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Old 01-08-2012, 09:41 PM
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Unhappy 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

First things first, this is my first post on JeepGarage, so please bear with me.

(Short version at bottom)

Alright, story of my WJ. i bought it off of a lady that had over heated it. The water pump went bad and she kept putting water in it. this caused the block to warp, radiator to crack, and warped the head/valves. we had the valve job done, the head machined, then it sat. for 3 years. we never made progress on it. recently, i decided to sell my truck and decided this Jeep was going to be my next vehicle. I pulled the motor, sent it to the machine shop. this cost me $715 in mill work. the rebuild kit was $525 with cam, lifters, etc. Valve job was... i think $150 (dont remember). I got the motor rebuilt, dropped it, everything was good to go. we had started the motor right then, and it ran beautifully, after indexing the cam sensor which we had forgot to do. Keep in mind this had 3 year old gas in it. we came back to the garage the next day, started it, and BOOM! dead cylinder. i put new gas in it, cleaned the coil pack grounds, where it grounds to the block (since they painted the block) and it got just a little better. today, we unplugged the injectors (which Ohm out at about 13.7 ohms) and cylinders 4 and 5 made no difference when unplugged. when idling, it is really rough. above idle (above 1000rpm) it runs fine. well, great i should say. we took the brand new spark plugs out, and cylinders 1, 2, and 3 are like new. however, the 3 towards the back, 4, 5, and 6 were covered in black soot. what do i do? we made sure coil has spark on all cylinders (kinda weak spark, but still spark) and we made sure the injectors have fuel. Snap-On OBDII scanner says right bank is running lean. however, Snap-On scanner says both upstream and downstream o2 sensors are fluctuating from .2 up to .9 in voltage.

IN SHORT:

WJ 4.0L runs rough at idle, above idle it is fine. have spark, have fuel, back 3 (4, 5, and 6 cylinders) foul plugs with soot. maybe one of those damn Pre-Cats is plugged?


ANY help is GREATLY appreciated!
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Old 01-09-2012, 02:43 AM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

Seems when i see idle issues (not 4.0 specific) I think vacuum leak, volumetric efficiency (low compression static and or dynamic... excessive leak down), I have seen Idle concerns caused by what I could only assume would be a valve guide or seat concern... with an overheat and recent work, not that I have really heard of this but maybe after a warm up and cool down cycle maybe it allowed a poorly installed valve seat to drop. Ultimately in theory you only need one dead hole to throw the rest of the bank out of whack... since your adaptive numerator has likely not been learned yet (numerous hard decel's) the pcm is not seeing that it is misfiring all it has to work with is O2 sensor information and if one dead hole is throwing that bank out (rear 3 cylinders iirc) then again in theory it could cause major misfire concerns in other cylinders in the bank DUMPING fuel into the cylinders to even out the "lean".

Ultimately if you have verified spark and fuel you need compression and timing yet, Naturally with one cam and crank with 3 good cylinders timing would be safe to say is good... I would not read into the lean code too much, misfires will cause an false lean reading because combustion doesn't occur to convert O into CO, NO and H2O (along with other incomplete combustion forms) so its turned into an oxygen pump tricking your O2 sensors into thinking its running lean.

I would get a compression tester on those cylinders, a leak down test would be wonderful!
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Old 01-09-2012, 04:22 PM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

Quote:
Originally Posted by 2005JGC View Post
Seems when i see idle issues (not 4.0 specific) I think vacuum leak, volumetric efficiency (low compression static and or dynamic... excessive leak down), I have seen Idle concerns caused by what I could only assume would be a valve guide or seat concern... with an overheat and recent work, not that I have really heard of this but maybe after a warm up and cool down cycle maybe it allowed a poorly installed valve seat to drop. Ultimately in theory you only need one dead hole to throw the rest of the bank out of whack... since your adaptive numerator has likely not been learned yet (numerous hard decel's) the pcm is not seeing that it is misfiring all it has to work with is O2 sensor information and if one dead hole is throwing that bank out (rear 3 cylinders iirc) then again in theory it could cause major misfire concerns in other cylinders in the bank DUMPING fuel into the cylinders to even out the "lean".

Ultimately if you have verified spark and fuel you need compression and timing yet, Naturally with one cam and crank with 3 good cylinders timing would be safe to say is good... I would not read into the lean code too much, misfires will cause an false lean reading because combustion doesn't occur to convert O into CO, NO and H2O (along with other incomplete combustion forms) so its turned into an oxygen pump tricking your O2 sensors into thinking its running lean.

