Misfire mystery with my 2005 Jeep Liberty

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Malicesin

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Hello!

First off, i'm sorry if I am posting in the wrong area. I have a 2005 Jeep liberty Renegade and it's been a great vehicle until recent. I'm getting P0306 CEL and at a loss on how to proceed. I've done all the scheduled maintenance on this vehicle since ~65,000 miles on the odometer and today she reads 140,000. About 8 months ago, when coming home the check engine light came on solid. I used both the dash trick to read the code and I have a little cheap scanner tool too and it showed P0306. I decided first off let's just push up the next maintenance interval and include plugs and ignition coils since the ones I have are OEM from factory (Note that all parts that I have used on this Jeep are only Mopar and sometimes Napa/gates items). I started with the plugs and replaced with NGK and gaped them according to the emission stick, replaced ignition coils, oil change, PCV, and coolant and radiator (Decided to get new mopar radiator since I did an order anyways and mine had a small leak).

After completion, I reset the CEL with my scan tool and within 2 weeks the light came back on again with P0306. I then tried using exclusively 91 octane as the owner manual states hot environments warrant using 91. After a few more weeks, the CEL would come back and I'd reset it only to have to come back. I then replaced the new installed plugs with newer ones thinking MAYBE something was wrong with them, I also did the same thing for the Ignition coils and returned them and got news ones, still after a few more weeks the light always comes back.

At this point, I knew I had to contact a mechanic as they have more advance diagnostic equipment than I do. I've taken the jeep now to 3 different mechanics INCLUDING the dealership and no one can tell me whats wrong with it. The dealership suggested a new PCM, I don't see how this would fix it and I told them that there is a code specific for a bad PCM. They ended up giving me a receipt with all the tests they did and it states nothing is wrong even though the light is still on. The other 2 were independent mechanics and I paid their diagnostic fee's and they also said they couldn't find whats wrong with it. $250 for dealership diag, $150 diag fee from 1 mechanic and $100 diag fee from the other.

There is no performance lose nor does the engine sound abnormal. No clicking or loud banging that would make valve train a suspected. MPG seems a little worse, like I think I used to average 18 mpg and now get 16 mpg.


I'm not trying to get this fixed for free or anything like that, I just want my baby happy and fixed. If you know of a good mechanic or shop I can take it to, please let me know.
 

BillyG

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I had a multiple cylinder misfire code and it turned out to be a bad upstream O2 sensor. If you can check the voltage output of each sensor and see if there is a problem.
 

duderz7

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I don't remember code exactly, but I once had a missfire code, but felt like it was a fluke due to condition of the components involved. I put a can or 2 of berrymans or maybe it was Lucas fuel system cleaner in the tank and half a tank later light went out. That was years ago. Mine was a semi clogged I injector I suspect, maybe yours is too?
 

LibertyTC

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Chevron techron concentrate is also a very good fuel injector cleaner.
I put 2 bottles in to a full tank and take the jeep for a good long highway boogie, try that out with a high octane fuel.
 

uss2defiant

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I think even if you had new coils installed, I would try switching them with a different one to see if the problem follows.
 

Malicesin

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Yeah not only were all new ignition coils installed, I've already swapped them to different cylinders and still have P0306. I've asked all the mechanics to measure the primary and secondary voltage of the coils and they all said, they don't do that as "it's not necessary", this includes the dealership.
 

daves06lrenegade

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140k Miles on the original fuel injectors may cause a P0306 code... The misfire can come from a bad coil pack, a bad spark plug, or a bad fuel injector... Injectors can and do become partially plugged or mechanically stick causing too little fuel to that specific cylinder (Cyl #6).... little or no fuel will cause the P0306... Because it comes and goes I don't think it is a valve problem but a compression check can rule this out...

The suggestion to use a valve/injector cleaner like Chevron Techron is a good one but remember that it may take a couple tanks of gas to remove the deposits...
Dave
 

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