I just finished the head replacement on my '01 Cherokee @140k because of combustion gasses in the coolant. This was confirmed by a test kit from Oreillys. Coolant loss was found, but was suspected from the radiator as I had replaced the original radiator last May when it failed and subsequently replaced the radiator two additional times because of leaks. Yes, that is three new radiators in about 6 months and now this one will make it four in about 9 months. The three were NAPA brand with lifetime replacement so I only paid for the first one. I suspected bad part quality as the reason for these failures, but now I'm not so sure that's the issue. I have never had an overheating problem with this XJ.





I had heard of electrolysis in the coolant causing radiator failures, but didn't have a reason to suspect it until the third replacement radiator started to leak. I did a lot of research into the issue and tested the coolant with a meter and discovered that I had electrolysis happening. I also discovered that combustion gasses in the coolant contribute greatly to this. In fact when I ran the engine the coolant showed over 1V from battery ground. That means the radiator is acting like a battery and eating the less noble metal, aluminum. I knew that I would be replacing the the 0331 head eventually, but I never suspected that it would be because of leaking radiators.




What I found so surprising is that the engine ran great and had no indication of a blown head gasket, so I assumed it had to be cracks in the head. Before the teardown I performed a compression test (cyl 1 - 155, cyl 2 -175, cyl 3 - 175, cyl 4 - 175, cyl 5 - 155, cyl 6 - 175). Here again I was surprised that cylinders 3 and 4 had great compression and cylinders 1 and 5 were a bit low. After the teardown I examined cylinders 1 and 5 closely, and found some flash rust towards the top of the bores, indicting the presence of a coolant leak. The head gasket was still good, no indications that there were any leaks. The block and head surfaces also checked out good. I wasn't able to find any cracks in the head, but I didn't invest much time in searching as it was going to be replaced anyways.




I went with a Clearwater stock replacement head for $650 including shipping. I got it in about a week from the day I ordered it. It came complete with dowel pins for the intake and a plug for the rear temp sensor (had to add thread sealant). I used a FelPro Permatorque gasket set to seal her up. After reassembly and before starting I did another compression test, The results were about the same as before, so OK. I also installed a new CFS copper and brass radiator from Rockauto ~$250 shipped. I replaced the high pressure power steering hose that has been slobbering for years and installed a new brush set in the alternator. I also discovered that the fan clutch is leaking fluid and have a new one on order.

I primed the fuel system a couple of time before attempting to start. When I stared her up it was just like every other time, popped off immediately. It set a code and was idling at 1k, so I checked the code. It was p0505 IAC system. Reseated the connector and cleared the code, problem solved. Only other issue was a small leak at the radiator from the trans cooler line, tightened the nut a little and that issue was solved. I checked the coolant for electrolysis and it checked good ~ 0.01v.

I've gotten really good at replacing radiators on the XJ, but if you need one replaced I'm not available.

 

Good write up! More knowledge base on the infamous 0331 cylinder head! I had the 99 with a different head, so was never able to enter your club. Now in the 250K range with the new owner!

 

I did a little triage on the old head and discovered something interesting.



#1 intake


#5 intake

 

When the head cracks in the usual spot, no combustion gases get in the coolant. I guess the photos in your most recent post are possible cracks in the exhaust port?

 

They are both intake ports in cylinders 1 amd 5. The large crack is deep, a pick drops into it. I could not visually find any cracks in the combustion chambers or exhaust ports. It's a very strange failure.