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P0301 Cylinder mifire and other issue Solved

Old 07-12-2019, 09:49 AM
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Default P0301 Cylinder mifire and other issue Solved

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.

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

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

Last edited by HostileJK; 07-12-2019 at 10:53 AM. Reason: Wrong word
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Old 07-12-2019, 10:43 AM
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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?
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Old 07-12-2019, 10:55 AM
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Ah nice catch, I only changed the crank sensor 😉
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