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Old 03-23-2015, 06:42 PM
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misfiring problems

I'm sure this has been discussed before so i'll apologize in advance. I have a 2002 tj with a 4.0 and gettin p0300, 301 and 306 misfire codes. Ive changed all spark plugs and boots, rotated injectors, checked fuel pressure and tested the coil (wich checked out fine). Still gettin codes so hopin someone could give me some insight on what to check next?

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Old 03-24-2015, 08:45 AM   #2
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On my 2001, I had that issue a few years back now and it turned out to be my PCM. Luckily a friend is a great Jeep mechanic (only works on Jeeps) and figured that out quickly, got a PCM from a junk yard and he replaced it for me and set it all up. Haven't had an issue since. Since you pretty much changed everything else, thats all I can think it could be at this point.

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Old 03-24-2015, 05:38 PM
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So if i get a junkyard pcm can i just plug it in and go or do i have to have it flashed?
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Old 03-24-2015, 06:26 PM   #4
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PCMs are year specific as well as different auto vs standard

At least for early years calif PCM are different than federal emissions PCM

You may also have odometer issue if you swap PCM

Finally if PCM came from a wrangler with sentry key you need at least one key preferably two it has already learned and the reader module in your jeep or it will not function
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Old 03-24-2015, 06:57 PM
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Ok thanks digger, i figured it was year specific but didnt consider the odometer issues. I dont have a sentry key so that wont be an issue.

But is the pcm my next best option or is their something else it could be that i havn't tried yet?
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Old 03-24-2015, 07:09 PM   #6
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have you thought about getting it checked out by a professional? usually for about one hour labor they will tell you what is wrong. if you guess one time and you are wrong you have most likely spent that hours labor.
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Old 03-24-2015, 07:10 PM   #7
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But if the junk yard PCM you buy had the sentry key feature activated by seeing three starts in prior vehicle with sentry key it will never function unless it sees its previously memorized chip key code
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Old 03-24-2015, 08:13 PM
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Quote:
Originally Posted by mdop22 View Post
have you thought about getting it checked out by a professional? usually for about one hour labor they will tell you what is wrong. if you guess one time and you are wrong you have most likely spent that hours labor.
So far ive been wrong 4 times and only spent about 40 bucks. Im actually an aircraft mechanic so i have a hard time sending anything to a shop when i know i can fix it, its just a matter of diagnosing it.

Speaking of diagnostics, is there any way to test the pcm to know for sure if thats it?
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Old 03-24-2015, 08:15 PM   #9
Knows a couple things...

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My '04 TJ's misfires & CELs were caused by a bad pre-catalytic converter causing the 02 sensor to misreport the true air-fuel mixture. The previous owner tried changing the 02 sensors which didn't help.

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