Sv_dude

I've had a pesky P0304 with a corresponding CEL for a few years. The jeep has run fine and the CEL hasn't been an issue so I've ignored it for a while. I've finally just got tired of seeing the CEL so it's time to start digging into it.

This is on a 2015 JK with an automatic transmission.

What I've done so far:
I swapped the following parts from the #4 cylinder to the #2 cylinder to see if the code will change from a P0304 to a P0302:
Fuel Injector
Spark Plug
Ignition Coil

The code stayed on P0304.

Since the code is always a single engine misfire and not a generic misfire, I think it's safe to rule out any sensor that gives data on the whole bank or whole engine such as camshaft position sensors, mass airflow sensor, crankshaft position sensor, o2 sensor, etc.

I don't fully believe that the engine is actually misfiring since it sounds normal, idles smooth, has decent power (considering its a fat heavy jeep with big feet), and overall acts like there's nothing wrong. I did however get a live data recording from JScan and it doesn't appear (according to the data) that cylinder 4 isn't firing at all.


One thing I'm unsure about is the reading from o2 sensor 2 (short term fuel trim), it's 99.22. From a bit of research, this is a default number if the ecu/pcm doesn't use this data. But I wonder if maybe it's used for our jk and maybe mine is just malfunctioning? Can someone check theirs and see if this data shows up?

I believe the engine sounds fine, here's a YT video of it:
There is a slight tick, I could be wrong but I believe it's normal. Using the screwdriver stethescope, I wasn't able to tell any difference in noise between the left and right bank or either cylinder.

Some other possible causes that I need to troubleshoot are possible faulty wiring, low cylinder compression, and maybe a leakdown test to rule out a head gasket. I doubt it's a head gasket since the oil isn't contaminated with antifreeze, and there's no white smoke from the exhaust. The cam/rocker could be an issue but I would think there would be a much more noticeable noise coming from the valve train?

Any opinions or comments are appreciated.

resharp001

Bad news man, I think you need a complete teardown and rebuild the timing!

From the video, I think the engine sounds normal. When I had the rocker tick my wife, who knows zero about cars, heard my jeep idling in the driveway from 50'+ away and asked me what that noise was. It's that obvious IMO.

I haven't had to deal with single cylinder misfire really and they can be pretty aggravating. Probably the 2nd thing behind electrical issues IMO. It also sux would have to pull the intake just to do a compression test on that side, but that is probably the point where I would be. That seems easier to confirm before trying to chase electrical gremlins.

Sv_dude

Ha! I'm completely fine if I don't have to do a timing repair. It's not the task I'd like to tackle any time soon. I think if I ever did have a timing issue, I'd hope that because the engine is an interference engine that it would just eat itself and give me the ammo I need to do an engine swap.
I do agree, a simple (yet tedious) compression check would be a lot easier than chasing sparks.

Sv_dude

I did a bit of troubleshooting today. I used the service manual from AllData. This is going to be a little bit of a long winded post since there isn't any decent troubleshooting posted online that I could find. I'll post all of the documentation up front then discuss my findings through each step.

There are 17 pages to the cylinder misfire troubleshooting from the FSM below:


















Also needed are the C1 and C2 pinout for the PCM. Only the C2 will be posted since the C1 is listed in the engine wiring harness diagram that will be posted below. Note: The C1 connection is on the left side while the C2 is on the right (as viewed from the engine perspective).



Engine wiring harness diagram:






Lastly, it wasn't needed but I have it so I'll post it. The Fuel system wiring diagram:

Sv_dude

1) Starting with step-by-step troubleshooting from the FSM was to check for active codes. This was obvious, there was an active P0304 for the cylinder 4 misfire.

2) There were no other codes whether pending/permanent.

3) I could not see anything obvious for mechanical conditions on the engine drive system. The engine idled smooth and does not vibrate so I believe I can rule out a harmonic balancer being out of balance. Tires and brakes are also in good shape.

4) Engine oil/filter are clean and within specifications.

5) I don't have a good vacuum leak tester so my method used is kind of hit or miss. Possibly more miss than hit since it's hardly a full proof method. I used a can of carb cleaner and sprayed each vacuum line connection, the throttle body, and around the intake plenum. While doing this I was listening for any changes in engine rpm since I assumed that if there was a vacuum leak, it would suck up the cleaner and burn it in the cylinders. I was unable to find any vacuum leaks.

6) Fuel quality is good. Not formally tested but I have no reason to believe that fuel is bad since it's the same gas I use in all my vehicles.

7) Check for clogged or leaking fuel injector. I swapped the #4 fuel injector to the #2 and the code didn't move to the #2 cylinder. In my opinion, this rules out the fuel injector as a possible culprit. I don't have a fuel pressure gauge or a fancy scan tool so I didn't follow this step exactly.

8) I did check for proper spark on the #4 cylinder using a spark plug test light that connects between the spark plug and ignition coil. When cranking the engine over, the light was bright and consistently flashing. I also held the ignition coil with the spark plug directly in the coil while having someone help me crank the engine over. The spark was bright and strong.

9) The spark plug itself looked pretty good for a plug with 15-20k miles on it. I did change the spark plugs at 50k miles (ish) when I first got the P0304 code.


