1990 XJ
RIP Skuggan

After months of diagnosing knocking and mucking about with flexplates and harmonic balancers I finally got around to borrowing a boroscope.
Sadly this means goodbye.
Cylinder 1- steam cleaned. ok, this is going great....
Cylinder 2-5 severe carbon built up on pistons, black coatings and chunks of carbon fluff in some places- ok, that can be cleaned out
Cylinder 6- WHAT? piston chewed up from the top- like a half moon shaped piece of the top has been worn off
Cylinder wall and side of cylinder was real rough where small chunks have been grindin against the wall I guess

I´m curious to know what can have caused this? What do you think happened?

All I know is that PO put way too long spark plugs in there, noticed that when I changed them.
Jeep was running but rough and with a knock that dissapeared when revving
It had been sitting alot recent 10 years or so.

(sorry bout the bad pics but boroscope was not that up to date and I didnt have my SD card in my pocket




The horror!





Not supposed to have graffiti on the cylinder wall I guess

 

Its a bit hard to make sense of still images, but I'll take your word that the cylinder 6 piston is missing a piece.

My first thought is fuel injectors. #6 may have had significant blockage leading to a clean condition and detonation.

 

thanks for your help!

yes it’s difficult- it looks like the pictures you get when you’re about to have a baby. But you would be more motivated to decipher your baby pictures than these from the inside of some random 4.0 guys motor-makes total sense!

so ”missing a piece” is a bit misleading- it’s the quench pad that has been worn off/down. It’s clear that the part of the piston that sits highest has been hitting something. Looking closer I can also see straight scratch marks across the top of the piston leading up to what is supposed to be the ridge of the quench pad.

Since it has a knock-could it be a worn rod bearing causing the piston to tilt and slam/grind against the head? Is this common?

will have a look inside the oil pan today for chunks of skirts etc. An engine rebuild is unfortunately not an option for me but I’m still curious to know what’s going on.