P0304, misfire, and rough idle

christopher.d.carr

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Hello to all JK folks, I have an 07 JK named Petunia (its my wifes, and I did not name her, but we love her just the same.) Anyway, shes got 114k+ and we replaced a cracked header with shorty stainless heads. Now theres a misfire, which started on Cyl 4, and I just replaced it, but theres still a rough idle. Anyone of the shadetree mechanics know what all I need to do next to make her a superstar? I am thinking she might need a new ignition. p0304 was the only code and it cleared after the plug wire was replaced. I know from experience that the most experience and know-how is on these forums. I had an explorer, f150 and f250 that the forum guys blew my local mechanic's mind. I need expert advice, she's going to "get the buisness" with upgrades eventually. Thanks for the help!

Chris-
Proud JK father to Petunia
 
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christopher.d.carr

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Hey thanks! Looking on these forum I generally find better advice and technical know how than the dealership and manufacturer.
 
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Chris

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So there's no more codes being thrown, but a rough idle?

When you drive it, does it run fine other than the rough idle?
 
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christopher.d.carr

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There was only the p0304, and it backfires too. I replaced the spark cables, which worked for 15 minutes, cleared the check engine light even, then she started running horrible again. I seafome'd her and now the random is all over the place. I haven't checked again for the codes that have popped back up. I am thinking coil pack distributor thing, or the crank sensor, but that does not rule anything out.
 

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If there's no code, it's hard to diagnose. However, if I had to guess, it's spark related, so either spark plugs (cheap and easy), or coil packs, which are also very easy to test as well.
 
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christopher.d.carr

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I think that theres something up with the heat coming off those stainless headers I put in. The misfire initially was p0304, so I replaced the cables with new. Now theres a ton of other things not firing correctly. I think that the ecu may shut down cylinders or something weird when it senses a problem.

Anyone ever had this issue with a crank sensor or with a bad coil/distributer?
 
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christopher.d.carr

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If there's no code, it's hard to diagnose. However, if I had to guess, it's spark related, so either spark plugs (cheap and easy), or coil packs, which are also very easy to test as well.
How do you test one? I imagine resistance or shorts to ground, but what should I see? A climbing resistance across the coil, obviously infinite resistance when taken from ground. I also ordered a "demon dui" or something like it, higher performance coil, and I hope its not just hyped up chinese crap. Seemed to be legit.

thanks too btw
 

Chris

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How do you test one? I imagine resistance or shorts to ground, but what should I see? A climbing resistance across the coil, obviously infinite resistance when taken from ground. I also ordered a "demon dui" or something like it, higher performance coil, and I hope its not just hyped up chinese crap. Seemed to be legit.

thanks too btw
If you put the headers on and it didn't happen until after the install, then it must have something to do with that.

You can test the coil packs like this:


I know that's for an older 4.0, but it's the same principle.

As for those coil packs you ordered, I wouldn't trust anything other than OE Mopar. I suspect those ones (like most everything else) are cheap Chinese crap. Hopefully I'm wrong!
 
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christopher.d.carr

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If you put the headers on and it didn't happen until after the install, then it must have something to do with that.

You can test the coil packs like this:


I know that's for an older 4.0, but it's the same principle.

As for those coil packs you ordered, I wouldn't trust anything other than OE Mopar. I suspect those ones (like most everything else) are cheap Chinese crap. Hopefully I'm wrong!
I installed the coil pack today, it's putting enough spark through a burnt wire to not idle too bad. I am almost certain it's the header heat that burns the wires through. I'll update the progress with that coil pack. It looks more like a distributor but there's no mechanical connection. Have any heat shield diys?
 

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I installed the coil pack today, it's putting enough spark through a burnt wire to not idle too bad. I am almost certain it's the header heat that burns the wires through. I'll update the progress with that coil pack. It looks more like a distributor but there's no mechanical connection. Have any heat shield diys?
I'll bet that is it then. The stock manifolds have heat shields, and by removing those (with the aftermarket headers), it's now burning the wires.

Get the headers ceramic coated if you want to do it right. That will make them so cool that you can touch them with your hands even after the most extreme driving. I've done it on many muscle cars, and believe me when I say that ceramic coating works really damn well. You could also fab up some sort of a heat shield using a product like this:

 
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christopher.d.carr

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I'll bet that is it then. The stock manifolds have heat shields, and by removing those (with the aftermarket headers), it's now burning the wires.

Get the headers ceramic coated if you want to do it right. That will make them so cool that you can touch them with your hands even after the most extreme driving. I've done it on many muscle cars, and believe me when I say that ceramic coating works really damn well. You could also fab up some sort of a heat shield using a product like this:

Wow! Ceramic is that effective? I have some exhaust wrap and I plan on that, but will it work better if I get it coated first? I appreciate your reply... I can't find a shop worth a damn anymore. The guys that did this were "professionals" I understand aftermarket stuff doesn't just plug and play, but it never occurred to me that they wouldn't even try.
I am gonna try that asap.

Thanks again!
 

Chris

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Wow! Ceramic is that effective? I have some exhaust wrap and I plan on that, but will it work better if I get it coated first? I appreciate your reply... I can't find a shop worth a damn anymore. The guys that did this were "professionals" I understand aftermarket stuff doesn't just plug and play, but it never occurred to me that they wouldn't even try.
I am gonna try that asap.

Thanks again!
Ceramic is amazing. It will blow you're mind how good it is. I had it down on my Corvette Z06, and after racing around the backroads at high RPM, you could literally touch the headers with your fingers and all they were was warm, not hot.

Exhaust wrap will work, but the issue with it is that it traps moisture and will eventually rot / rust your headers. I guess this is something I had forgotten about with headers, and it was fairly common on most American muscle cars as well. If you didn't get new plugs with insulated wires (which is also an option if you can find them), the headers would melt the plug wires, and then you'd have misfire issues.