BruceB

If you get a bore scope, get the #6 piston to bottom dead center and take a look at the cylinder walls for scoring, I would assume that the valves are sealing well if you have high compression pressure, so that would indicate either the rings on the piston have failed in an unusual way, or have carbon build up so bad that it isn't letting much blow by. If that is the case, then the cylinder wall will look scored differently from the others. In any case, sounds like you are going to have to go into the motor but before that you may want to try the "steam clean" method of cleaning out the cylinder heads..
https://www.cherokeeforum.com/f30/st...-4-0-a-226299/
hope this helps

cruiser54

Doesn't look like #6 plug is firing.

Joshua White

My point was we've got one cylinder with an odd issue while the rest act normal, if it had something other than stock pistons wouldn't the other cylinders behave oddly?

Joshua White

Thanks for the link Bruce, I've got no problem grabbing a scope as I progress down this path of testing and such.

The thing is I jumped on the opportunity to buy another 2wd XJ in much better shape than this one so I've got another Jeep to transfer parts to with a better motor and chassis. So I'm weighing options at this point, but it looks more and more like I'm going to dive into the swap

Joshua White

I happen to have another set lying around that are almost new :-)

Joshua White

This is my thought as well, seems like that cylinder is not firing for some reason. Whether the spark is being blown out, or missing entirely for some reason...it looks like it's just not firing, and the way it runs leads me to that conclusion as well.

bluejeep2001

Seems like I got your pictures mixed up..obviously 6 is the wet one due to not firing...but the compression has nothing to do with it. ..Don't know why its high but unless you have a bad cap or wire it just might be a bad plug..or its firing but not burning due to improper fuel air ratio due to whatever is causing your high compression

ijeeep

Good.

On several occasions we have experienced spark plugs that looked serviceable and bench tests good yet drivability issues persisted.
As part of the diagnostic procedure the coil/wires/etc. were tested and replaced if needed. Visually the plugs were serviceable, they fired blue hot "on the bench". No amount of cleaning & sand blasting would resurrect the plugs. The only cure was replacement.
Our pet theory was and is that under the correct conditions of damp plug and weak spark, one of the many chemicals added to gasoline flash formed a semi conductive ground path between the plug tip and casing.

awg

So lay that slug on the grounded engine and run it. It will be immediately obvious whether its firing and the color of spark give additional clues

alternatively, a spark tester is another very cheap tool

It may or may not be the case, but improper firing of a plug can be greatly influenced by compression and other factors with the combustion chamber at the moment of ignition, (so it may fire externally, but not under "duress")....pointing to a peek inside

Joshua White

I have done this, spark is present. I will do it again and compare to the other plugs then report my findings

awg

have you also tried pulling each spark plug lead, one at a time, and see how engine RPM alters accordingly

In theory, this should isolate (a single) poorly functional cylinder

I use a reflective tape on damper for this, so the RPM count is very accurate

Joshua White

I have, and again cylinder #6 has no noticeable change while all others affect the run condition, It seems to be isolated to that cylinder at this point.

I'm leaning heavily towards just ripping into this 4x4 swap project so I can pull the motor and get it on a stand for a rebuild anyhow

Idunno

Did I miss the post where you swapped plugs and/or injectors with #6 cylinder? You mention no change in idle with #6 injector connector removed so I'm assuming that got swapped out as a first approach to see if the problem followed the injector (or plug).

Joshua White

You didn't miss it, I came here for some brainstorming before I continued down that testing path. Being that I already have another XJ to transfer all the drivetrain into it's a battle of effort/time at this point. Do I spend more time trying to figure this out when IMO we have enough signs pointing to some kind of top end issue or at least a very gummed up cylinder? I'm leaning towards no, I think my time would be better spent ripping these XJs apart and getting them melded into one solid rig with a good amount of extra parts including this motor in question sitting on a stand :-)

BruceB