P0301, P0302, P0303, P0304, P0305, P0306 (the dreaded misfire) and how I solved it

Since this forum and thread in particular was instrumental in solving my own misfire issue, I thought I would post on my experience and what I went through. First, I recently bought a 2005 LJ with a 6-speed tranny with about 150k miles. Shortly after buying it (as it always happens) my check engine light came on, read the codes and it was a p0306 code. So I thought a misfire, how hard could this be to fix? My Jeep started laughing at me I think, especially as I began down the path of figuring it all out. My approach was start small and simple, then go from there. Mostly I followed this thread as directed and tried everything.

  • So I ran to the store and bought new plugs after reading this forum and got the Autolite XP-985 plugs. Gapped them and installed them. Still misfiring....
  • Ran back to the store and bought a coil rail (now I realize that a MOPAR one may be the way to go) but I just bought one off the shelf. Still misfiring.... Then I reinstalled the factory MOPAR one when the new one didn't work. Still misfiring...
  • Rented a NOID light kit and tested the injectors, seemed normal, but swapped #6 with #1 anyway. Still misfiring....
  • Performed a compression test, all cylinders tested normal and equal in value. Still misfiring.....
  • Tested for manifold gasket leaks with both propane and spray cleaner, found none. Still misfiring....
  • Cleaned the throttle body and IAC valve, some crap in there but it really wasn't that bad. Still misfiring....
  • At this point, I am thinking it must be something either very random or serious. So instead of throwing parts at it, I took it to a friend's shop to take a look with all the special tools and such. He retested everything I had done and found no problems with the above tests that I did. Then he used a dial indicator and found that the exhaust lobe on the cam was worn down significantly. Problem solved! Or so we thought, note Jeep is still laughing at me. So I had the shop install the new camshaft and check the head and everything. The cam was definitely worn down and needed to be replaced. This also accomplishes replacing the intake/exhaust manifold gasket which is on the list. So after the cam was replaced it did seem to run better and smoother. But you guessed it STILL MISFIRING....
  • So then we replaced the crankshaft and camshaft position sensors. Still misfiring....
  • At this point the shop has given up and I thought I'd bring it back and keep trying things. Cause all this while the Jeep ran fairly well. But still misfiring....
  • I was talking to my neighbor one day, and telling him all about my woes of trying to fix a "simple" misfire. He mentions that he has a nephew who works for Chrysler and has been involved with Jeep engineering over the years. The nephew's son is also training to be a mechanic at this time. So my neighbor said he could put me in touch with them and see if they could help me. Their suggestions came back to look at all of the above things. But also to check all the wiring harness and connections to make sure there is no shorts or anything. And I had done that and all seemed normal to me. But I rechecked again and everything still seemed normal.
  • But it made me think with all the PCM issues on these years of Jeeps, could it be that? These jeeps do have an individual circuit on the PCM for each injector, so it would be possible that particular circuit went bad. Checked and my Jeep has a factory refurbished PCM on it already. So I am now thinking at this point of dropping the money to get a new PCM from Wrangler Fix. But as I am twiddling my thumbs on that one....
  • I got new H4 headlights and was cleaning the plugs off with electric connection cleaner and thought maybe I should hit the PCM connections. So I did, let them dry out. Plugged it back in and haven't had a misfire since that point!!!!! That was about 3-4 weeks ago and I have been driving the Jeep a fair amount to try and feel more confident that it is actually fixed. Now my Jeep is happy and content and purring like a kitten. Now my family and I can enjoy the Jeep as it was intended, out and about exploring this great open world we live in. And in between those times with my family, I can spend countless hours addressing all the other little issues I find or want to improve upon...
Hopefully my story will help someone with their random misfire issues in the future. Thanks to everyone here that shares their expertise and help so that I could figure out the problem.
 
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Hopefully my story will help someone with their random misfire issues in the future. Thanks to everyone here that shares their expertise and help so that I could figure out the problem.

That's quite the story, and quite the first post. Thank YOU for sharing that. It's posts like this that end up becoming very helpful to others in the future, so thanks again (y)
 
That's quite the story, and quite the first post. Thank YOU for sharing that. It's posts like this that end up becoming very helpful to others in the future, so thanks again (y)
Yeah I thought it might be helpful for someone some day. When I went through my process I got to a point that it was going to be a really random thing that fixed the problem. Which turns out it was. Jeep is still running strong still by the way with no more misfires.
 
