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Old 09-01-2010, 01:22 PM   #1
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Misfire question

Hi guys, I'd like some input-mainly to validate my own conclusion before I start buying parts.

Last week I was driving home from work-70ish on the interstate and all of a sudden she (2002 Wrangler-4.0l) fell on her nose and started misfiring-vibrating like crazy under throttle and the the check engine light started flashing. I slowed down to about 55 and she was better but obviously still missing. I was meeting my parents for dinner so I checked the codes and pulled a 300 (misfire detected-duh), 301-cylinder #1 misfiring and 304-cylinder #4 misfiring. When I came out of the restaurant I wanted my dad to listen to her. Started up and ran smooth as a kitten purring. I periodically rechecked the codes over the next few days and they were still there but I hadn't felt the engine misfire. So I chocked it up to moisture since I had cleaned up the engine the day before at the car wash.

But its never that easy. Coming to work this morning it misfired a few times. Nothing as severe as the first time but I could feel the occasional stutter on the interstate. I'm thinking coil rail instead of injectors since cylinders #1 and #4 are companion cylinders and share a coil pack. Unfortunately it looks like I have to buy the whole rail instead of just one coil pack.
So aside from changing the plugs while I've got it apart does anyone have another suggestion or does this sound on target?

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Old 09-01-2010, 01:33 PM   #2
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This might sound too simple but check your battery and connections. A weak battery will cause an engine to run rough. If your battery is almost shot the alternator isn't enough to keep the engine running smoothly.

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Old 09-01-2010, 01:38 PM   #3
Knows a couple things...

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Originally Posted by pokey View Post
This might sound too simple but check your battery and connections. A weak battery will cause an engine to run rough. If your battery is almost shot the alternator isn't enough to keep the engine running smoothly.
X2. The TJ's alternator won't put ANY power out if the battery is dead, and it will put out weak power if the battery is weak. Without a good battery, the engine either won't run or it will run badly depending on the battery's condition.

Yes we know that SOME vehicle alternators and all generators will run fine with a dead or weak battery, just not the type of alternator used in the Wrangler.

If it's not the battery or a loose/dirty/corroded battery connection (no, a visual check is not good enough to tell if it's ok), then odds are fairly good that either the crankshaft position sensor or camshaft position sensor has gone bad.
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Old 09-01-2010, 02:16 PM   #4
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Always 1 & 4 - companion cylinders, not other cylinders?
Just washed it down.

Remove the coil rail - look closely at the boots and plugs on those 2 for signs of dirt. Clean as needed.
Reassemble with a touch of Silicone Dielectric grease on the boots.

And - have a friend with good hearing listen carefully for the telltale "snap" "snap" of the coil breaking down.

If it starts misfiring constantly, check underneath with an infrared thermometer on those 2 cylinders. One will be too hot, the other too cold - compared to the other cylinders.
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Old 09-01-2010, 03:18 PM   #5
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Battery was replaced in June. Crank and cam position sensors have both been replaced within the year. Both sensors threw codes specific to them and when the battery went your right it wouldn't do anything. I'll pull the rail and inspect as recommended. Thanks.
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