We have a '92 two-wheel drive 4.0L Jeep Cherokee four door with 235,000 miles on the odometer. We purchased the vehicle in the year 2000 from the original owners. I have noticed the last few months the coolant temperature gauge wanting to climb past the normal midway mark on the dash. Then one day after a twenty minute drive it finally decided to boil over (the needle almost pegged and then about two minutes after shutdown the coolant boiled out past the overflow bottle).

First off I decided to replace the thermostat, which I did along with the sensor in the thermostat housing. But funny thing is, when I removed the housing I found the thermostat was missing! To make a long story short, I found the vehicle would idle for 30 minutes or more and not overheat, but after a 15 minute drive the coolant temperature passed the midway mark again and almost hit the red zone before i returned home. So I did a compression test, expecting to find two adjacent cylinders low (indicating a blown head gasket or worse). All six spark plugs looked pretty normal (brownish//tan) which surprised me. Well, not really, because the motor seems to run just fine, both at idle and on the highway.

So the compression test results were (front to back): 150, 155, 150, 145, 130, 150. That does not indicate much of a problem so I ordered a combustion gas leak detector kit online.

I brought the coolant temperature up to normal (where the electric fan actually kicked on) and placed the fluid tester over the overflow bottle filler. After 2-3 minutes, the test liquid was still pretty blue (see photo). I was surprised again not to find it turning yellow pretty darn quickly.

I now plan to drive the vehicle for 15 minutes and repeat the combustion gas test. Being the fact the vehicle does not overheat at idle, maybe the test will not show valid results until I perform the same procedure after a brief highway cruise. To be continued ...

 

Update: Forgot to mention in my first post that the upper and lower radiator hoses were replaced just in case one or both hoses were collapsing. This change did not improve anything.

Now this AM I had to drive 40 minutes before the coolant temperature needle approached the red zone. I noticed it took 15 minutes of driving to reach the middle 210 mark. Then after a total of 25 minutes I pulled over and checked with the combustion leak fluid at the overflow bottle with the engine idling and the electric fan now operating. The fluid remained blue.

I drove another five minutes and noticed the temperature gauge needle finally began to approach the red zone. After another five minute drive (about 55 mph) the needle was just shy of the red zone and I pulled over and stopped. As soon as I put the gear selector in park and with the engine still idling, the coolant pressure overcame the radiator cap and things began to boil over onto to ground. I tried to test the boil over vapors with the fluid, but it was not easy. My attempts at this time either valid or invalid did not provoke a change in the blue fluid. I then went across the street to purchase coffee and a burrito and waited an hour for things to cool down. I did not replace any of the spilled fluid, started the engine ,and drove five minutes home. At home I checked again with the fluid over the overflow bottle at idle and the fluid remained blue. The electric fan was still on at this time.

I let the motor idle a few more minutes and when I saw the gauge needle begin to rise once againt above the 210 mark (and with the fan on) I decided to raise the idle to 2,000 rpm to see if that would cool things down. After about 2 minutes at the higher rpm it seemed instead of cooling down further, the temperature gauge still was climbing slowly toward the red zone. I then aborted further testing and shut the motor off. End of story for today. Photo is the fluid color after the first 25 minutes of the drive.

 

Update: Expected a fluid color change to yellow, but actually turned green instead. I didn't realize I have a dual chamber tester. I believe I should have filled both upper and lower sections with test fluid for more accurate results. I might have to perform another test using that method.

 

Update: Performed the test again using both upper and lower sections which both turned green. Now it's time to remove the cylinder head and find out what is going on inside.

 

Blue to green MAY indicate a slight mixing of combustion gasses..don't know for sure. Since your jeep came with no thermostat...the previous owner obviously had issues with overheating. The older heads are pretty robust. I would be more inclined to replace the radiator and the fan clutch.before ripping into the head. Do you see mush when you look into oil filler cap. Simple clutch test..have someone run engine then shut off. The fan should stops pinning almost instantly . Just thinking....if you had the truck since 2000 and didn't know it had no thermostat then the radiator,water pump fan clutch and everything else is 25 years old....Time for some replacement maintenance

 

I pulled the cylinder head and at first glance I don't see any obvious defects. It took several days for the disassembly, so I'm not sure if that slight rust color in the middle cylinders was during driving operations or happened when I loosened the intake manifold a few days ago. The casting number on the head is 7120.

 

Update: Let me just mention I don't find any locating pins to orient the cylinder head properly on the block. I don't see a hole for them either (or else I am mistaken).

 

yes your block and head castings seem a little different from my 98 series. You could always get the cheapest bolts..4 of them, that thread into the block and cut them so they stick out about an inch. Then cut a slot in the body of the bolt shank for a screwdriver to unscrew them once your head is aligned. You should chamfer and smooth the shanks so the head slides on easy with no scratching. You might also need a magnet to pull out once fully unthreaded. If you make them too long the head will not fit between the firewall overhang. If you are pulling the motor out completely then the bolts can be as long as you like. I say buy cheap bolts because they are easier to cut than old headbolts.