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kbrown 02-14-2012 12:35 PM

WJ Cylinder 4 misfire mystery.
about a year ago i bought a 2000 grand cherokee 4.7 for a fair deal, or so i thought. knowing it had a "blown head gasket" i proceeded to purchase the vehicle. After pulling the heads off the passenger side head was done for, so instead of taking risk buying a new head and putting it back together i went ahead and ordered a new engine(assembled), 0 miles so i felt that it would be a smooth problem free vehicle engine wise for a while. well i was wrong. after getting the engine in it ran AMAZING i loved it and for what i paid i couldn't have been happier. a few days later i took the jeep on a road trip (180miles) to see a show about two hours away all highway, constant 70mph not beating on it because it was new.
on my way back i had a check engine light come on, i was so close to home (5 minutes away) i just went on home scanned the code. it came up as multiple cylinder misfires. after clearing the codes it gave me a cylinder 4 misfire code. i assumed coil pack and/or spark plugs. replaced the coil pack and plugs, it was still there. so we started playing around with it. moving injectors and coils to see if the code would follow, nope stayed on cylinder 4. after brainstorming, we proceeded to test the connections to the coil and it was receiving power. now i have a jeep with a cylinder 4 misfire and rough idle. with no other vehicle my only option was to drive it. (i'm a student and i work no other options) i found this forum and I've been on here as a guest about 100 times looking for answers via search. i found it could be the crank position sensor or cam position sensor. replaced both with autozone parts. no go still a misfire on cylinder 4 and it was the ONLY code. after researching more and more i found jeeps like oem sensors. well off to the dealership it was, two new oem sensors and still a misfire. now the only conclusion i could come to was pcm. so online i ordered one brand new. hooked it all up and start it up, still a misfire! i take it for a ride and it was misfiring sooooo bad, way worse than the original misfire. bring it down reset it and it was back to a cylinder 4 misfire, did a compression test and it checked out perfect, compression test and leak down test were perfect as well. fuel pressure test was on the money. i ran a direct route from the pcm to the cylinder 4 injector and from the pcm to the coil and it still misfired. the code does not follow the injector nor does it follow the coil. ive replaced spark plugs muliple times. i've watch cylinder 4 spark, i have also watched cylinder 4 shoot fuel into the cylinder. all grounds are good i've cleaned them multiple time hoping for something to work. there is also a new coil on it and a new injector.

current codes: cylinder 4 misfire

randomly the check engine light will go off but it still has the rough idle.

the only modification is K&N cold air intake.

the only possibilities i can come up with:
grounds (free) not leaning towards this though.
however if one was bad i would think it would have a code for the faulty sensor. knowing now that jeeps like oem parts, the cost to replace all of this is $$$ and being a student im not sure if i want to risk it and it still misfire.

anyone who can contribute information besides "search" thank you in advance!

Scottina06 02-14-2012 02:37 PM

Re: WJ Cylinder 4 misfire mystery.
this is usually a symptom of a rocker arm coming loose or coming off. Was the original plug from that cylinder black? If so...that rocker arm is probably came off. Its a simple fix if you have any auto knowledge. You will need a miller tool #8387 or 8516. You need this to help you compress the spring and insert the rocker arm. The shop will charge you $650 to do this and that's the usual rip off dealer. I suggest getting a haynes or equiv. The parts are $20 for the arm and you might as well replace the lash adjuster too.

Escape 02-14-2012 03:29 PM

Re: WJ Cylinder 4 misfire mystery.

Also if you can, scope the cylinder to have a look and verify that everything is moving and the valves are functioning properly as you turn it over by hand.

kbrown 02-14-2012 04:12 PM

Re: WJ Cylinder 4 misfire mystery.
Rocker arm is on, it runs lean I'll have to snap some shots of the plugs when I pull them out.

kbrown 02-14-2012 08:24 PM

Re: WJ Cylinder 4 misfire mystery.
Anoy other ideas? There's not a single noise from it either. Weird situation

Escape 02-14-2012 08:38 PM

Re: WJ Cylinder 4 misfire mystery.
Did your new engine come with any warranty?
Where are you may be close to someone who could look at it.

I think you have covered all your bases...make sure to scope it also and then swap everything around 1 more time (coils, injectors, plugs, ect).

Have you tested the engine harness itself, bad wiring?

kbrown 02-15-2012 10:38 AM

Re: WJ Cylinder 4 misfire mystery.
located in Connecticut just south of Rhode island.

i have checked all wiring, the pcm communicates with the injectors and coils because if i pull the plug on either of them the code gets picked up. i have taken a wire and tapped into the harness right where the harness plugs into the pcm and ran it directly to the coil, i have also done the same with injectors to assure it is not being interfered with by a broken wire. i've swapped everything about 10-15 times no lie. i've been on this hunt for over a year.
i do have a warranty for my engine however, they say i need to prove it is bad/malfunctioning. i cannot do so because it passes the compression test and the leak down test. i have no other way of proving it is bad without disassembling it and doing so will void my warranty.

Frango100 02-15-2012 11:13 AM

Re: WJ Cylinder 4 misfire mystery.
I would have a look at the camshaft tone ring for any damage and if its not loose.
I red once on the net a story that the tone ring was wobbling a bit and in that way caused a misreading of a certain piston position.

kbrown 02-15-2012 12:00 PM

Re: WJ Cylinder 4 misfire mystery.
never heard of that, but it cannot hurt to check, ill good this stuff and probably get to it after the weekend.

kbrown 02-23-2012 02:24 PM

Re: WJ Cylinder 4 misfire mystery.
new engine ships out tomorrow

Scottina06 02-23-2012 02:35 PM

Re: WJ Cylinder 4 misfire mystery.
what was the problem

kbrown 02-24-2012 08:21 AM

Re: WJ Cylinder 4 misfire mystery.
not sure honestly, i finally gave up and took it to a shop, the guy is real good and he said its not fuel, and its not in the ignition, its nothing electrical it must be internal so i called the company and told them what was going on and he wanted to send me a new head but i told him a motor is more reasonable and he sent it out

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