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Dreaded Misfire P0306 - ‘03 TJ 4.0L

5566 Views 15 Replies 6 Participants Last post by  Shark_13
As the title states, I’ve got a misfire in cylinder 6. Diagnosed from turning the key 3x to show code P0306. Specs are as follows.
-2003 TJ
- 4.0L
- automatic trans
- ~120k
- recently replaced the alternator and battery
- engine has a newer head gasket
- common rear main leak
- no other major engine repairs/issues (I’ve owned it for 14 years)
- Jeep does sit in a garage most of the winter

All I’ve tried to do so far is change the plugs (autolites) and no change. Old plugs were worn, but no signs of damage/fouling. Symptoms other than the code are rough idle(exhaust sounds like the engine is “chugging”, if that makes sense?), and seems to misfire/briefly hesitate under acceleration around 50-55mph. Otherwise runs ok at cruising speeds. Drove it for an hour, 50mi this morning and code wouldn’t clear. Running the same as it was before.

My skill level is swapping basic parts with simple tools (alternator, starter, plugs, etc.). I understand that these can be annoying to diagnose and throw parts at. So, trying to do what I can at low cost before bringing it to a shop for them to do the same. I’ve read a few write ups that say it could be plugs, coil pack, injectors, or compression issues, among other things.

So, any ideas to narrow it down based on these specific symptoms would be helpful.

Thanks
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the CEL is on so lets try a scan, they are free at many auto part stores, don't know till we look.

As you mentioned a misfire can be several directions to check,
1st is always plugs, inspection or replacing.
Next lets try a can of seafoam get that at the part store when you get scan.
KNowing if there are any codes to guide us would be next.
Checkig fr a vacuum leak - use the carb spray trick for a easy check of the intake manifold.
A Diyer trick is to swap injector with another cylinder, see if code follows.
Last is A compression test.

If you are lucky enough to have a schraider valve on the fuel rail, do a pressure test then with key in RUN engine off (KOEO) observe fuel pressure drop in 5minutes, post high PSI and reading in 5min.
 

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If this wasn't happening when you parked it, I would suspect two things beyond the obvious tune up items, plugs,wires,cap, rotor. (I replaced my entire ignition system distributor and all, didn't help);
1) a mouse chewed on a wire, lol. Seriously, check the grounds, the TJ has a body to engine ground at the back of the engine. Take it off clean and replace. There is a battery to engine ground . Fuse box to engine/body ground. The engine management harness runs from the PCM/ECU behind the engine and then forward on both sides. Look for damage and loose connections.
2) fuel system degradation. If you have the factory fuel injectors. I would replace them. Simple process and relatively cheap if you use reburbished units, https://www.ebay.com/p/193688592
Anything that can cause marginal fuel pressure can cause skips. Unfortunately, the main fuel filter is in the tank. 300$ part, as it's commonly sold as the pump.

There are other more complicated items that can cause this issue. Bad cam/crank position sensor comes to mind.
Other then the refurbished Bosch injectors, use only OEM parts. Too many people report issues with the knock offs.

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Discussion Starter · #4 ·
the CEL is on so lets try a scan, they are free at many auto part stores, don't know till we look.

As you mentioned a misfire can be several directions to check,
1st is always plugs, inspection or replacing.
Next lets try a can of seafoam get that at the part store when you get scan.
KNowing if there are any codes to guide us would be next.
Checkig fr a vacuum leak - use the carb spray trick for a easy check of the intake manifold.
A Diyer trick is to swap injector with another cylinder, see if code follows.
Last is A compression test.

If you are lucky enough to have a schraider valve on the fuel rail, do a pressure test then with key in RUN engine off (KOEO) observe fuel pressure drop in 5minutes, post high PSI and reading in 5min.
I like your thought process. I know an OBDII scanner can provide more info, but I do have the CEL code - P0306. With what's going on in the world, not planning to go to any stores right now. Swapping injectors seems easy enough as a next step. Not sure what exactly you mean to suggest with the sea foam. I've heard of people spraying it into their throttle body, it creates a big cloud of smoke, but doesn't do much else.
 

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Discussion Starter · #5 ·
If this wasn't happening when you parked it, I would suspect two things beyond the obvious tune up items, plugs,wires,cap, rotor. (I replaced my entire ignition system distributor and all, didn't help);
1) a mouse chewed on a wire, lol. Seriously, check the grounds, the TJ has a body to engine ground at the back of the engine. Take it off clean and replace. There is a battery to engine ground . Fuse box to engine/body ground. The engine management harness runs from the PCM/ECU behind the engine and then forward on both sides. Look for damage and loose connections.
2) fuel system degradation. If you have the factory fuel injectors. I would replace them. Simple process and relatively cheap if you use reburbished units,
Anything that can cause marginal fuel pressure can cause skips. Unfortunately, the main fuel filter is in the tank. 300$ part, as it's commonly sold as the pump.

