P0301 Cylinder mifire and other issue Solved *Updated*
 

Posting this to help others

My ride:
2007 JKU with a 3" lift riding on 35's with a 3.8 Liter and a 6 speed manual.

Symptom(s):
Cylinder 1 misfire code
Light stuttering when first started
Lag and cutout at 3k Rpm's and above
Random engine shutdown, always able to start back up after a key cycle
Seemed to get better when warm

Testing:
Change #1 wire, no improvement
Changed #1 plug from NGK iradium to and AC Delco Iradium that I had, big improvement. Lag and cutout and 3k and above nearly gone but some issues yet at idle when cold.
Changed back to the old wire, better yet.
Installed a NEW Iradium plug, issue back in full force. Jeep no longer likes NGK plug for some reason??? Odd
Checked the intake plenum bolt in even of air leak, bolts pretty loose so I torqued them. Issue slightly better but not good enough for me.
Swapped the coil side wires only from 1 and 4 (They fire at the same time) Now the code is pointing to #4 meaning #1 is the issue due to the other end of the wire still being on #1
Moved wires back and removed the NGK and went back to the new Delco plug, Major improvement and hardly and issue at idle but still randomly killing at any speed.
Went out and beat it for a while and came back and checked codes, nothing new.
Picked up a Crank sensor as I know then can fail without tossing codes, No random dying for a couple days. Will update further if it comes back.

My conclusion:
Most of the issue seems to be tied to the NGK plugs, slight air intake plenum leak, and a failing crank sensor with 170,000 miles on it. At this point I am unsure why it became allergic to the NGK plug as lack of compression nor coolant seem to be at play here, I can only think the Coil may be getting a little weak and causing the NGK plugs not to fire correctly? Unless there is another intake leak I am not able to find near #1.


Anyhow I Just wanted to slap this out here to possibly help others with similar issues. Peace out and have a good weekend :beer:

Glad to hear you got it fixed.
Lots of people have had misfire trouble with the NGK plugs in the 3.8 engine.

At one point you said cam sensor. Then you said crank sensor. Did you change one, or both?

Ah nice catch, I only changed the crank sensor 😉

***UPDATE***

Still had a couple of odd idle fall outs and slight stumble with the ac on so I started to investigate further after it died a few times while at idle (With the ETC and traction control lights on). Here is what I found.

The rear bolt for the pipe coming from the intake to the EGR had let go or someone stripped it sometime before I owned it and the flange was open a couple MM

How I fixed it: the bolt holes are tapped into aluminum and so I got a self tapping thread type bolt that was slightly larger and courser thread than the original and carefully ran that into the old threads and the bolt grabbed and cut new threads nicely and tightened up great.

Next I was poking around looking for other leaks and found the breather hose going from the rh valve cover to the air box was laying off down by the manifold but I am pretty sure the fact I was laying over this area when fixing the EGR bolt I pushed it off, damn lifted Jeeps :)

Anyway I hope this information helps someone with issues like mine, I don't think this JK has ever ran so smooth as it does now on idle and going through the gears and my MPG has improved a bit as well. Thanks for reading :wave:

If you want something to work on all the time, buy a Jeep

True story some days haha, I'll take my 4.0 back in my TJ any day.

As an FYI, most of us stick with the stock Champion plugs and OEM Mopar wires. The days of hotter wires and hotter plugs are gone with computer controlled ignitions where those "performance" wires/plugs often cause problems.

*Update*
I hate when no one updates a repair post so I thought I would update this after a month that it is still running strong with no misfires or stalling.
Thanks