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So my 2004 LJ threw a check engine light on the way home from Denver. I stopped by a Napa in Moab and they let me borrow the code checker. It was P0303
I read about the rough idle after a hot soak and I did notice a rough idle a couple times early in the trip, the temp was mid 90s.
I'm home now and I found this detailed thread. It appears that my Jeep already has the Mopar Heat shield installed. I'm wondering if I should buy the $10 wire cover, or I could probably use any kind/brand of heat protection.
I did notice that the #3 injector was twisted putting the wires closer to the heat shield. I have it pointing straight up now (like the other 5). I might wait a couple of weeks and see if the problem reoccurs.
Here are 2 pics showing the original position of the injector. Also, just to confirm, does it matter if I twist them? They seem to move easily.

IMG_7800 (Medium).JPG


IMG_7801 (Medium).JPG
 
@Nightmare9 , The heat soak is going to be related to heat on the injector itself not the wiring. If it comes back I would start with swapping plugs 3 and 4 and injectors 2 and 3 and see if the misfire stays at 3 or moves to 2 (injector) or 4 (plug).
 
I threw P0301 this morning. Solid CEL.

I’ll work through the trouble shooting laid out here (thanks everyone) but in the meantime would there be any risk in still driving the Jeep? From what I’m seeing as long as I don’t go blinky I shouldn’t be causing any damage but wanted to verify.
 
I threw P0301 this morning. Solid CEL.

I’ll work through the trouble shooting laid out here (thanks everyone) but in the meantime would there be any risk in still driving the Jeep? From what I’m seeing as long as I don’t go blinky I shouldn’t be causing any damage but wanted to verify.

It really depends on whether you have a constant misfire or only a low RPM misfire. You could be dumping fuel in that cylinder and not igniting it so it's just pushed out the exhaust which would be bad for your cats. FYI, my son-in-law has been running with a misfire for 2 years now.
 
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At some point all 2000+ TJ 4.0 owners will have a misfire CEL. They can be very difficult to troubleshoot and expensive to just throw parts at.

Mine was a cylinder #1 misfire, but this would work with any single cylinder misfire. Random misfires (P0300) would be troubleshot differently. I hope this helps others.

Note: A P0303 may be caused by a Heat Soak issued which Jeep put out TSB 18-031-03 on it. Basically all you need to do is wrap a piece of insulation around the #3 injector and zip tie it on. The Jeep TSB calls for this product to fix the problem: https://www.amazon.com/gp/product/B007O3QHDK/?tag=wranglerorg-20

*********************************​

I've been having a p0301 misfire for almost two tears. Never random or any other cylinder. But since my California registration is due and this is my year to pass smog, I figured that I better work on it ... I had until October, 20 to get everything good. Here's how I troubleshot and solved the issue:

1) Since this is a waste spark system (2 cylinders fire at the same time) and only cylinder #1 ever failed, I ruled out the Coil pack. I guess it could have been the #1 boot, but it looked good.

2) I pulled the spark plugs from #1 & 2 (#1 looked good), checked the gap and swapped them.

Drove the Jeep and after about 150 miles the code popped again. It had a pending misfire at about 50 miles, but at around 150 it went hard. Checked the monitors and Catalyst and Evap were not ready. Note: In California, the Evap monitor does not need to be ready to pass the smog test.

3) Swapped injectors #1 and #2.

Drove Jeep with same result as above.

4) Used a NOID Light to check injector signal to rule out the computer and injector wiring. All good.

5) Performed a dry compression tests on all cylinders, #1 was 125, about 10 below the next lowest, #3 ... A wet test yielded about the same results. Not enough for me to call it a ring problem.

6) Performed a leak down test to all cylinders, #1 leaked just a hair faster than the others, again not enough to call it bad.

7) Replaced both valve springs for cylinder #1.

Drove Jeep with same result as above.

8) Replaced the intake/exhaust manifold gasket, maybe it had a leak I wasn't detecting. Note: these are MUCH MUCH easier to change if you spend 30 minutes and remove the front fender.

Drove Jeep with same results as above.

Analyzed past troubleshooting and determined that the problem must be in the head ... valve or head gasket.

Removed the cylinder head and noticed a small burn on the cylinder #1 exhaust valve. My guess would be that once the valve rotates onto that area it reports a pending misfire code and once it rotates there again it pops a hard code.

Since I had the 0331 cylinder head that was prone to cracking, I decided to replace it with a re-manufactured TUPY head. Total job completed by myself —- about 6 hours.

Drove Jeep for 10 miles and had ALL monitors (except Evap) went ready. Immediately went to the smog shop where it passed with flying colors. Yeah!!!

