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Old 06-18-2009, 12:45 AM   #1
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HELP..cylinder 6 misfire.

So here's the deal. Months back I was driving and my jeep began to surge and ran very rough while driving. The engine light began to blink so I pulled over and shut the jeep off. I started it minutes later and the engine light was on but was not blinking anymore. I was on my way but still running very rough while driving and at idle. When I got home I ran a diagnostics test and it stated: P0306 cylinder 6 misfire. I decided to clear the code and it was fine. I changed the spark plugs thinking that it may make a difference. It has been running great for months without any problems and then all of a sudden today.....same thing. Light blinking and then it stopped. Engine light still on and the code reads the same thing....cylinder 6 misfire. I guess my question is.....what else could it be?? I would think if it was a major problem wouldn't this be a continuous problem?? Wouldn't the engine light stay on always?? Let me know what you guys think. This girl needs some help. Thanks!!

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Old 06-18-2009, 12:52 AM   #2
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Try this. Take one of your plugs wires and change it with the cylinder 6 plug wires. If it still misses on cylinder 6 you have ruled out the wires being bad. If it miss fires on another cylinder the wire is bad and it is a easy fix. IF that doesn't work post back and we can try some more things!

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Old 06-18-2009, 12:56 AM   #3
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You should check the basics of dist. cap, rotor, plug wires etc. But I had a similar problem where it would run rough then clear up for a month or two then do it again. Ended up being the upstream o2 sensor going out. Eventually it got to where I couldn't drive it without it acting up. Unfortunately with it being intermittent at best it was very difficult to track down until I unplugged the sensor and it ran fine for a few days, so I replaced it. No more problems. If they weren't 60 bucks I would suggest trying it as a guess!
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Old 06-18-2009, 07:39 AM   #4
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The same thing happened to me(except #3) and it turned out to be a bad fuel injector. Good luck.
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Old 06-18-2009, 09:08 AM   #5
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Good to try swapping a plug but you don't have a cap, rotor or wires on your 02 TJ.

There is a rail system that connects your plugs. Easiest thing to check is to ensure that it is fully seated and tight in the rear (near #6) and that the plug is fully connected to the boot.

Try that first.
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Old 06-18-2009, 07:22 PM   #6
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what Jd said is a good idea. Swap an ignition wire and see if the code is set for another cyl. Also check the connection at the coil and see if the terminal is rusty. Also check the cyl that shares that coil, the plug wire and the terminal at the coil. Also check to see if the plug itself isnt cracked (the white part)
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Old 06-18-2009, 07:29 PM   #7
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No coil. Coil packs. No wire, caps on the rail.
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Old 06-18-2009, 07:32 PM   #8
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what Jd said is a good idea. Swap an ignition wire and see if the code is set for another cyl. Also check the connection at the coil and see if the terminal is rusty. Also check the cyl that shares that coil, the plug wire and the terminal at the coil. Also check to see if the plug itself isnt cracked (the white part)
As Stevens243 said immediately above your post, the OP's TJ doesn't have ignition wires so they can't be swapped around. And there are three coils, each two cylinders sharing one of the three coils.

At this point, I'd swap two of the coils around to see if the problem follows the coil. If not, I'd be thinking about a compression check to make sure you don't have a valve problem.
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Old 06-18-2009, 08:52 PM   #9
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Between coil and plug
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Old 06-18-2009, 09:14 PM   #10
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Good call
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Old 06-18-2009, 09:49 PM   #11
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Well, thank you for all the input you guys. I will give some things a try and let you know how it goes. If i need anymore help you can count on hearing from me . Thanks again!!
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Old 06-18-2009, 11:49 PM   #12
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The entire coil rail is a sealed unit, sorry, individual coils can't be removed or replaced.

Symptoms, CEL on, low power, high HC.

Use your ear to start with - or somebody with good hearing. Listen close to the coil rail for a "snapping" sound. If there is one, replace the entire assembly. Dealers get about $225, CarQuest and Napa get around $150.

I wouldn't trust one from the Chinese stores, Krappen, AutoChina etc. They may be slightly cheaper, but all they sell is low quality junk you'll replace within a year.

Another way to check the coil assembly - from the bottom, use a laser type thermometer to measure the heat on the bottom of the pre-cats.

What I've found is, even though it may run OK, one cat's bottom end will read about 700 degrees, the other one about 200-300 degrees.

When an engine is running fine, BOTH will read about 450-500 degrees. That's not an official test, but so far all those I've tested with the uneven readings, the coil rail was bad.
I even keep a good test rail now, it's becoming more and more of a problem now that they have some miles on them.

Let us know --.
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Old 06-19-2009, 08:33 AM   #13
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The entire coil rail is a sealed unit, sorry, individual coils can't be removed or replaced.
Does that include all model years Rich? I could have sworn someone on a different forum swapped theirs around to isolate a bad coil.
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Old 06-19-2009, 02:15 PM   #14
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I can't say for sure, but ALL I've seen. Why would Jeep go to the expense of making 2 kinds?
Maybe they are talking about Pontiac or Fords? Some of those are interchangeable. Maybe a one-of-a kind Jeep prototype?

It sure would make things a bit, no alot, easier if they were interchangeable though.