10) The compression on cylinder 4 was really good at about 130-135 psi. I didn't test all 6 cylinders to verify a 25% variation since the engine runs smooth and healthy minus this pesky P0304.


11) I did previously swap the #4 spark plug, coil, and fuel injector to the #2 cylinder and found no change.

12) For cleaning the carbon from the valvetrain, I did not have the mopar stuff that the fsm calls for. I did use seafoam in the intake through the brake booster vacuum hose. I got the infamous could of smoke afterwards and this did not do anything to clear the misfire code. I had absolutely no hopes of this fixing anything but for the sake of troubleshooting, I did it anyways.

13) I removed the left bank valve cover and took a peek at the camshafts and rockers. Everything felt smooth, nothing smelled of burnt oil which would indicate a blocked passage, the rockers felt solid. Overall, everything under the lid looked great!


This image doesn't really show much but I took the photo so I'll post it.

14) For checking the ASD relay circuit, I couldn't do it per the fsm since I don't have a fancy scan tool to actuate this relay. I did however verify that fuses M19 and M20 were good, and applied 12v power to pins 85/86 of the relay and verified continuity between pins 30/87.






Now, to do a more thorough test of this circuit I did verify that pins 85 and 30 on the tipm had a constant 12v. I also verified continuity between pin 86 on the tipm side and pin 80 of the pcm C1 connection.

15) I don't have a set of back probes so I went through steps 16 and 17 in lieu of using a test light and a back probe.

16) I checked for continuity between pin 2 on the coil plug and ground.

17) I also had continuity between pin 1 on the coil plug and pin 73 on the pcm C2 connection. Resistance was right at 3 Ohms. It is barely within spec but I think it's fine since I didn't use the specified adapter the manual calls out for, I instead used a small piece of safety wire in the harness and attached the dmm probe to that which adds a little bit of resistance.

18) I made sure to inspect the harness, plugs, terminals, and general condition of the wiring harness as I'm going through it. I also sprayed every connection with a cannon plug electrical cleaner. All wiring looked good to me.

From here, the FSM says to replace the pcm. Not exactly what I want to do.

I went ahead and put everything back together to button up the mess for the night. After everything was put back together, I figured I'd check the live data and also clear the code.

Note: I noticed previously that the misfire only happened at an idle. As soon as I gave the engine a little bit of gas, the misfire would go away. This was confirmed by watching the live data on the JScan app.

While watching the live data this time, I noticed that the misfiring was still happening only at an idle, but it wasn't every engine revolution as it was before. This time, the misfire was much more rare and would only happen once every couple seconds. This may seem like a lot but previously I would get a misfire count for every revolution of the engine. I'm thinking that this is because of one of two possible reasons.




1) While cleaning/inspecting the electrical connections, the connection got better but still not perfect. Maybe I need to inspect the wiring harness a little more thoroughy?

2) I did use a dielectric grease on the intake plenum o-rings before putting the plenum back on. Maybe the o-rings aren't sealing that well and the grease slowed the vacuum leak down a bit?

I'm not completely sure what the problem is yet but I'll replace the intake plenum o-rings since they didn't look too great to start with. I'll also inspect the harness connections again. I think if neither of these two things fix the issue, I'm safe to say that it's the pcm.

resharp001

One note. When timing is lined up, as it appears to be via your cam phasers, the left side cams are not in a neutral position so I'd suspect in that situation you could have a bad rocker that does not reflect any play. On the left side you would really need to back the cam off to a neutral position for a good assessment, BUT, I really don't think you have a rocker issue cuz there'd be no doubt in your mind with the noise.


***not sure if you just paid for the alldata subscription for this purpose or not. I wish I had thought to offer my login since I just paid for a month myself.

Sv_dude

Yea, the data was from AllData. I had paid for a subscription already and actually just had an annual subscription expire. I figured I’d just do the month for now.

I replaced the intake gaskets for the upper plenum only. Still getting the pesky p0304. At this point in the troubleshooting, the manual says to replace the pcm. I am not 100% confident that’s the issue. Either way, I’m not going to replace the pcm. The engine runs fine and I’m not seeing any adverse effects of the misfire. So for now I’m just going to leave it. If the engine dies/explodes or otherwise, it’s getting a v8 swap.

Something else a little interesting is that when I do an advanced scan on JScan, I get this:



Im not sure what the deal is with the b or u codes. But the p0456 code catches my attention. An evap leak can cause a misfire from what I’ve read. But the code is a permanent code and it doesn’t list as pending like the p0304 does after I clear the codes.

Not too sure what to do about that one.

sarguy

I assume you checked the grounds on the O2 sensors ? They all ground to a single stud on the passengers side of the block. I fought the same issues for 2 years and the code would come and go. Replaced sensors multiple times. Finally my AC stopped working. While diagnosing the issue found no ground to the AC clutch. After alot of digging found all the O2 sensors and the AC clutch all went to the single ground. Mine was corroded bad. With an O2 sensor unhooked I could move the wires on the ground stud and the ground would come and go. Without pulling off the manifods I couldn't replace the ground there so I cut it and used a transmission bellhousing bolt. AC fixed and not one sensor alarm since. I know others have seen the issue and I think someone mentioned to me to check this on mine when I was chasing the issues.