Diagnosing uncommon WJ 4.0 misfire P0302 - JeepForum.com
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post #1 of 8 Old 05-08-2021, 07:38 PM Thread Starter
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Diagnosing uncommon WJ 4.0 misfire P0302

There's 10s of misfiring posts, but bear with me, this one is a bit different. I'm trying to find a logical way to diagnose this code that keeps coming back on my Jeep. Wondering if someone may have experience on this same scenario. First, some info:
  • Even compression on all cylinders
  • Spark plugs 5000 miles
  • Coil 5000 miles
  • Mopar crankshaft position sensor 500 miles
  • New injectors

Normally I drive like a bit over an hour and the car goes into 5 cylinders, with a clear misfire shake and lack of power. The code comes up, it flashes the check engine light, and goes on like that. Now the interesting part: I pull over, turn off the engine, turn it back on immediately and the misfire isn't there anymore. I runs smooth on all 6. Check engine light remains (as code is stored) then it may go away after some mileage.

I have replaced the crankshaft position sensor due to this. I have also replaced all injectors diagnosing this. Code is always for cylinder 2.

I may request a new coil as it's under warranty, but I can't get past the detail that a restart of the car makes this go away. Like, no temperature change, immediate restart. Almost like an ECU condition is triggered and then reset when turning the vehicle off and on again. Leaning towards replacing the ECU, but since it's pretty expensive, trying to be a bit more sure if the issue.

Anyone has experienced anything like this?

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post #2 of 8 Old 05-08-2021, 08:27 PM
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some thoughts-

What brand and number spark plugs are you running?
Check the injector connector - tight, corrosion free, wires not frayed pinched, maybe swap #2 injector with another inj see if code follows.
With your scanner look at freeze frame when code sets- maybe a clue.
Add a can of "seafoam" fuel treatment can't hurt and maybe some varnish dirt on #2 inj.

If you decide to go with a PCM contact "waterluvr" he is our go to member for PCM's.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 8 Old 05-09-2021, 09:08 AM Thread Starter
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Thanks for the response. I am using Champion RC12ECC, they are gap'ed at 0.40.
Looking at it, that injector harness is not great. The plastic protector is brittle and breaks the moment I handle it, so that could be an indication of wires that are not amazing either. I was thinking of plugging a multimeter and testing continuity from whatever the computer connector for injector #2 is, to the clip for the injector, and move things around by hand to see if I see a change in continuity. Resistance measuring should indicate corrosion too. I just gotta figure out which pin to test from the computer to the clip.
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post #4 of 8 Old 05-09-2021, 09:26 AM
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I had a similar issue. In my case, the exhaust header was not bolted down correctly. Check that the studs are tight when the jeep is cool.

I would also question the brand of the coil, as Jeeps are sensitive to that as well.

Good luck!

PS, getting O2 sensor data might be helpful to diagnose this as well is you have access to a scanner with live data

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post #5 of 8 Old 05-09-2021, 04:31 PM Thread Starter
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Thanks, that's interesting. I do not hear any sort of exhaust leak around the header though. I will take a look at that either way.

So, last night I started thinking of carbon build up, as per the bulletin that's floating around. I took it out and drove it like an *******. 3500 RPM, floored it to over 4k RPM, etc. About 20 minutes of high RPM driving.

Today I did the following:

- Cleaned up ground connections, battery to the block, block to the ECU. I used a steel brush, sand paper, etc. They are solid, I measured continuity and resistance.
- Took coil rail off, took spark plugs off, measured the gap, swapped #2 plug to #3 cylinder, just in case. Spark plugs look great, no carbon build up, no wetness of any kind, tan brown color. I measured the coil rail and all 3 coils throw the same value.
- I disconnected and removed the ECU. I checked connector 2 pins #15 and #9, those being injector #2 driver and coil for cylinder #2 driver, and they looked fine, no movement, no softness that would indicate cold solder on the board. I sanded them, I cleaned the connector too. I WD-40'ed all electrical connectors.
- I measured connectivity and resistance from that pin #15 on the ECU to the injector #2 clip. I moved the harness around, I shook several parts of it, etc. Cable seems solid. I did the same with the coil driver, aok too.

Then I decided to do some long distance driving. I went out to pick up a Sea Foam spray and put half the thing through the throttle body. Did about 80 miles at various speeds and RPMs, but mostly driving normally (that means like a grandma in my case). Then did another 80 miles back and put the rest of the Sea Foam can. The car did not misfire at all.

Not that I believe 100% that it's fixed, because this is so random that I may deem it fixed and then it just happens again, but I am having a good feeling about this one.

Can someone with experience in these engines tell me if, given some misfiring due to for example the exhaust valve not closing properly (as per the factory bulletin), would the ECU shut off signal to the injector as to not spray gas on a cylinder that's not firing? That would explain the quick reset I get by turning the engine off and immediately on. Thoughts?
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post #6 of 8 Old 05-10-2021, 10:10 AM
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Newer jeeps will turn off injectors if it senses a missfire to keep from damaging the cats but I don't think these year models do could be wrong though.
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post #7 of 8 Old 05-13-2021, 11:08 PM
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Have you checked the wiring in the loom that passes behind the valve cover? It like to rub through back there and cause a short.

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post #8 of 8 Old 05-14-2021, 10:31 AM Thread Starter
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Originally Posted by Bigrigr View Post
Have you checked the wiring in the loom that passes behind the valve cover? It like to rub through back there and cause a short.
Yeah, looks good though. I even measured resistance between pin 15 of the middle connector at the ECU to the #2 injector, while moving different parts of the harness. It looked ok.

I have to drive it more to be sure, but it appears that this misfire has evolved into a "pending" ECU misfire code on cylinders #3 and #4. This is actually good, because it means there is not something wrong with the #2 cylinder itself.

I swapped spark plugs between #2 and #3, so a bad spark plug could have been the issue. Cylinder #4 I actually took the spark plug off and gapped it at 0.40, so I may have affected it, not sure.

Check engine light isn't on with the "pending" codes, I can only see them with the scanner app. My next move is to swap all spark plugs for NGK ZFR5N (I have the OEM Champions right now) in an attempt to rule out bad plugs. After that I'll replace the coil if things do not change. I was also thinking of wrapping that coil to minimize temperature exposure.
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