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P0306 , I'm losing my mind.

6K views 7 replies 6 participants last post by  Dobber91 
#1 ·
I've read all 49 threads that pop up when searching for p0306 in the tj section. I have a 2001 4.0 manual with about 225000 miles. I bought the jeep with about 220000 about 2 years ago. It was a very clean 1 owner jeep for the miles but had some cylinder misfires when I bought it. Come to find out it had a leaking valve and cracked head. I put a rebuilt head on and the problem went away for a while. Now I have a cylinder 6 misfire popping up. It seems to run okay for a while, then my CEL starts flashing for a minute then just stays on. I've cleared codes several times, but it keeps coming back.

Things I've done and changed
-Completely Cleaned Intake
-Replaced IAC
-Replaced TPS
-Replaced Coil Pack
-Replaced Spark plugs with Champion 985's gapped at .035
-Checked electrical plugs and connections
-Swapped spark plugs between cylinder 6 and 1 to verify I didn't have a bad one
-Swapped injectors between cylinder 6 and 1 to eliminate a bad injector, changed the orings when I put the new head on
-Had a smoke test done recently for an evap leak, fixed the vacuum leak
-Tested fuel pressure and check valve, it checked out good

That's all I can think of for now. Do you think a bad O2 sensor or CKP could cause a misfire? I'm open to suggestions. Thank you for reading and for all the other good info I've picked up from this forum.
 
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#3 ·
A crank sensor would be more likely be causing a random misfire, not always #6. An O'2 sensor would be the same thing as fuel trim affects every cylinder equally. You've covered the basics and aren't pulling any circuit codes, so I'm leaning toward a mechanical issue. Next would be compression and leak down tests.
 
#5 ·
Does your scan tool read live scan data?

Can you report the fuel trims data when it is running rough and/or list the freeze frame data for the P0306?

Do you have a vacuum gauge?
Have you checked for a vacuum leak on the #6 intake runner? Exhaust leaks?
Was the #6 spark plug whiter or blacker or different than the other spark plugs?
 
#6 ·
I like the idea of good compression or better yet leak down on all six cylinders for comparison

One other possibility is damaged intermittent injector or harness to just that injector

Easiest way to eliminate that is make a new one injector harness to hook to just suspect cylinder then hot for new harness to bat plus via fuse and ground to that injector wire back at PCM plug and while there make sure the PCM plug is clean and tight

That is the only single cylinder things you have not tried that I can think of
 
#7 ·
Thanks for ideas. I don't have a vacuum gauge and my code reader won't let me see freeze frame or live data. I put the new head on a few thousand miles ago because one if my valves was leaking and I had a crack between the 3 and 4 cylinders. I'm heading to Colorado Sunday so I took it to a mechanic this morning, hopefully they can give me an answer pretty quick. I'll keep y'all posted.
 
#8 ·
Well the mechanic called and said one of the valves on cylinder 6 was acting up. I guess he put a gauge on it while it was running and said it just bobbled all over the place.
 
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