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Old 11-30-2010, 08:08 PM   #1
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misfire codes runs badly

OK I am having problems with my 2004 Rubicon it's a 5 speed if it helps. I keep getting misfire codes #1 cylinder and sometimes for others as well p301 it starts running vary badly. Dealer I got it from just keeps doing a carbon cleaning but this keeps coming back I really like the Jeep and have always wanted one but right now I just want to drive it off a cliff. I have been thinking maybe it needs injectors or a coil pack but it looks pretty new or possibly a crank position sensor not sure what to do. Thanks for your help

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Old 11-30-2010, 08:37 PM   #2
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If the crank sensor was bad it would quit running not miss I would lean toward a coil pack If you had a pair of insulated plug wire pliers you pull plug wires off of the coil pack when it was acting up and see if there's no spark then you will know.

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Old 11-30-2010, 09:07 PM   #3
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It's got the coil rail so there isn't any plug wires just the short boots.
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Old 12-01-2010, 05:58 AM   #4
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It was running so bad on the way home this morning that I stopped and ran a can of throttlebody cleaner thru it and its now running fine except the CEL is still on.
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Old 12-01-2010, 06:01 AM   #5
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Maybe it was a dirty IAC valve which you cleaned with the throttle body cleaner. Delete the codes and the cel will go off.
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Old 12-01-2010, 10:47 AM   #6
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The coil rail is hard to diagnose without just swapping it out. If the coil rail is shorting internally, it affects 2 companion cylinders. It still runs fairly smooth but it lacks power.

When they first start to fail, sometimes it only lacks power under heavy acceleration, but it soon lacks power at all loads. But there are a couple of ways to test it without swapping it out.

1. Have someone with good hearing stand next to it, hood open, while you rev it - snap it. If they hear a snapping - spark jumping somewhere, replace the rail - it's shorting internally.
That still doesn't mean it's good if they don't hear anything.

2. Next, use a laser thermometer (harbor freight $29), run it at 2000 or so for 3 or 4 minutes, then go underneath with the thermometer.
Measure the temp of the pre cats. They should both be about the same temp - around 300 degrees. If the coil rail has been shorting it's been dumping raw fuel into the cat - the cat will run very hot - sometimes in excess of 700 degrees. The other cat will probably be cooler than normal, 200-250 degrees. If yes, replace coil rail.

And then there's always just trying a new one.

If you have to buy one, pay that little extra for a good one. NAPA, CarQuest sell rails made by Standard Ignition - decent quality - same mfgr the factory uses.
But there are cheap knock-offs - Chinese trash - Autozone and the like sell them.
You'll save around $30 over a real one.
If you count your savings you'll really save money on them, every time you buy one you'll save $30 - in a years time you'll save at least $90! They don't last very long.

If you buy quality and pay that extra $30, they'll last years, it will only cost that extra once. (I wish the government understood that principle.)
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Old 12-01-2010, 03:18 PM   #7
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going to try to get a new coil rail this weekend and see if that fixes it. Thanks for your help guys
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Old 12-03-2010, 07:26 PM   #8
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Thanks for your help again. Today I put a new coilrail on from carquest and its idling better than it ever has and if feels like it has about 40 more horsepower. Thanks again.
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Old 12-20-2010, 06:42 PM   #9
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Misfire code P0301

I had a similar issue with my 2002 Wrangler SE, 2.5L and all I had to do was replace the spark plugs.

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