P0306 Cylinder Six Misfire in 3.8L

stainlesself

stainlesself

New member
Hey all, recently I received the dreaded P0306 code for my 2011 Jeep JKU, I know it is a cylinder 6 misfire, it would usually go on for about a day than off for about 2 weeks until last night when the check engine light started flashing for the first time.
I have replaced the following already
- Coil Pack
- Spark plugs (although one could've failed prematurely)
- Mass Airflow Sensor
- Spark Plug wires.
I do also have a small evap leak and and major evap leak but do not see the correlation to that being a misfire, any advice or experience from people who have dealt with this would be much apperciated!

Thank you!
 
Check fuel injector wiring and ignition coil wiring. Make sure they're tight and no rodent damage.
 
Fix the vacuum leak. Vacuum leaks lead to erroneous info to the computer which can lead to erroneous signals to the system which gives false readings to the sensors which leads to erroneous info being fed to the computer and so on and so on.

Another source of grief (at least on the 3.6) is bad cam position and crankshaft position sensors.

Erroneous info and/or bad sensors can cause all kinds of grief you wouldn't think was related. For example, I found out today that a loose connector for the steering wheel airbag caused the right front brake to overheat.
 
GP NOIR said:
Fix the vacuum leak. Vacuum leaks lead to erroneous info to the computer which can lead to erroneous signals to the system which gives false readings to the sensors which leads to erroneous info being fed to the computer and so on and so on.

Another source of grief (at least on the 3.6) is bad cam position and crankshaft position sensors.

Erroneous info and/or bad sensors can cause all kinds of grief you wouldn't think was related. For example, I found out today that a loose connector for the steering wheel airbag caused the right front brake to overheat.
Click to expand...
agree with this, I've had issues with my JKU remote start starting then immediately shutting off, also my horn quit working.
Took the steering wheel cover off, found a wire on the horn wire had completely come out because the factory wire was too short (when I pressed the horn on that part, it pulled the wire out), spliced in a longer wire, fixed the connector and now the remote start works like a charm.
I got lucky finding that 1st, I was about to go down the remote start rabbit hole issues and repairs.
 
GP NOIR said:
Fix the vacuum leak. Vacuum leaks lead to erroneous info to the computer which can lead to erroneous signals to the system which gives false readings to the sensors which leads to erroneous info being fed to the computer and so on and so on.

Another source of grief (at least on the 3.6) is bad cam position and crankshaft position sensors.

Erroneous info and/or bad sensors can cause all kinds of grief you wouldn't think was related. For example, I found out today that a loose connector for the steering wheel airbag caused the right front brake to overheat.
Click to expand...
If an evap leak was large enough for the unmetered air to cause a misfire it would be across multiple cylinders, not just one.

Op- does it only miss when it’s hot after driving for a bit? If so check the “new” coil pack you just installed and check the wire. Also check the injector wiring as Waco mentioned. Could have gotten nibbled on to cause the misfire intermittently
 
cozdude said:
If an evap leak was large enough for the unmetered air to cause a misfire it would be across multiple cylinders, not just one.

Op- does it only miss when it’s hot after driving for a bit? If so check the “new” coil pack you just installed and check the wire. Also check the injector wiring as Waco mentioned. Could have gotten nibbled on to cause the misfire intermittently
Click to expand...
Perhaps. However, the leak introduces variables that can make troubleshooting difficult. You don’t know how the computer is going to interpret erroneous information. I’ve had single cylinder misfires that couldn’t be fixed until a minor vacuum leak was chased down and repaired.

Another example, my Jeep was throwing codes for the O2 sensor, #2 misfire, showed the ESC light, Bank 1 cam shaft position sensor, crankshaft position sensor, ABS no torque signal and the right front brake started grabbing and overheating- all at the same time.

What fixed the engine codes, ESC light and ABS no torque signal was replacing the cam shaft position sensor. What fixed the brake problem was fixing a loose wire connector to the airbag.

If you have a known problem, eliminate it first. If I’d looked at the airbag when the light first came on, I would have saved myself a lot of grief and money trying to fix the brake. I didn’t think the two were related. In this case, fix the evap leak before throwing ignition coils and spark plugs at the misfire.

Also, get yourself a code reader, even if it’s something as simple as a Blue Driver.
 
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Forgot to mention- misfires can be due to low cylinder pressure. Might be a good idea to perform a cylinder leak down check.
 
cozdude said:
If an evap leak was large enough for the unmetered air to cause a misfire it would be across multiple cylinders, not just one.

Op- does it only miss when it’s hot after driving for a bit? If so check the “new” coil pack you just installed and check the wire. Also check the injector wiring as Waco mentioned. Could have gotten nibbled on to cause the misfire intermittently
Click to expand...
So, it only misses when it first starts up, when it is driving sounds smooth and clean as day. and accelerates normally.
I checked the wiring yesterday but didn't notice anything damage or eaten
The Evap Codes it is throwing out is P0455, P0456, P0457, along with the Cylinder six P0306 code
 
 
stainlesself said:
Awesome Thank you, it does seem that the Evap Leak may be the culprit behind the misfire I will do a thorough check this weekend and report back with what I find!
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This is the more common cause of the JK evap large leak.
The area circled in green gets broken, no need to drop the tank to repair it (there is no replacement part, you have to buy a new tank).
IMO this happens when the line (marked in red) is too tight to the body mount and has no room to flex, be sure to clip the zip tie and replace it with a looser fitting tie.
I used FlexSeal for my repair and it's still holding lol
Others have used JB Plastic Weld

tank_nipple.jpg
My FlexSeal repair after loosening the line and holding the 2 pieces together for a tight (as possible) fit.:
FlexSeal_05_14_2023.jpg



another member post with better images.
www.wayalife.com

Does this look right?

Getting a small leak Evap code P0456. Had work done to the jeep and they had to remove the fuel tank to do the work. So I'm a little suspicious of the timing of this CEL. Crawled around under there the other day and pushed on all the connecters, to make sure they were seated properly. I saw...
www.wayalife.com www.wayalife.com
 
jeeeep said:
This is the more common cause of the JK evap large leak.
The area circled in green gets broken, no need to drop the tank to repair it (there is no replacement part, you have to buy a new tank).
IMO this happens when the line (marked in red) is too tight to the body mount and has no room to flex, be sure to clip the zip tie and replace it with a looser fitting tie.
I used FlexSeal for my repair and it's still holding lol
Others have used JB Plastic Weld

View attachment 408105
My FlexSeal repair after loosening the line and holding the 2 pieces together for a tight (as possible) fit.:
View attachment 408107



another member post with better images.
www.wayalife.com

Does this look right?

Getting a small leak Evap code P0456. Had work done to the jeep and they had to remove the fuel tank to do the work. So I'm a little suspicious of the timing of this CEL. Crawled around under there the other day and pushed on all the connecters, to make sure they were seated properly. I saw...
www.wayalife.com www.wayalife.com
Click to expand...
Awesome thank you! I will definitely take a look there and see if there is a crack or anything hopefully this is the cause of all the Evap codes but glad to have a starting point!
 
I very highly doubt the evap leak is a cause of the misfire. I had an evap leak for four years and never had a misfire.
 
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