Sorry you are having a difficult problem to diagnose. However, you can diagnose it yourself by the process of elimination, plus time is on your side unless this is your daily driver.

My neighbor and I just finished diagnose a misfire on his Impala and this is my suggestion:

Stop buying parts because you are throwing money at things that may not be problems. You will find out what precise parts to order later. Get a good ODB2 reader that can read off misfire as it happen. Misfire is caused by the combustion process (the entire fuel/air/compression/ignition chain) that isn't operating as efficiently.

Put your Jeep back in the condition that you first had the misfire. Hook up the ODB2 and see when the misfire occurred. Sometime after the engine warms up, the misfire goes away.

Check the electrical: Since you know which cylinder does not misfire, swap up the sparkplug wire to cylinder 2. If the misfire doesn't go away, move to next step.
Swap the sparkplug from the good cylinder to number 2. If misfire is still present, move to the next step.
Swap the ignition coil from the good cylinder to number 2. if misfire is present, you don't have electrical issue, move to the next step

Fuel: swap the injector from the good cylinder to number 2. If misfire is present, you don't have fuel problem. Move to the next step

Air: This is tricky. Check the intake to cylinder2. I think your intake as the gasket that can be hardened and cannot seal, or the plastic intake plenum has a crack thus has air leak. You can make DIY smoke machine to check for intake leak. Since the misfire is isolated to cyl2, you can check the leak in the cylinder plenum.

Fuel Again: Check the long term fuel trim and short term fuel trim when it's at cold, warm idle, and driving. You need an decent ODB2 for this. Most sub $100 units work fine. Fuel trim should be +-5% but you want it to be as close as zero as possible. A better ODB2 can show the fuel pulse for each cylinder.

Compression: Do a compression check and leak down test. Compare the Cyl 2 numbers to all the good cylinder. Do cold compression and warm compression when possible. They will have different number but you need to compare cyl2 to the rest of the good cylinders.

Leak down test will show if the valves are the problems. Again compare Cyl 2 to the rest.

Since the compression test takes a lot of time, do the other tests first. If you find some odd numbers during the compression test, it can get complicated and expensive, unless you do the repair yourself.

For the Impala, the misfire went away after the engine warms up but it was still there. The misfire was much more pronounced after cold start. No fuel problem, no vacuum intake leak, no ignition issue, no timing issue from the variable cam. But we found out the misfire cylinder was compression was consistently 5 psi less than the rest. The leak down test showed no valve leak. So something caused the compression problem. The engine head was removed. The culprit was spalling in one of the intake cams. That was it, just a few psi from a spalling cam will trigger the misfire, especially with these finely tuned engines we have today. Hopefully you never have to get to this point. The key for us was to isolate the problematic system and to observe the misfire behavior. Spalling cam is a symtom of car not driven a lot to keep the oil circulated or bad manufacturing defects. Unfortunately, the car is out of warranty.

Tips for ODB2 monitoring. If you have a good ODB2 like we had, you can observe the misfire counts in each cylinder. Depending on the CPU, most computer will not flag the misfire until the rate of count exceed a certain amount in period of time, usually a few hundred misfire in a few seconds. You can have occasional misfire but the small number of misfires won't trigger the CPU. In the Impala case, the cylinder was expecing a certain amount of fuel/air but it didn't compared to the rest of the cylinder. The modern CPU can detect this minute defect. It will take time and patience to find it, especially when you have time to work it.

Good luck.

 

sounds more like the failing head issue when the valves leak. its NOT the valve seat. its the valve guide bore. they have been known to change relationship to each other do to be a poor design. Fiat redesigned these heads after 2013 about the six month.

Does it tick? meaning the left bank is sounds like a old VW. this was a well know issue and those that failed early on were covered and then an extended warranty . Though once past the regular warranty if heard of people that were told to pound sand.

less often its a valve train issue; followers keepers etc.


leak down is the test they would do to see if its past if i recall the 25% leak. it is always the exhaust valves as well. can be 2 or 6. even some its both sides. though Fiat never owned up to the right bank, in writing.

my 12 had this issue early on at 20k both 2 and 6 leaked like the titanic, this was back in oct of 2013. sounded like a Vw that had never had ts valve lash set. They replaced the head and it ran better then when i bought it new. gained mpg. this and other issues have left me to point now i am done wasting money on any new vehicle, especial Fiat. as i had other issues that were seriously a danger. Fiat denied any responsibility. even after the jeep was impounded for 30+ days a black box investigation. That showed the SRS going off was never recorded, only a fault code to the clock spring. The jeep deployed air bags and the SRS never recorded it said NOTHING went off, no incident meaning no collision. They sent this to legal and basically told me to F Off.

 

details of what 222Doc mentions, anyway a compression and compression leak down would be my next step.

.

 

Has good compression and passes a leak down test.

 

You do these test and the work? Most people its bit more then they can do since the intake must come off to do anything to the left bank.

Perhaps its a valve train/ cam issue is about all i can think of.

what were the numbers on the leak down test?

 

If you are sure the problems are not electrical or the intake air or anything upstream of the intake port, you can do a final check of the intake plenum before replacing more expensive parts. Verify with the ODB2 that the misfire is isolate to the single cylinder and not other by watching the misfire count. If you get blips of misfire on the adjacent cylinder then they could share common cause, maybe the intake, head, of bad head gasket. If you are absolutely sure the ODB2 only indicate cyl #2 misfire then you have to do the more difficult work of checking the valve lash, rings, and cam lobe on cylinder #2 by removing the intake and possible the head.



USB borescope is cheap from Amazon. you can use it to check the condition of cylinder 2 and compare to a good cylinder to ensure the cylinder wall is goo.



What are your compressions for all cylinders? Is there any inconsistent number with cylinder #2?

 

Only code is p0302?
you replaced all plugs, what brand rating, Swapped coils, and injectors, compression and leak down were good*, You have checked for other codes with a SCANNER and looked at live data? Did you look at freeze frame?

* Were there any suggestions/hints in the TSB as to how to do compression tests?