Thread: Intermittent misfire

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  1. #1 Intermittent misfire 
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    Hello everyone, I'm new here so this may not be the best place to post this but it seemed fitting. I've have a 98 grand prix gtp, all stock, no mods. I keep getting a p0304 cylinder 4 misfire. Now before we jump through the usual hoops, let me explain. The light will only come on if I let the car idle for 10 minutes or more. If I start it up and leave immediately I can drive the entire day without the code returning, once it comes on it stays one until erased. It does not affect the performance of the vehicle I average 19.6 mpg as it always has and it runs smooth under load and otherwise. I have been through the usual suspects. I have replaced the coil packs, spark plugs, plug wires, I've replaced the fuel injectors as well, yes they were flow matched and all the parts are original GM parts. I have also replaced the lower intake gaskets with the zzp aluminum ones, I am getting very good compression on all cylinders and I've replaced all the engine/transmission mounts. Light still comes on, super annoying to look at lol. What am I missing here that could be doing this. I have a stage 2 zzp performance kit ready to go on this vehicle but I would like to track down this issue first.
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  2. #2 Re: Intermittent misfire 
    GrandPrix Junkie
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    Check the spark plug wire routing back there and make sure #4 isn't laying on the O2 sensor or anything else for that matter.

    What type of spark plugs are you running?
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  3. #3 Re: Intermittent misfire 
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    Quote Originally Posted by FordMan77 View Post
    Check the spark plug wire routing back there and make sure #4 isn't laying on the O2 sensor or anything else for that matter.

    What type of spark plugs are you running?
    The spark plugs are ruthenium ngk plugs, heat range 4. I have also ran a set of the autolite 104 plugs heat range 4 as well, and the acdelco OEM copper plugs but I cant remember the part# issue remains. I can verify all plug wires are secured in the plastic factory retainers and do not appear to be touching each other or the oČ sensor. I have also checked plug wire resistance and swapped the wires around but code always shows cylinder 4.
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  4. #4 Re: Intermittent misfire 
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    I gave my boy my '06 GT and he just started getting the same code on cyl 5. For the '06 however it does it when accelerating hard. Changed pretty much everything Kenny did to no avail. He did however remove the mufflers just before it started. Hope this does not take from Kennypowers original question.
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  5. #5 Re: Intermittent misfire 
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    Removed belt from the super charger and codes went away. Figure it is a back pressure issue.
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  6. #6 Re: Intermittent misfire 
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    Have either of you replaced the cat yet? a 98 and an 06 with a stock cat are likely well past the useful mileage and it's probably clogged up at this point in life.
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  7. #7 Re: Intermittent misfire 
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    If it is excessive back pressure then something is plugged up.

    SMGPFC Member #1
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  8. #8 Re: Intermittent misfire 
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    Quote Originally Posted by FordMan77 View Post
    Have either of you replaced the cat yet? a 98 and an 06 with a stock cat are likely well past the useful mileage and it's probably clogged up at this point in life.
    I was teetering on that, mine is a 98 but only has 122,000 miles on it so I never that it would be the issue. I have a new down pipe, high flow cat and OČ sensors, I should have them on by the end of the week. I will be back with an update.
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  9. #9 Re: Intermittent misfire 
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    Quote Originally Posted by tonic View Post
    Removed belt from the super charger and codes went away. Figure it is a back pressure issue.
    Does your supercharger rattle at all at idle? When I first got my car I would get a multiple misfire code and the car missed badly when on the accelerator really hard. But ran fine otherwise. Supercharger coupler was bad, it solved quite a bit of issues. You might try that, get the green one from zzperformance if you do replace, much better than OEM, they have a whole kit for the job. I think if backpressure was the issue it wouldn't matter if the belt was on or off the supercharger, under load it would still be a problem even with no boost. Also if your car has more than 150k on it, change the lower intake gaskets, they're junk plastic and prone to sucking coolant into random cylinders, common 3800 problem especially if you've been using the orange DeX cool coolant, it's highly acidic if not treated properly.
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  10. #10 Re: Intermittent misfire 
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    Thanks Kenny, intake gasket is less then 2 years old. Tried running with straight manifolds, still misfire under load. I will try super charger coupler.
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