I would get a compression tester on those cylinders, a leak down test would be wonderful!

thanks for the fast reply! i had tried a compression test, found that all cylinders were good, except 5&6. they were low, 6 was about 30 lbs of compression... leakdown test was successful. very very little air leaking out, and not from intake or exhaust... broken compression ring maybe?

also... the gauge i had... was crap. but it was the only one i got. the schrader (spelling?) valve was missing so it was hard to tell. before i get too crazy, im going to get a better gauge from the Snap-On guy, and see what that says. ive checked the pushrods for those cylinders seeing as how they in the motor when they were overheated. thinking maybe they would be bent.
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Old 01-09-2012, 04:49 PM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

Coil?
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Old 01-09-2012, 06:10 PM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

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Originally Posted by Sqall12001 View Post
Coil?
I believe there are 3 coils on the 4.0L. One for 1&2, one for 3&4, and one for 5&6. I would think that if it was a bad coil it would be bad on both cylinders for that coil (meaning 3, 4, 5, & 6).
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Old 01-09-2012, 07:02 PM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

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Originally Posted by bsmith0612 View Post
thanks for the fast reply! i had tried a compression test, found that all cylinders were good, except 5&6. they were low, 6 was about 30 lbs of compression... leakdown test was successful. very very little air leaking out, and not from intake or exhaust... broken compression ring maybe?

also... the gauge i had... was crap. but it was the only one i got. the schrader (spelling?) valve was missing so it was hard to tell. before i get too crazy, im going to get a better gauge from the Snap-On guy, and see what that says. ive checked the pushrods for those cylinders seeing as how they in the motor when they were overheated. thinking maybe they would be bent.
The schrader valve is the same one used in a car valve stem if you can find one of those lyin around. low compression your looking at a mechanical failure, also consider if one of the push rods didnt seat in the rocker it wont open the valve and you will have low compression (mainly if its an intake valve you would have little compression as it would suck a vacuum instead of air so when the piston comes back up there is no air to compress).

With that compression it really is sounding like your going to need to pull it back apart. I would try to dump a teaspoon of oil down into the low cylinders and see if that affects the compression.
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Old 01-10-2012, 02:11 AM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

Coil has spark on all cylinders, i pulled the plugs, and the coil, grounded them to the valve cover, and they all sparked. also, injectors all sprayed.

At higher RPM's if i plug and unplug the number six injector, it will make a little difference. at idle, it does nothing.


ive tried oil to seal the rings, it did help a little. no air is escaping through exhaust or intake valves. i got a brand new gauge from work, and by gauge i mean compression tester. im going to try to do the compression test again and make sure, but if i still have low compression i guess i am going to drain oil and antifreeze, drop the oil pan, take the head off, pull the number six piston and rod out and inspect it. i hope its not a broken ring and it scored the cylinder wall... then i need to have it bored another ten over, totaling 30 over :/
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Old 01-10-2012, 03:21 AM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

Stuck open egr maybe. I'm just throwing out some random guesses btw, lol. Educated but still a bit random
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Old 01-10-2012, 03:35 AM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

no egr valve on the 4.0, but it does fit the symptoms.
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Old 01-10-2012, 08:13 PM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

Quote:
Originally Posted by 2005JGC View Post
no egr valve on the 4.0, but it does fit the symptoms.

Alright, so i used the new compression tester and found #6 has no compression unless theres oil in the cylinder. Cant remember how many pounds though. i pulled the head off, to find that the head that was machined 3 years ago, is messed up. the compression is leaking out of the cylinder and the head is not sealing for the #6 cylinder. I'm taking it to the same machine shop that did the head on our backhoe, and my block. They also machine parts for many many hotrod builders that buy parts from where i work. they are a very reputable machine shop. 2moro morning ill drop it off and see what they say. i believe i found the problem.

no cracked ring as of right now, because the piston does not budge in the cylinder.
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Old 01-10-2012, 08:19 PM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

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Originally Posted by bsmith0612 View Post
2moro morning ill drop it off and see what they say. i believe i found the problem.

no cracked ring as of right now, because the piston does not budge in the cylinder.
Good to see you might have finally found the problem plaguing you. Best of luck to you.
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Old 01-10-2012, 08:29 PM
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Re: 99 WJ 4.0L Rough Idle, fouls plugs in last 3 Cylinders

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Originally Posted by Reichminister Goebbels View Post
Good to see you might have finally found the problem plaguing you. Best of luck to you.
Thank you

I sincerely appreciate all of the help and responses from anyone and everyone, even the guy that said EGR :P lol thanks to all.
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