Saw this and decided to shut down for the night before it got too involved ..i know it's the reason for the miss fire codes, bit what caused only 3 outa 6 to get that much build up? First 3 were burnt amd last 3 were good? ?First burnt one starts at the front and works backwards towards cabin

 

My first thoughts are leaking valve guides or worn rings allowing oil to get by in those cylinders. I would start with a compression test and a leak down test. Post the results when you are done. Hopefully I'm wrong and one of the 4.0 experts will chime in soon with more suggestions.

 

Yup, compression test is the place to start.

The spec. for the 4.0 is 120-150, with no more than a 30 psi variation between cylinders.

Perform a dry test, then a wet test on any cylinders that show out of spec compression.

Post the results here for comment.

 

Can someone explain the difference between a wet and dry compression test?

 

Dry, just hook up gauge and turn over, see what you got.
Wet, pour tiny amount of oil into spark plug hole (cylinder)and hook up gauge and turn over again..see what the difference is..am i right guys?

 

Yes, sir!

 

+1. If the compression reading comes up substantially after adding a bit of oil to the cylinder, the rings are suspect. The oil helps the rings seal temporarily.

 

Just finished the dry test and they ranged from 155psi to 170psi...bout to do wet if i can jusy find a way to cleanly get the oil in there

 

Dry
1-160
2-170
3-170
4-155
5-160
6-170

Wet
1-185
2-195
3-210
4-180
5-180
6-190

 

Those readings look fine.
What mfgr/part# are those plugs? Single platinums by chance? How many miles on them?
Your waste spark ignition fires plugs as pairs: 1&6, 3&4, 2&5, single plats wear three plugs faster than the other three because half of them fire reverse polarity. Just tossing it out there.
I'd screw in fresh set of NGK and see what happens. Make sure the coils are seated right, making good contact, no cracks or arc marks where they contact the plugs..

 

What codes did you get? Can you post the exact code(s) here?

Can't see the plugs too well because the pic is so small, but are the plugs "burnt" or are they sooty fouled?

If they're sooty/fouled....

With a split exhaust (Catalytic Converters and O2S's) you may be getting a lean signal to the PCM on that bank cylinders and the PCM is adjusting the injector pulse width to the rich side to compensate, thus fouling out the plugs.

 

Dont know how many miles on them but they are champion (RC12PEC5)..just installed some more champion copper plugs though for now

 

Very first i was getting P1494, found dry rotted hoses and got that one cleared..drove it around the block a few times and got P0171, cleared it and drove around the block again, thats when i got p0300,p0301,p0302,p0171..cleared and did further troubleshooting, thats when i pulled the plugs and made this post..yesterday did compression test and installed new plugs..everything is running smoother for the most part. Not perfect like I'd like..p0171 came back (no cel though if that means anything) and that's when i did more searching and found a few post you made about the IAC and O2 sensors..pull 1-3 up stream sensor and its got a little bit of caked on carbon




I know you've had some problems with my picture(been posting from phone) but the picture is of just the head of the sensor..i know its first in line in the exhaust but isn't this even a little too dirty for a sensor? And i will pull the IAC when i get some MAF cleaner amd a box of q-tips later on today

 

P0171-1/1 FUEL SYSTEM LEAN

When Monitored: With the engine running in closed loop mode, the ambient/battery
temperature above 20 deg. F and altitude below 8000 ft.
Set Condition: If the PCM multiplies short term compensation by long term adaptive and a certain percentage is exceeded for two trips, a freeze frame is stored, the MIL illuminates and a trouble code is stored.

Possible Causes:

INTERMITTENT CONDITION
RESTRICTED FUEL SUPPLY LINE
FUEL PUMP INLET STRAINER PLUGGED
FUEL PUMP MODULE
O2 SENSOR
O2 SENSOR SIGNAL CIRCUIT SHORTED TO GROUND
O2 SENSOR HEATER OPERATION
TPS VOLTAGE GREATER THAN 0.92 VOLTS WITH THROTTLE CLOSED
TP SENSOR SWEEP
MAP SENSOR OPERATION
ECT SENSOR OPERATION
ENGINE MECHANICAL PROBLEM
FUEL FILTER/PRESSURE REGULATOR (HIGH)
PCM

Most of these possible causes are for a lean condition across the board but yours is limited to 1/1 so I would go ahead and replace that 1/1 O2S and see what happens.

You can see on that sooty O2S that the PCM is dumping fuel in 1-3 to compensate for a lean signal.

Here's a pic of your exhaust with the NTK O2S part numbers.

Edit: Do not mix Bosch O2S with NTK O2S.

 

Damn Ken, you're a smart guy! You are the guru of post RENIX XJs. Between what you and Cruiser provide there is no reason why members should have a crappy running Jeep.