There are other more complicated items that can cause this issue. Bad cam/crank position sensor comes to mind.
Other then the refurbished Bosch injectors, use only OEM parts. Too many people report issues with the knock offs.

Sent from my Pixel using Tapatalk
Good suggestion with the injectors. Obviously cheaper for me to throw parts at it than paying someone else to do it. Injectors seem inexpensive enough to give it a shot. Perhaps even the coil pack. Luckily it's only one cylinder, and not the that indicates it's jumping between them. Really hope it's not the fuel pump.
 

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put seafoam in fuel tank, its a detergent and would clean any inj varnish, that could have them stick open.
 

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The seafoam treatment should also take care of any carbon buildup on the valves or in the combustion chamber which is another possible cause for misfires, though usually that will usually manifest itself on multiple cylinders. Search for "Italian Tuneup".
 

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While the sea foam for the tank is a good idea, it didn't work for me and the concentration level needed to clean them as well as the re-furbed ones, would likely cause catalytic converter issues .
You can, clear the code and move the injectors, but that could mean doing the whole job twice.

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Discussion Starter · #9 ·
An update...I brought it to a shop to diagnose because I didn’t want to chase my tail. They said it was throwing codes 301 and 306, and because those are on the same coil, they were confident it was the coil pack. Replaced that and it still idles like crap and eventually threw more codes. Now they’re 301 and 300. So, I decided to swap out crank position sensor, and no improvement. I’m at a loss at this point. Planning to call the shop tomorrow but frustrated they didn’t seem to diagnose it and just threw parts at it. Any ideas?
 

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maybe look at LIVE data on scanner.
Do the fuel pressure tests.
the mentioned vacuum leak ck.
clean the IAC.

You have not changed pugs in14 years? that may be another place to check.
 

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I would definitely do a compression test. Hopefully it will eliminate a major mechanical issue, but you will know if you're wasting time with something else.

Missfire detection, especially of older systems is wonky at best. The system has difficulty determining the difference between the two cylinders in the same crank position.

While there is nothing wrong with doing a seafoam type system cleaning, it's probably not going to cure a consistent misfire on a single cylinder. I'm not sure if you've tried swapping a fuel injector, but that's worth a try.

ALSO: back on the old 4.2 (and might still be the case on yours) there is a vacuum connection hookup for the heater box etc back near cylinder 6. If you have that, pull off the hose and cap the connection on the manifold and see if there is a difference.

Check the tigtness of the intake manifold bolts. I have actually had that cause a stubborn idle miss. (on one of my 4.2s)
 

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Discussion Starter · #12 ·
maybe look at LIVE data on scanner.
Do the fuel pressure tests.
the mentioned vacuum leak ck.
clean the IAC.

You have not changed pugs in14 years? that may be another place to check.
I changed the plugs at the beginning of all this and no change. Old ones looked fine, too. And, I don't have a scanner, but that's a good idea.
 

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Discussion Starter · #13 ·
I would definitely do a compression test. Hopefully it will eliminate a major mechanical issue, but you will know if you're wasting time with something else.

Missfire detection, especially of older systems is wonky at best. The system has difficulty determining the difference between the two cylinders in the same crank position.

While there is nothing wrong with doing a seafoam type system cleaning, it's probably not going to cure a consistent misfire on a single cylinder. I'm not sure if you've tried swapping a fuel injector, but that's worth a try.

ALSO: back on the old 4.2 (and might still be the case on yours) there is a vacuum connection hookup for the heater box etc back near cylinder 6. If you have that, pull off the hose and cap the connection on the manifold and see if there is a difference.

Check the tigtness of the intake manifold bolts. I have actually had that cause a stubborn idle miss. (on one of my 4.2s)
I called the shop that did the coil pack swap and they're going to look at it and drive it a bit free of charge. Of course as I was driving to the shop it started idling GREAT. Not enough miles to clear the code on its own, but definitely better. Someone suggested a coil pack connector. Apparently some of the 4.0's need an adapter to sync the firing order because the replacement packs aren't the same as stock. Anything like that I was able to find was for a Grand Cherokee, not the TJ, though.

I figured the crank sensor would be an immediate improvement. Is there any chance it could take some miles for it to adjust or sync with the engine, which would explain the improvement from yesterday to today?
 

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Discussion Starter · #14 ·
Overdue update...shop checked it over and said they found a dead O2 sensor. Started to run ok for a bit but it’s throwing misfire codes again. At a loss as to where to start looking now.

Seems to really struggle at idle and take off from a stop.

Any ideas?
 

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Discussion Starter · #15 ·
Update...Jeep has been sitting since fall. Battery died, which reset all monitors. But I randomly decided to try and fix my broken HVAC control. Wouldn’t you know the vacuum line at the firewall on the passenger side was completely severed? I was able to patch it together. Tried starting the Jeep and it wouldn’t go. After trying a bit I got it running. It idled a little rough, but much better. Drove it around for 20 miles and started/stopped it a few times. Noticeable improvement. Could the rough idle and misfire really just been that stupid vacuum line?

🤞🤞🤞
 
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