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Stumped by cylinder 6 misfire - SOLVED

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3.3K views 16 replies 6 participants last post by  EWC  
#1 · (Edited)
New to the forum and Jeeps, I hope someone can point me in the right direction. 2015 JKUR 3.6l 6MT purchased by me six months ago with 77,000 miles. A few weeks ago at 80,000 miles I replaced the oil cooler, spark plugs, and coils with OEM parts, and did an oil change. About 250 miles later I had a check engine light come on for P0306 Cylinder 6 Misfire. I cleared the code and immediately started using Jscan to monitor misfire counts, and sure enough it was showing an intermittent misfire on cylinder 6. All other cylinders stayed near zero. Unfortunately I didn’t monitor this before doing the work above. From what I can see, the misfire would occur when I was at very light throttle, such as going downhill, but not off-throttle engine braking. I pick up 1-3 misfires when upshifting, but the count really climbs when at part throttle and light engine load. The misfire count hit over 300 in about 30 minutes, with most of those while descending a mountain pass. It does not occur during idle, acceleration, or cruising. I was towing a small 1500lbs teardrop when I got the CEL, but the misfire continues to show (without throwing a CEL) driving around town. The Jeep drives great, idles smooth, and doesn’t have any rocker ticking that I can hear.

Maybe unrelated, but about 50 miles later I had P06DE “Engine oil pressure control circuit stuck on” CEL, and thanks to Jscan I was able to confirm the oil pressure was staying around 60psi during cruising on the highway around 2,000rpm. This I understand normally means the solenoid on the oil pump has failed requiring an oil pump replacement.

The fact that the misfires occur during light throttle and low load made me suspect a vacuum leak. I flashbacked to when I was struggling to get the upper intake manifold back on and suspected maybe I pinched or damaged the gaskets. So I pulled the upper intake manifold off and didn’t find anything obvious. I replaced with new gaskets, and also swapped coil #6 with coil #1 while I was in there. I buttoned it back up and took it for a test drive – no change, still getting an intermittent misfire on cylinder #6 under the same conditions.

I pulled the upper intake manifold again and ran a compression test – between 160psi and 170psi on all cylinders. Aware of the known rocker arm issues on the 3.6l I decided to remove the valve cover to check. Cams look fine and there is no obvious rocker damage that I can tell. The rockers have a small amount of movement if you twist them with your finger around where they connect on top of the spring, but no side to side movement that I’ve seen in youtube videos. Nothing unique about the cylinder 6 rockers compared to cylinders 2 and 4. I do see some metal “fuzz” on the magnetic portions of the cam sensor tone rings which indicates some steel has been wearing, but maybe this is just from break in? Oil galley plugs were loose but not backed out.

Just out of curiosity I put pressure on the cooling system for 24 hours to see if I get any seepage into the cylinder, but when I checked with a borescope it was dry.

I’m not sure where to go next. I’m debating pulling the lower intake manifold and checking it for cracks, and replacing the lower gaskets. Maybe swapping injectors to see if the misfire moves? I have not pulled the camshafts to directly inspect the rockers but maybe I should? What am I missing?

EDIT 5/8/25 - SOLVED. Took it to a shop that diagnosed it as a cylinder head issue. Rebuilt cylinder head and now misfire is gone. Hope this can help anyone with the same symptoms.
 

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#2 ·
You know you're getting air, you know you're getting spark - that leaves one thing.
Sounds like you're thinking in the right direction with swapping injectors, it'll give you an opportunity to inspect the injector harness while you're there.

Edit: That last lobe on the left looks different to me, like the lifter is sticking almost - It doesn't seem to have the characteristic lead-in mark in the middle. That said, I've built engines but I'm not a diag guy so keep that in mind.
 
#3 ·
Thank you for the reply. It’s hard to tell in that picture but I took a close look at the cam lobe and it doesn’t seem too different to me. The surface is smooth with no ridges.

I’ll try looking at the fuel injectors next. When I did the oil cooler I did not disconnect the fuel line, so maybe some debris ended up in the #6 injector while I was moving it in all sorts of angles to get the lower manifold out of the way.
 
#5 ·
I was getting an intermittent misfire on just one of my cylinders. Sometimes real bad where it would shake, others times it would go unnoticed until I used JScan and saw hundreds of misfires as yours does. There was never any real consistency of when it would misfire. I finally replaced all the injectors with the newer version of the mopar injectors and the problem was solved.
 
#6 ·
I might end up going this route. I ordered Fel Pro fuel injector O-rings from Amazon in order to get them quickly (FEL-PRO ES 73046), and they seem to be all one size instead of the two different sizes, despite the description saying they match this application. Now that I read the reviews, it seems like I'm not the only one who had this problem.

Meanwhile, Mopar OEM O-rings from the dealer are almost the same price as the updated fuel injectors which I believe include the O-rings. So it is tempting to upgrade while I'm in here. Did you notice a difference with the new injectors compared to stock before you had a misfire? Did you install the updated lower manifold too?
 
