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Old 05-28-2012, 05:24 PM
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Need Help! 4.7L that can not be fixed!

Hello everyone. Im new to this forum and hoping to get some expert advice. I have a 2005 Grand Cherokee 4X4 with the 4.7L engine. It is setting a code P0300 (poor/mildly rough idle but smooth above and runs perfect above 1000rpms) and nothing seems to correct it. I've had it at three shops and the following has been completed:
1. New OEM spark plugs
2. All new coils
3. Injectors removed and flow tested and inspected (all ohm-out correctly and flow at spec)
4. Crankshaft Sensor replaced
5. Camshaft Sensor replaced
6. All 4 O2 sensors replaced (left upstream showed signs of buring lean/white)
7. Replaced MAP sensor
8. 2 De-Carbon services performed
9. Complression test shows all good and consistant
10. Cylinder leakdown test showed 2-5% leakage for all 8 cylinders
11. Fuel pressure is 58psi
12. Engine vacuum is consistant at 17psi at idle (checked for leaks and found none)
13. EGR has been removed and cleaned. Not plugged up and not leaking and exhaust or vacuum.
14. Jeep dealer just installed a new ECM and now it stalls with new codes (they have it back)

With all the work and tests that have been done, we can see no reason that it should be setting a code P0300 for misfires on a non-specific cylinder. I cant register it until it can pass emissions and this is in my way.

I've read on the internet of only one other case like this and it looks to be still unresolved. Does anyone have any ideas? Thanks in advance.
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Old 05-28-2012, 05:45 PM
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Re: Need Help! 4.7L that can not be fixed!

Wow. That's a lot of stuff done just to clear that code. A friend of mine has the same Misfire code on his '96 Ram and they could/can not figure it out either.

Have you tried using the search feature on the forum using that code as a key word?

If you unplug the battery cables, let sit for 15 minutes, then hook them back up, does the code come back immediately?
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Old 05-28-2012, 06:00 PM
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prob headgasket...... or internal coolant leak..... pull plugs out and put a scope in and check for piston washing

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Old 05-28-2012, 07:30 PM
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What are the new codes? Had anyone ever checked the timing?

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Old 05-28-2012, 08:18 PM
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cam sensor?

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Old 05-28-2012, 08:19 PM
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Re: Need Help! 4.7L that can not be fixed!

The 2 responses above mine do merit investigation, it is possible cam timing is off ever so slightly causing the vehicle to mix some exhaust in with the intake charge. I think it is related to either a very small internal coolant leak or the already mentioned cam timing issue. What is the mileage on said motor??

EDIT- just looked at the original post again, I would check the cam timing for sure, I know it ain't easy or cheap but neither was shelling out all you did already for a vehicle that still ain't fixed. Rough at idle has me thinking cam timing, a coolant leak would have to be pretty large to affect idle quality, some smoke and/or coolant loss would be evident.
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Old 05-28-2012, 09:25 PM
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intake leak? like manifold all it is nylon

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Old 05-29-2012, 12:13 AM
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Re: Need Help! 4.7L that can not be fixed!

Carbon buildup is a huge problem on 4.7's and 3.7's and more times than not a decarb doesnt work. Honestly you may be looking at headwork here... I question the leakdown test as I have had a 2 4.7l's and 1 3.7l that had good leakdown after about 65psi when the pressure in the cylinder clearly seated the valve and leakdown dropped to almost nothing but before about 65psi I had nearly 50% leakage... in this case the fact that there is almost no leakage at 100psi doesnt mean anything because your volumetric effiency is down, at a low warm idle is when this will cause the most problems because the speed of the engine is slow enough that the leakage is magnified, instead of spinning so fast that it builds enough pressure to seal the cylinder quickly it will allow oxygen to escape causing a poor air/fuel mixture. The fact that you have a low engine speed misfire leads me to believe it is a volumetric effiency issue.

Ultimately the 3 mentioned cars got reman cylinder heads, 2 being warranty and 1 being a car for the sales department . I put new valves in 1, swapped lifters, rockers, plugs, coils, and injectors to no avail, I am not sure if it was a valve seat falling out that I didnt catch while I had the valve out, or if there was a valve guide concern, honestly I am not sure the exact cause myself, all of that to say I have seen something nearly identical a couple times and it took a cylinder head to repair the concern... you may be lucky and get a machine shop to pinpoint a failure for you if your not ready to buy reman heads.
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Old 05-29-2012, 05:10 AM
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Re: Need Help! 4.7L that can not be fixed!

dont know anything about az emissions but are dealers authorized to do the em. insp? perhaps they will pass you just based on the amount of problems and money youve spent. i know in md. it used to be if you failed em you had to spend at least 450. to try and correct the problems and if after spending the 450 it was still failing they would grand you a waiver
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Old 05-29-2012, 05:32 AM
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il emissions also worked like that. idk if they still do....I'm in process of putting my 4.7 together due to a had head gasket. while I had the heads off I had them looked over and installed new valve seals. some cleaning resurfacing and welding cost me $500 for just heads...

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Old 05-29-2012, 07:21 AM
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Re: Need Help! 4.7L that can not be fixed!

air filter element?? blocked exhaust pipe, check the end piece
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Old 06-12-2014, 03:02 AM
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Re: Need Help! 4.7L that can not be fixed!

possible valve seat is coming loose. its a common problem on the 3/4.7's, especially if it gets over heated. a leakdown test may not show it with a high loss but it may explain the 2-5% drop mentioned in the OP
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