the issue is around all 1999-2001 4.0L's that used 0331 non-TUPY revision heads. that's XJs, WJ's and TJ's.
The company that made the heads for Jeep didn't use enough nickel in the iron casting, so the metal turned out to have ductility problems.
The head usually starts cracking around cylinder 3-4 area, and you can't tell if its cracked unless you pull the valve cover off. If you think about it, as the head heats up during combustion, then cools down when you park it outside at -20°F, the metal expands and contracts. You do this thousands of times over a few decades and you've got a whole bunch of expand/contract cycles. That's the cycling that causes a head that isn't ductile enough to crack.
TUPY revision heads fix the issue. If you crawl on top of the motor, take the fill cap off the valve cover, and look directly down with a flashlight, you want to see "TUPY" cast in the cylinder head. If it's a 00-01 TJ and it doesn't have that marking, it's a "bad" 0331 head.
It's also worth noting not all non-TUPY 0331 heads crack. Some are just fine, some a ticking time bomb. it all depends on the thickness of the casting in the problem areas...and theres really know way to test it...so it's just best to avoid the potential issue.
Or to avoid pulling the valve cover, just remove the oil cap and shine a flashlight down in it and look. The only good thing about the weak spot is it can be seen from the cap.
The head just cracked on my sons Cherokee about 2 weeks ago. I had checked it a few times in the past and it was fine but after driving it and it got a little hot on me that day I pulled the cap as it was running and could see water bubbling from the crack as the engine was running.
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I heard that cherokee's had problems with the I6 heads? something about they warped and were not that reliable.
Does the TJ's have the same problem with the I 6 engine?
The #0331 head did have a slight tendency to crack sometime around 130-180Kmiles. This is the "early" head, through mid-2001 - the later head with the "TUPY" foundry mark is revised to correct this.
The crack, if it happens, can be seen merely by removing the oil fill cap and looking in - it usually cracked on the upper surface, between the #3 and #4 cylinders. I am not informed as to how well stop-drilling and welding works.
The "TUPY" mark may be found in the same location, it will be raised from the surface of the head.
The cylinder head casting ID number can be found outside the valve cover, top surface of the head, above and between the #3 and #4 exhaust ports.
The TUPY 0331 head is a direct replacement for the earlier 0331 head. Also, the 0630 and 7120 may be adapted to fit - the intake ports did not move or change very much, but the exhaust ports did reduce in size with each change. An adapter plate would be needed - use min. 1/8" thick steel (mild steel okeh,) and two exhaust gaskets - the one correct for the head goes on the head side, the one correct for the manifold on the manifold side. Open up the plate with ports matching the later gasket with the smaller ports. Use screws 1/4" longer, get flat washers to take up the difference (retain and reuse the bowl-shaped washers directly against the manifold flanges, or replace those with true Belleville spring washers - they're there for a reason!)
The reason for this is the reduction in port size - you can actually end up seeing inside the port if you put the later exhaust on the earlier head, as the primaries won't fully cover the port.
How do I know if the head is cracked? What would be a good indication that it is cracked?
White smoke out of the tail pipe! running rough!
or is there something else that is a telltale on it?
On my jeep, the exhaust was fine. When the heads crack on the 4.0s, the coolant doesn't get into the combustion chamber, so you won't see any changes in the exhaust.
The symptoms for mine were:
1. Mysteriously vanishing coolant.
2. Milky-looking oil under the valve cover and on the underside of the oil fill cap.
3. Low oil pressure
4. Clacking noises that I assume were coming from the lifters. Also, I assume the bearings in my motor were shot due to all the coolant mixing with the oil and the low oil pressure.
Here's what my leak looked like. If you look closely, the crack resembles an upside down "U".
OK, If I find a TJ with this crack in the head, what does it cost to replace it? Lets say that I replace the head myself. and then I have a dealer fix it. I am looking for negotiations on a bad TJ.....in case I find one.
and how long do you have before you really need to replace the head once it goes south.
With 20 years in the US Navy, and nine+ years underwater on Nuc Subs.