I'm having issues on my wifes '03 3.7 Auto 4x4 (125K miles) KJ. It's had the CEL of P0306 for a while now... I've changed the Coil Pack (#6), changed plugs, replaced the Pass Valve Cover gasket to fix the oil leak, Checked the engine Compression and all cylinders are w/in spec, replaced busted Vacuum lines on both Cylinder sides to Air Intake ... (I have not changed any of the O2 Sensors, Cat Converters, Fuel Injectors) ... Up until yesterday the CEL was constantly ON. Yesterday it started sputtering when at a stop and the CEL was flashing. She drive it home ~ 2 miles.

When at home I pulled the Throttle Body and Idle Air Control Valve off to clean them and remove all the carbon. I had planned on doing this anyway as recently it started sputtering a bit at idle. Checked all the vacuum lines in the Engine bay and they were all in good shape. I cleared the CEL codes and took it for a drive. It idled much better and ran fine. Only drove it around for about 3 miles and did a lot of sudden stops, quick acceleration, sudden stops type of cycles. All was fine. No stumbling or stuttering. (Side note: CEL came on this AM. Not flashing but a solid CEL)

One other thing I've noticed is that there is an unusual amount of white smoke coming out the tailpipe on the KJ especially when it's cold outside. Not the normal slight white puffs until it warms, but lots of white smoke. She also looses a bit of coolant consistently (~ 1 cup a week) and it's nowhere to be found in the engine bay. No leaks f/the radiator, water pump or thermostat housing. I'm really concerned about a leaking or blown head gasket. Wouldn't that reveal itself on the Compression Test I ran? I'm at a loss... don't want my wife to be driving around in a ticking time bomb.

Any other suggestions? Could there be any of the sensors that are causing this? Should I swap fuel injectors (#6 and #5) to see if the miss-fire code follows to Cylinder #5?, hate to throw more parts at this one but would like to get it fixed. Also I've noticed that there is always a bit of brown pudding like substance in the Oil Fill tube, on the plastic filler plug (not sure what it's called) that is under the Oil Fill Cap. But on checking the engine oil and when doing oil changes the oil does not have this, or I would assume Coolant in the Engine (not good). The old engine oil is just black and oily (as you'd expect).

Thanks...

I had this code on my 07 along with the Multiple cylinder misfire code(P0300 I think). It ended up being that the previous owner put in the wrong spark plugs and then forced the coil packs onto the plugs. This damaged the coil packs but they worked most of the time just fine. CEL would only come on when it would rain outside. Drove me nuts for awhile. Had to put new proper plugs in and replaced the coil packs. It's been working good since. Not sure if that helps you.

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07 KJ Sport 4x4, 3.7, 42RLE, Black, and mostly stock for now.
Westin Tail Light Guards and Gutter Guard Inserts.
Wish list is: Frankenlift II Premium, 245/75R16 tires, Al's UCA,
Skink Super Sliders, Full set of Skidplates, ARB Bumper, Warn 8000,
ARB lockers, and I'm sure that I'll come up with more...

Thanks for the info. I replaced the plugs a few months back and even tried replacing the #6 Cylinder Coil Pack and it didn't change. I'm going to buy a new set of plugs and before swapping them out do a thorough SeaFoam cleaning. I'm wondering if I have a stuck valve or possible broken valve spring. I would think however if I had a broken valve spring the car wouldn't run, or would really run horrible... and it's not doing that. It does have a bit of a stumble when idling, but runs ok.

When I did do my plug replacement the only plugs I could find w/platinum and I'd prefer some good NGK V plugs... copper and not platinum, but we'll see what I can find.

had this problem on an 05 company owned liberty. TSB for valve retainer things, changed those, no fix. Mechanics went through ti and said that everything was fine, only thing that could be wrong would be the cam. Brought to stealership, it was the came, cheap booty pressed on lobes onto a thin steel tube, must have rotated. it was like 6k out of warranty!! It ended up clogging a cat so they replaced cat under warranty, but the cause of the problem, the poopy cam that tweaked, was not covered and we had to pay. very lame, IMO. owner does not like jeep anymore and started buying total POS subarus now instead of jeeps!!! jeep should have paid in full, IMO. I beat the bag out of my 4.0Ls, they have way more miles, and they never have problems as long as you don't redline them into a water/mud hole.

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Civilisation exists by geological consent, subject to change without notice. -Durant

Update... Had a friend over who is a much better mechanic than me w a lot more high$ diagnostic tools. His MAC Scantool brought up codes showing both fr/rr pass O2s & Cat Con toast. While running a vacuum check the engine would barely hold 13 lbs of vacuum and the gauge needle was bouncing all over the plac (Not Good). When I started the KJ up it was knocking really hard (this had not been happening prior to this) & after about 5 minute of idling while running diagnostics, I think it dropped a valve. Started banging hard everytime the #6 cylinder would fire & did that about 8-10 times b4 I could get her shut off. I'm thinking major rebuild or salvage motor. Only 3.7 I could find yesterday they want $2K for a 103k mile running motor. Don't think I want to go that route & I don't know anyone who'd be willing/able to rework this motor for a price that's doable. Don't want to invest $3-4K into an engine on a vehicle that bluebooks for ~$8.2K. Rest of her KJ is darn near flawless.