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Old 08-13-2019, 11:48 PM
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P0304 at my wits end

Okay everyone need some help here.

06 Rubicon unlimited 4.0 throwing a p0304 cylinder 4 misfire code. This code popped up about 2 years ago life got in the way so the Jeep sat now I'm trying to get things sorted out.

Here's the laundry list of things that I have done in terms of parts replacement trying to chase down this error.

1.Alternator was replaced with brand new
2. Coil rail has been replaced
3. Plugs were replaced with app985. I have moved the cylinder 4 plug to a different location and the misfire does not follow the plug.
4. Fuel injector number 4 was replaced.
5. Fuel rail was removed and wrapped in heat tape to make sure I was not getting a heat soak issue.
6. I was having a problem with it throwing belts and had to replace the harmonic balancer as the rubber had separated and worked its way off the shaft.
7. I have replaced the odpa with a new unit that is supposedly upgraded to avoid squealing monkey issues.
8. I replaced the crank position sensor however I did not replace with a Mopar unit.
9.I did run a can of mopar CCC today sprayed in through the intake letting it hot soak for an hour before restarting

So after all of the above I still get a flashing check engine light randomly at idle. I am at a loss at this point.

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Old 08-14-2019, 07:15 AM   #2
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Pull the valve cover and inspect the rocker arms and valve guides on cylinder 4.
Back trace the fuel injector wire for any problems.
Have you run a compression check on cylinder 4?

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Old 08-14-2019, 08:09 AM   #3
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Agree with performing a compression test.
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Old 08-14-2019, 08:42 AM
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Agree with performing a compression test.
Iíll try to track down a gauge today and get that done in the next day or two
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Old 08-14-2019, 11:25 AM   #5
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A leakdown test would provide more information. Piston rings? Exhaust and/or intake valve(s)?
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Old 08-14-2019, 12:07 PM   #6
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A leakdown test would provide more information. Piston rings? Exhaust and/or intake valve(s)?
That's what I would do, along with a compression test.
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Old 08-14-2019, 06:22 PM
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Dry
Cyl 1 150psi
Cyl 2 150psi
Cyl 3 150psi
Cyl 4 105psi
Cyl 5 150psi
Cyl 6 160psi

Wet
Cyl 1 155psi
Cyl 2 155psi
Cyl 3 165psi
Cyl 4 115psi
Cyl 5 160psi
Cyl 6 175psi

I do not have access to a leak down tester. I did remove the Schrader valve from the compression test hose and introduced shop air to cylinder 4 with it at one I believe was top dead center. I then had air coming out the throttle body.
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Old 08-14-2019, 08:37 PM   #8
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Sounds like a valve related problem on 4.
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Old 08-14-2019, 08:57 PM   #9
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If you happened to be on TDC of the overlap cycle (going from exhaust to intake stroke) you would actually have both valves open slightly. That would cause air to come out the TB & also the exhaust. That would be easy to check. If that turns out to be the case you just need to go around one more revolution to get TDC on the compression stroke & recheck pressurizing the cylinder.

If it is a leaking intake valve it is more than likely carbon build up & you might be able to decarbonize it. Worth trying…
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Old 08-14-2019, 10:22 PM   #10
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Do you hear significant "lifter noise" or piston slap?



Wouldn't a less than normal leak down on cylinder 4 also raise issues about cylinder wear, piston ring condition, or even the possibility of a damaged piston?


I had cylinder 4 error codes for about two years and everyone told me it was related to typical valve noise and lifters before my cylinder 5 piston shattered. We were treating lifters and the problem was piston/cylinder/ring wear.
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Old 08-15-2019, 07:13 AM   #11
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Remove the valve cover and inspect the rockers, springs, guides on 4. You can also test the valve sealing against the head with compressed air by getting the piston to tdc, loosening the rockers and filling the cylinder with compressed air. Since your other cylinders look good I doubt you have a ring problem on 4 because lubrication related issues would most likely occur on more cylinders. My 2001 4.0l had the cracked head with coolant leak that caused the same issue. You don't happen to be loosing coolant do you?
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Old 08-15-2019, 08:17 AM
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Do you hear significant "lifter noise" or piston slap?



Wouldn't a less than normal leak down on cylinder 4 also raise issues about cylinder wear, piston ring condition, or even the possibility of a damaged piston?


I had cylinder 4 error codes for about two years and everyone told me it was related to typical valve noise and lifters before my cylinder 5 piston shattered. We were treating lifters and the problem was piston/cylinder/ring wear.
No lifter noise or piston slap. The only odd noise it makes is when it does actually misfire you hear a little loopy idle
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Old 08-15-2019, 08:18 AM
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Remove the valve cover and inspect the rockers, springs, guides on 4. You can also test the valve sealing against the head with compressed air by getting the piston to tdc, loosening the rockers and filling the cylinder with compressed air. Since your other cylinders look good I doubt you have a ring problem on 4 because lubrication related issues would most likely occur on more cylinders. My 2001 4.0l had the cracked head with coolant leak that caused the same issue. You don't happen to be loosing coolant do you?
I am not losing any coolant. I will pull the valve cover off but Iíll have to pick up a new gasket first so thatís going to have to wait till this weekend
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Old 08-15-2019, 08:22 AM
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I’m going to be honest we’re going wheeling in three weeks on a charity event for autism awareness. I really want to run the LJ instead of taking out my jkur which is not set up as nicely. I guess I can’t see it doing anymore harm to the motor so I’ll probably wheel it and then pull the head when I come home if looking at the valve cover this weekend doesn’t reveal anything.

That opens up a whole can of worms though about sending my head out to be rebuilt our buying a rebuilt one online

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