#8 ·
Well, still frustrated by this misfire… First, my curiosity got the better of me while I had the valve cover off so I pulled the rocker arms from cylinder 6. Not only are they in perfect shape, but much to my surprise they are the updated design with the floating pin! So one of the previous owners has been in here before. Overall that makes me happy that I won’t have to worry about rocker arms in the future but it doesn’t help with the current problem.

So, I bit the bullet and purchased six of the updated fuel injectors and also added the updated lower intake while I was at it. As I mentioned above, the dealer price for fuel injector o-rings was nearly the cost of new injectors! I figured it couldn’t hurt at this point and if I get an extra mpg out of it all the better.

Well, I put everything back together and there’s no change. On a 30 minute test drive I picked up over 100 misfire counts on cylinder 6. All others were very low or zero. So I’ve still got some work to do.

I think the next step is checking the wiring, in particular the fuel injection harness for cylinder 6. I checked the easy spots like the grey connector on the passenger side and the PCM connectors. Looks like the upper intake manifold will be coming off yet again…

Appreciate if anyone has any thoughts I haven’t check yet.
 
#9 ·
Yup. You have a wiring harness issue to either the #6 fuel injector or the #6 ignition coil.

Ironically, I changed plugs on my wife's 2014 JK just a couple of weeks ago. The ignition coil plug on her #4 cylinder broke when I was unplugging it. Despite being only at ~75k miles, none of the ignition or fuel injector harness had ever been touched. After 10 years, that one particular plug was just brittle enough that I broke it.

Interestingly, it was very difficult to see at first the connector was broken. It plugged back in OK, but just didn;t seem right. It was when I unplugged it again to double-check it that it actually came apart in pieces.

MOPAR makes an ignition coil connector repair kit. $60. It was a PITA to repair that connector, but it completely solved my problem .... and clearly prevented forthcoming P304 codes.......

HTH

You need a wiring diagram and a test light.
 
#10 ·
Update, but no progress... I was out of town and busy with other projects so just getting back to the Jeep this past weekend. Fortunately it's not my daily driver, but we use it to get to some very remote campsites, so I want to make sure it's 100%.

Since my last post I pulled the upper intake manifold again, and removed the wiring harness for the injectors and ignition coils. I checked continuity and didn't find any issues, even with flexing the harness. I did a leak down test and saw about 15?% (not the best leak down tester, just a Harbor Freight one I bought long ago). No sign of any leakage past the intake or exhaust valves as far as I could tell. I also opened the radiator cap and didn't see any bubbles (not sure if that is definitive?).

I had a theory that maybe the foam under the upper intake manifold on bank 2 might be preventing the upper and lower intake manifolds from mating properly, so I reassembled without the foam. Still no difference...

I dug in again and pulled the upper intake and valve cover yet again to double check and still no sign of any bad rocker or soft lifters. Borescope inspection of the valves down the spark plug hole doesn't show me any obvious signs of a burnt valve.

Here's the behavior I'm observing with JScan:

Idle - no misfires (idles smooth)
Accelerating - no misfires (power seems normal)
Cruising - no misfires
Off throttle between shifts - 1 to 3 misfires on Cylinder 6 each time
Light throttle (such as decelerating or going downhill) - Cylinder 6 misfire counter ticks up
Revving engine in neutral sitting in driveway - Cylinder 6 misfire count climbs rapidly

All other cylinder misfire counters are 0 or single digits.

I'm at a complete loss at what to try next. I am seriously considering taking it to a shop. At this point I'm willing to pay someone if I can get this problem solved and the Jeep running reliably. Anyone know of a place in the Los Angles area that is reputable and competent?
 

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#11 ·
I vote for the harness failure.
I had upwards of 20 CEL codes happening on our 2015 JKUR, and once I repaired the engine harness my electrical troubles went away.

I also replaced the CSPS, coil packs, spark plugs, TIPM, battery terminals, battery, and even had a new ECU on hand to test it with before finally realizing/accepting that it was the harness.
I bought new injector connectors and new coil pack connectors on Amazon for pretty cheap.
Don't skip the connectors behind the oil cooler (temp & press) - this is where my main failure was discovered.
 
#15 ·
Thank you. I don't have a warranty so hopefully this is not the case, but at this point I'll gladly spend whatever it takes as long as it's properly diagnosed. Did your misfire occur under the same circumstances: light throttle, but not under load? Did it run rough at all?

Here are some borescope pictures of the exhaust valves in cylinder 6. It's a cheap Amazon borescope so forgive the quality of the photos. I don't see anything obvious but maybe I'm missing something.
 

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#17 ·
SOLVED - Well, I gave up spending all my free time trying to diagnose it myself and brought it to Jeep Solutions in Long Beach. A rebuilt cylinder head later and the misfire is gone. They found an issue with the valve seat and described this as a common problem with the 3.6l. Jscan is showing 0 misfires after a long drive. Happy to be back on the road.
 

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