Next pull the IAC sensor located on the drivers side of the throttle body. Check to see if this is clogged up with oil residue , carefully remove the big stuff, you can use carb cleaner to finish off the rest of the residue, wipe off with a paper towel or rag. The (for lack of an accurate description) piece that goes inside the TB should be able to move in & out. If it doesn't you may need to replace it you can't clean it up.

    TB -
    Now open the blade on the TB, how black is it? You can remove it and clean it up, the air galley for the IAC will likely be full of crap.

    Do you still have the CATS? If so are they the original CATS?



    What bolt on's do you have and did you just do a mod prior to the issue?

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    pewter
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    Thanks for all the info, and yes original cats are still on the car. No recent mods either. Forgot to mention I cleaned the Maf and didn't help. The car also is down on power and runs like crap. I'll try the other things you mentioned above. Much thanks

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    Do you use a K&N filter?

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    Typically MAF, IAC, TB, or cats wont target one or two cylinders, they would target one bank or all cylinders. Also if you have a misfire on two adjacent cylinders then one of those cylinders is the root misfire and the other is showing up as an imbalance and just registering as a misfire. So get a scanner and see which cylinder has the most misfires in that cycle and address that cylinder. Usually to diagnose a single cylinder misfire you start swapping components from one cylinder to another one at a time to see if the misfire follows. To test injectors hook up a fuel pressure tester and perform an injector balance test and record the pressure drop on the cylinder in question. It shouldnt be more than 10 percent from the rest of the injectors. That would also be a good time to check the fuel trims to see if its running rich or lean.

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    I had this issue several months ago. Got the bottom of the car wet driving home in slush, car ran like crap. Turned out the whole passenger side bank would randomly cut out. Turned out to be 02 sensors. i had no O2 sensor code just P0300 random misfire.

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    op any updates?

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    Quote Originally Posted by xerocks View Post
    Typically MAF, IAC, TB, or cats wont target one or two cylinders, they would target one bank or all cylinders. Also if you have a misfire on two adjacent cylinders then one of those cylinders is the root misfire and the other is showing up as an imbalance and just registering as a misfire. So get a scanner and see which cylinder has the most misfires in that cycle and address that cylinder. Usually to diagnose a single cylinder misfire you start swapping components from one cylinder to another one at a time to see if the misfire follows. To test injectors hook up a fuel pressure tester and perform an injector balance test and record the pressure drop on the cylinder in question. It shouldnt be more than 10 percent from the rest of the injectors. That would also be a good time to check the fuel trims to see if its running rich or lean.
    now that's a pretty solid first post.......


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