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

CodaMan

California Prisoner (for only a while longer)
Supporting Member
Mar 9, 2017
2,749
California (for now)
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

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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!!!
 

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Chris

Administrator
Staff Member
Sep 28, 2015
34,400
Salem, Oregon
Fantastic write-up, one that should be very helpful as well. Seems like every week someone is posting with a p0301 error code!
 
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Chris

Administrator
Staff Member
Sep 28, 2015
34,400
Salem, Oregon
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.
Best advice ever. Replacing the intake / exhaust manifold gasket is royal pain in the butt unless you remove the fender (ask me how I know).
 
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CodaMan

CodaMan

California Prisoner (for only a while longer)
Supporting Member
Mar 9, 2017
2,749
California (for now)
Best advice ever. Replacing the intake / exhaust manifold gasket is royal pain in the butt unless you remove the fender (ask me how I know).
The middle bottom bolts really suck and the other bottom ones are no cake walk, but when the fender is removed you can do them with your eyes closed.
 

Chris

Administrator
Staff Member
Sep 28, 2015
34,400
Salem, Oregon
The middle bottom bolts really suck and the other bottom ones are no cake walk, but when the fender is removed you can do them with your eyes closed.
Yep, I agree completely. I did it the first time with the fender on, and that's a serious pain in the ass.
 
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Bigbill

Member
Mar 11, 2017
39
Connecticut, United States
I have had a noticeable engine roughness or misfire since she was new in '99(TJ). She's been back to the dealer on warranty but they never fixed it. At 124k it still runs the same. I chalked it up because I drive a Subaru most of the time. There so smooth running. The TJ was the misses.

My old 55 willys with the 134f head is so simple to fix. I been thinking of putting the TJ body on the 55?
 
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CodaMan

CodaMan

California Prisoner (for only a while longer)
Supporting Member
Mar 9, 2017
2,749
California (for now)
I have had a noticeable engine roughness or misfire since she was new in '99(TJ). She's been back to the dealer on warranty but they never fixed it. At 124k it still runs the same. I chalked it up because I drive a Subaru most of the time. There so smooth running. The TJ was the misses.

My old 55 willys with the 134f head is so simple to fix. I been thinking of putting the TJ body on the 55?
I never really could feel the misfire, but it would produce a CEL. In most of California we have to smog vehicles every 2 years and you can't have a CEL.
 
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Chris

Administrator
Staff Member
Sep 28, 2015
34,400
Salem, Oregon
I can relate. I had an XJ with the 0331 head and had to replace it myself. I'm familiar with this whole 0331 head saga.
 

Mike_H

Rust Belt Heavyweight
Supporting Member
Feb 28, 2017
3,780
Grand Rapids, MI, United States
Mine came on recently. It has been running pretty rough when I first start up, but smooths right out with a tap of the throttle pedal. I need to clean the IAC. On an 04, can you switch ignition coils? How about plug boots?
 

Scott Davidson

Member
Supporting Member
Aug 11, 2018
39
Banks county Ga
Mine threw the po303 code. Mine had the 0331 head, cracked on number 3 cylinder placed it My issues were heat soak after operating temp and park for ten minutes then crank back up, it ran bad rough for long enough to toss the code. I installed a louvered vent in my hood after cutting big holes to allow for heat to get out. No more issues. Motor runs 10 to 15 degrees cooler.
 
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astjp2

TJ Enthusiast
Aug 22, 2018
730
Utah and Alaska
How does the exhaust valve rotate when there are no rotators on the top of the beehive springs? The valves are locked into place by the valve keepers and dont rotate. At least my stroker head, the head from my 98, the 258 head all had fixed retainers on all of the valves. I think that you had something else wrong with the head but I do not believe it was a valve rotation issue. Glad its working for you. Tim
 
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CodaMan

CodaMan

California Prisoner (for only a while longer)
Supporting Member
Mar 9, 2017
2,749
California (for now)
How does the exhaust valve rotate when there are no rotators on the top of the beehive springs? The valves are locked into place by the valve keepers and dont rotate. At least my stroker head, the head from my 98, the 258 head all had fixed retainers on all of the valves. I think that you had something else wrong with the head but I do not believe it was a valve rotation issue. Glad its working for you. Tim
I don't agree. Valves do rotate under normal operation albeit slowly. The rotation action helps keep the valve and seat clear of debris to ensure good sealing. That's why we lap the valves so the entire face of the valve will seal with the seat and not just one little part. And yes, some have rotate mechanisms to ensure they rotate at a predictable rate, but all valves rotate.
 

astjp2

TJ Enthusiast
Aug 22, 2018
730
Utah and Alaska
I have tore down 50 year old engines,and the valves had the original marks where they didnt rotate. If they don't have rotators the valves are not supposed to rotate.