Gotta qualify that - I'm talking I-6 4.0, '02 and up - or whenever they started using them.

Maybe 4 bangers are different? Dunno.

Or the V6? Haven't even looked at one - yet.

Even if they were interchangeable, I think the trouble is in the straps inside the rail, not the coils themselves. No way to determine that, just a hunch. If you could swap coils and the problem didn't change cylinders, what then? Internal rail insulation? An injector? Or the valve problem you've mentioned before?

So far I haven't heard of any way other than the temperature method to diagnose them, other than swapping parts. DRB's miss it, scopes don't see it (yes I tried,) because the coil is double ended and both ends fire at the same time - one is the "hot" spark, the other is a waste spark to burn unburned fuel. You can see the coil discharging fine - but to where?, the wrong cylinder?

In the "old days" we simply pulled a plug wire to test the insulation.
So far MSD hasn't been interested in making an aftermarket coil rail system with GOOD insulation - I'm sure it's too many ARB hurdles and too limited a market. Yes, I talked to them several times about it.
Others would never even understand - Accel would just paint it yellow and charge twice as much, LOL.
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Old 06-20-2009, 12:44 AM   #15
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Jerry - here's an interesting article I found. It's involving a misfire code, and cam and crank sensors and their codes.

The most interesting part is the patterns he shows - and the double firing when the cam and crank sensors weren't synchronized absolutely correctly. I haven't tried hooking up my scopes like he did.

It seems the more we know the less we understand!?
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Old 06-20-2009, 11:22 AM   #16
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Have to agree with JeepNJim; the easiest/ cheapest thing to check off the bat is the fuel injection.
Check the connector to #6. Reseat the connector, and try to clean up the two pins to ensure they make a good connection.
If that doesn't work, check the seating of the fuel injection rail.
I had a similar problem on a 99. Cleaned out the fuel injectors and reseated connectors and she runs like a charm. Not as complicated a task as it sounds.
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Old 06-20-2009, 12:02 PM   #17
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Jerry - here's an interesting article I found. It's involving a misfire code, and cam and crank sensors and their codes.

The most interesting part is the patterns he shows - and the double firing when the cam and crank sensors weren't synchronized absolutely correctly. I haven't tried hooking up my scopes like he did.

It seems the more we know the less we understand!?
Post it up Rich, I'm anxious to read what you found.
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Old 06-20-2009, 05:30 PM   #18
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Darn, I must be getting too old. I thought I put up the link, but obviously not!
And to make things worse, I didn't book mark it.

I'll try to find it.

Duh!
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Old 06-20-2009, 05:36 PM   #19
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Found it!

http://motorage.search-autoparts.com...30/article.pdf

Fortunately I remembered the words I used in the search, it was still highlighted.
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Old 06-21-2009, 10:21 AM   #20
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That was a pretty cool article, thanks for posting it Rich! However, I'll have to look more closely at the camshaft position sensor to figure out his toothpick alignment method since I've never had to align it. It's too bad you probably can't get a collection of 4.0L trouble articles like that one, they'd be really educational and cool to read. I liked that Motorage test at the end too but was bummed when it didn't include the correct answers at the end to verify your answers.

Thanks again for posting it!
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Old 07-16-2009, 10:13 PM   #21
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i am having the same problem with my 98 I-6.. did the hole "tune up" thing with plugs, wires, cap, rotor and she ran fine for a month.. now the code is back.. so with what i have read you guy say to check the coil, injector, or the o2 sensor next? is this about right?
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Old 07-17-2009, 01:28 AM   #22
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This may not be the trouble - but I've noticed using the wrong plugs can set that code. Use what the underhood label calls out, gap it like it says.

When installing them, use a torque wrench to specs. Reason is setting it to the proper torque also positions the ground electrode to the least offensive position in the chamber. Believe it or not, when wrong plugs are used, or the wrong torque, that ground electrode makes a "shadow" for the firing/burning inside the chamber. That "shadow" can cause misfire, excessive emissions, carbon build-up, ping, even rob some of the power - and may even set codes - (dunno for sure on the codes.)
Torquing to spec positions it in the right spot. (And, always use a dab of Anti-Seize on the plug threads.)

When building race engines we always marked and indexed the plugs to get them in the right position - it could make the difference between winning and disgrace.

Also - on the later 4.0 TJ's with the rail type ignitions - make sure you use a good amount of Silicone Dielectric insulating grease on the plug boots. About the same amount you'd use toothpaste on your toothbrush, not just a tiny dab. Seems the boots like to let the spark jump out to the head - causing misfire and codes. The insulating grease helps keep it contained.
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Old 07-17-2009, 03:35 AM   #23
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Could moisture cause the problem? Mine did the same thing on #5. Like the OP, I cleared the code and it runs fine.
I also appreciate the tips so far, thanks.
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Old 07-17-2009, 10:35 AM   #24
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If the spark is jumping out past or through the boot - then yes, moisture could help it jump.
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Old 07-18-2009, 07:19 AM   #25
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If the spark is jumping out past or through the boot - then yes, moisture could help it jump.
Is the boot replaceable? Drove it 150 miles and up to 8500 feet yesterday. Ran fine, I just don't trust it yet.

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