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Unread 06-01-2013, 01:49 PM   #1
fordguy101
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Join Date: Feb 2013
Location: manchester, TN
Posts: 25
3.7 v6 2005 jeep liberty

hi, ive been having issues with my jeep misfiring and jerking around 45-55 rpm or it will shake bad at stop sign,light other then that its a smooth ride(unless u smash the gas)....now ive had it to the dealer wanted to change the pcm or chase wires...got the codes so figured i do the small stuff myself...spark plugs,coils,cleaned tb, changed tps, changed cam,crank sensor...no dice...so finally broke down too it to the shop....had it half a day and said it was the pcm they scanned it and got no feed back on port 4 which goes to cylinder 4 ding ding we got our misfire problem....


now i read that the pcm rarely goes bad the code we are getting is p0304 anyone have any luck with this? and could it very well be the computer causing all this?

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Unread 06-02-2013, 01:21 AM   #2
AVR2
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1993 ZJ 
 
Join Date: Apr 2007
Location: UK
Posts: 2,193
Well, that code is simply "misfire cylinder 4", which could be caused by any number of things. Assuming you've replaced the most likely causes - plug and coil - and you've also changed all those sensors, and it's still misfiring, then you have to start looking at the wiring or the PCM. Given that the PCM controls the coil packs and the injectors, then yes, it could be the cause.
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Unread 06-02-2013, 08:20 AM   #3
streetglideok
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2005 KJ Liberty 
 
Join Date: Jul 2011
Location: Colorado Springs, Colorado
Posts: 2,344
PCM/ECMs rarely go bad, however they do once in a while kick the bucket. I would venture to say most of them are replaced due to poor diagnostic abilities, only a portion of the ones replaced are really bad.

That said, a good shop should be able to easily pinpoint the cause of your misfire. They should verify there is power to the ignition coil, and that the pcm can ground and unground it causing it to spark, and there should be power to the injector, and that the pcm is grounding the injector to make it spray fuel. If those things are good, then they should make sure that the injector and coil works properly, the plug is good, there is compression and valvetrain is sound. However if there is an issue with the pcm controlling the coil or injector, they should check for operation at the pcm connector. If that checks fine, the pcm isn't your problem, its wiring or a connector. If you still have an issue at the pcm, then the proper thing to do before condemning the module is scope the cam and crank sensors, make sure their patterns are correct and in synch. 1-2 hours to verify this. Whether they actually do this or not, is another story. There was a horror story on the jeep kj page on facebook recently of a shop throwing everything at it when they thought it was a similar issue. Turned out to be a loose valve seat.
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Unread 06-03-2013, 08:13 AM   #4
fordguy101
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ouch...yes ive had plugs,coils done...did all 6 of each...still problem...ive had it to dealer then a friends shop who i know does good work....to both point me to the pcm saying port 4 wasnt getting anything to it...i noticed i have a check enginge light..erase codes or reset battery as soon as u start it engine light back like it never got erased in the first place
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Unread 06-03-2013, 09:47 PM   #5
fordguy101
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well buddy has a junk yard and had a spare pcm for a jeep liberty laying around...threw it in to try it out(i know needs to be reprogramed and fired right up no misfire sounded good til it died haha...anyways thanks for all the help looks like ill be replacing the pcm anyone know what dealer charges?
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Unread 06-07-2013, 10:25 AM   #6
fordguy101
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all fixed ordered a plug and play pcm threw it in....no problems since (knocks on wood) thanks for all the info and help
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