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Cylinder 3 Misfire after spark plug change.

This is a discussion on Cylinder 3 Misfire after spark plug change. within the General Help forums, part of the LSx Technical Help Section category; I installed my new nitrous kit, and changed the Bosh Platinum plugs that were in my car to NGK TR6's ...

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    Member first01SS's Avatar
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    Angry Cylinder 3 Misfire after spark plug change.

    I installed my new nitrous kit, and changed the Bosh Platinum plugs that were in my car to NGK TR6's gaped @ .038". I also changed the spark plug wires while i was into it. I went to start up my car, and got a horrifying surprise of rough idle, and "P0303 Cylinder 3 Three Misfire Detected." A buddy came over (who happens to have HP tuner software) scanned my car and showed me the shocking 150+ misfires (in a relatively short period of time) all happening at idle. When the car is under load, wide open or otherwise anything above 2000 RPM, there is NO MISFIRE. I've tried swapping out coils, wires, spark plugs, etc, and i have not been able to slow it down. I've tried doing a compression test, and everything seems good. I think i may have broken the AIR gasket on the stock manifolds that i have. I'm at my wits end, and i've spent two days on this. I'm ready to give up, and... dum de dum dum: take my car to the dealership............. Please help a newbie out. (I'm not car stupid, i'm just sick of jacking with it.)

    FYI: My car is completely stock except for a SLP lid and a quicktime cut out.

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    Member edbird's Avatar
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    I'm thinking it can only be plug,wire,coil,or injector since none of the other cylinders are involved.Have you tried to swap an injector?

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    Slow'er'Ass Mr. Luos's Avatar
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    Move injector.

    A slight pain in the ass...but not too bad.

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    Could the misfire be related to the exhaust leak I created with the missing AIR gasket?

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    Smiles for 9.5 Years cammed goat's Avatar
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    I tuned up my GTO cuz of a misfire. After the tune up the miss was still a little bit noticeable but I cleaned out the MAF and the miss went away. Maybe you disconnected a wire while you were changing the plugs? I would also replace the AIR gasket just to be sure everything under the hood is there.

    Also, did the wires click when you installed them?

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    Yes, all 8 of them are very secure. Also, I can feel I've lost quite a bit of power, especially on launch.

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    Smiles for 9.5 Years cammed goat's Avatar
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    Hmmm...is the MAF clean? Symptoms sound like mine before I cleaned my MAF. Would bog down on launch from a dead stop.

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    HP Tuner specifies 1 cylinder as culprit. If it were the MAF wouldn't it be more random? Cleaning can't hurt, but since I'm at work, pondering this problem, I'm just throwing that bone out there..

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    Phantom Black Metallic
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    Try replacing the AIR gasket and clean the MAF. I re-read your first post and, as an LsX newbie myself, your symptoms sound dead-on with the ones I had over the summer.

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    Also, if it helps, when the car is cold (sitting for more than 5 hours) there is no misfire on startup, at idle.

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    Smiles for 9.5 Years cammed goat's Avatar
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    Quote Originally Posted by first01SS View Post
    Also, if it helps, when the car is cold (sitting for more than 5 hours) there is no misfire on startup, at idle.
    That kinda sounds like O2 sensors? I could be wrong.

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    Whatever it is, I'm afraid to spray it with this misfire. Also, with such a degradation in N/A power, I'm very irritated with it. Also, cleaned the MAF while at lunch; no change. Could there be an air leak through the threads in the new spark plugs/spark plug holes? If so...... what can i do about it.
    Last edited by first01SS; 01-19-2009 at 12:44 PM.

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    Slow'er'Ass Mr. Luos's Avatar
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    Have you moved a few spark plugs around??

    Trade #3 and #5 or something?

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    Of course. Sitting here in my office, it just dawned on me; and work with me guys, and let me know what you think:

    The AIR pipe is directly adjacent to cylinder 3. Say, hypothetically it was sucking in fresh air through the AIR gasket leak...

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    Slow'er'Ass Mr. Luos's Avatar
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    Quote Originally Posted by first01SS View Post
    Of course. Sitting here in my office, it just dawned on me; and work with me guys, and let me know what you think:

    The AIR pipe is directly adjacent to cylinder 3. Say, hypothetically it was sucking in fresh air through the AIR gasket leak...
    Figured you had already tried that.


    Get that AIR gasket fixed. Could be it. Although, it would have to be a fairly decent leak.

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    I just bought the gaskets today. I'm covering all possibilities now, because at 10:30 at night, when i get home, options will be few for trial and error.

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    Replacing the gaskets made no change. Also, I think I may be loosing compression through the spark plug hole, and it didn't show up on a compression test because the compression tester has a rubber o-ring on it. Is there any kind of sealing soluition that I can put around the spark plug (like high temp RTV) to seal around the plug?

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    Smiles for 9.5 Years cammed goat's Avatar
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    Have you checked the threads in the head to make sure they are not cross-threaded? I have seen mechanic use pipe tape on their pressure testers when they do comp tests.

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    I did the test myself; and did not use pipe thread tape. Tester went into spark plug hole, tightening clockwise.

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    Wow. So, you have checked the coil on number 3, swapped coils and got the same results, replaced the AIR gasket and there has been no change?

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