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Hole in piston #2.

R

resq229

NAXJA Forum User
Location
Waynesville, NC
I tore the motor down in my 98 2 door XJ and found a hole in piston #2, what or any ideas on what caused this?

I bought it cheap knowing that it had a possible bad motor and the lady that i got it from said she might have ran it hot for a short distance, but would that cause a hole in the center of piston 2? anything else could have caused it?


bustedpiston_zps169aa16c.jpg
 
WOW!
 
Possibly running very lean, and that may have caused the hot temps along with a bad cast on the piston making it weaker in that area to melt. From looks of the pic it looks like it was punctured because of the edges concaving inward but this may be shadows of light. How was the head? Valves all there? Wrist pin still attached to rod? Like I said if the piston is melted not punctured then she might have had an injector leaned out. Which would also cause higher temps!
 
It did still run, the head is fine and looks good, valves good too, the wrist pin and rod are still attached to the piston and it moves up and down freely. i really see no melt at all and the edges of the hole seem sharp like something was in the cylinder that caused the puncture.

The tops of the valves and combustion chamber have tiny nick marks where it chewed up i assume bits of broken piston, but really the head still looks usable.
 
does it look like the metal was soft when it broke? Is the dirt on the piston sorta cracked like slag or old paint? Edge of the hole sagged/stretched downward or a clean break/fracture?
 
Pull the valves out of the head from that cylinder--one may have stuck "open" and taken out the piston, usually caused by a bad valve guide.

There is a TSB on 99-01 4.0 exhaust valves being slow to close due to carbon build up--what is the production date of your XJ and what is the head casting number on it?
 
Valves seem fine and open and close with no binding, my jeep is a early 99, the head is an 0630, i see no signs of a blown gasket or water in the cylinders.

i see nothing melted at all, basically just looks like something busted thru it? there is nick and scars but i dont know if its was from the piston parts that busted.

better picture.
piston2_zps3cd1b23c.jpg
 
that's really odd. I'd have to see a super closeup of the edge of the hole and not sure I'd be able to tell even then, but it sorta looks like it might have softened and then the compression pressure blasted the semi-melted/softened metal out.

It doesn't line up with a valve, and the 4.0 is non interference anyways isn't it? So I doubt it's a timing issue.
 
Probable cause is "Holy Jeep Gremlins", the only answer that fits this one!

I have one idea, a large chunk of hard carbon got loose top side and fell in through the intake valve, and cracked the piston on compression impact, the aluminum not being as strong as the iron head took the damage, then it failed after multiple combustion cycles.

I found about 2-3 coffee cup fulls of golf ball sized rock hard carbon in the top side of the head on the 89 Renix I bought when I pulled the head! I was shocked!!
 
Too much nitrous, not enough fuel?

Just kidding but to me it looks like something was getting very hot in the middle of the piston and it gave way under compression.
 
The plugs were gone when i bought it, the ladys brother had done a compression test and who knows what he did with the plugs.

The only real clue i have is that she claims she did run it hot for a short distance,, but who knows what her short distance was, she really never said how long or far.
 
There is no sign of melt down or heat stress in the photo, it looks mechanical.

xjtrailrider said:
Too much nitrous, not enough fuel?

Just kidding but to me it looks like something was getting very hot in the middle of the piston and it gave way under compression.
Click to expand...
 
Last edited:
That makes sense.
 
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