Since the successful smog test I have driven about 2000 miles with ZERO problems. In fact the engine runs much smoother now.

Relief!!!
Thanks. Excellent write up.
 
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At some point all 2000+ TJ 4.0 owners will have a misfire CEL. They can be very difficult to troubleshoot and expensive to just throw parts at.

Mine was a cylinder #1 misfire, but this would work with any single cylinder misfire. Random misfires (P0300) would be troubleshot differently. I hope this helps others.

Note: A P0303 may be caused by a Heat Soak issued which Jeep put out TSB 18-031-03 on it. Basically all you need to do is wrap a piece of insulation around the #3 injector and zip tie it on. The Jeep TSB calls for this product to fix the problem: https://www.amazon.com/gp/product/B007O3QHDK/?tag=wranglerorg-20

*********************************​

I've been having a p0301 misfire for almost two tears. Never random or any other cylinder. But since my California registration is due and this is my year to pass smog, I figured that I better work on it ... I had until October, 20 to get everything good. Here's how I troubleshot and solved the issue:

1) Since this is a waste spark system (2 cylinders fire at the same time) and only cylinder #1 ever failed, I ruled out the Coil pack. I guess it could have been the #1 boot, but it looked good.

2) I pulled the spark plugs from #1 & 2 (#1 looked good), checked the gap and swapped them.

Drove the Jeep and after about 150 miles the code popped again. It had a pending misfire at about 50 miles, but at around 150 it went hard. Checked the monitors and Catalyst and Evap were not ready. Note: In California, the Evap monitor does not need to be ready to pass the smog test.

3) Swapped injectors #1 and #2.

Drove Jeep with same result as above.

4) Used a NOID Light to check injector signal to rule out the computer and injector wiring. All good.

5) Performed a dry compression tests on all cylinders, #1 was 125, about 10 below the next lowest, #3 ... A wet test yielded about the same results. Not enough for me to call it a ring problem.

6) Performed a leak down test to all cylinders, #1 leaked just a hair faster than the others, again not enough to call it bad.

7) Replaced both valve springs for cylinder #1.

Drove Jeep with same result as above.

8) Replaced the intake/exhaust manifold gasket, maybe it had a leak I wasn't detecting. Note: these are MUCH MUCH easier to change if you spend 30 minutes and remove the front fender.

Drove Jeep with same results as above.

Analyzed past troubleshooting and determined that the problem must be in the head ... valve or head gasket.

Removed the cylinder head and noticed a small burn on the cylinder #1 exhaust valve. My guess would be that once the valve rotates onto that area it reports a pending misfire code and once it rotates there again it pops a hard code.

Since I had the 0331 cylinder head that was prone to cracking, I decided to replace it with a re-manufactured TUPY head. Total job completed by myself —- about 6 hours.

Drove Jeep for 10 miles and had ALL monitors (except Evap) went ready. Immediately went to the smog shop where it passed with flying colors. Yeah!!!

Since the successful smog test I have driven about 2000 miles with ZERO problems. In fact the engine runs much smoother now.

Relief!!!
 
My P0303. When my 04 shoots me this code it coincides with a large misfire and limp mode. Has happened 3x recently. After first time i replaced 6 spark plugs that were sorta white like running lean and a bit over gapped but otherwise looked good. All 3 events happened when on the highway, climbing a hill at 70+ MPH and under load (not towing) just heavy in the throttle.
Just insulated injector 3.
engine is 184k
Any suggestions?
 
My P0303. When my 04 shoots me this code it coincides with a large misfire and limp mode. Has happened 3x recently. After first time i replaced 6 spark plugs that were sorta white like running lean and a bit over gapped but otherwise looked good. All 3 events happened when on the highway, climbing a hill at 70+ MPH and under load (not towing) just heavy in the throttle.
Just insulated injector 3.
engine is 184k
Any suggestions?
Is it always cylinder #3? Of it is and it's not heat soak them follow the troubleshooting steps in the first post before throwing parts t it.
 
Is it always cylinder #3? Of it is and it's not heat soak them follow the troubleshooting steps in the first post before throwing parts t it.
Gave it hell 67 miles home today with the new heat shield and no codes. One time it was 1 and 3 that was the first time. Always 3 since
 
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Gave it hell 67 miles home today with the new heat shield and no codes. One time it was 1 and 3 that was the first time. Always 3 since
So far. Two trips 67 miles heavy in the throttle and no code with heat insulation installed. I have the insulation held snug with a plastic zip tie currently. Seems with the amount of heat that won’t last. Suggestions? Mechanics wire? Aluminum tape?