I've done my searching online (this forum and others) and have stumped a local mechanic so I am coming to you guys for some help.

I live in Montana but purchased this 1997 Cherokee in Phoenix (drier climate means better condition used vehicles). It has an automatic tranny, AC, and a 4.0.
Besides this overheating issue and a few minor problems like the electric window motors and locks, it is in extremely good shape, especially considering its age and the 304k miles.

The problem first manifested itself while my wife was driving. The "check gauges" light came on, the temperature gauge buried itself in the red and the coolant boiled over. She said the AC stopped working as well, but I have been unable to replicate it.

Since then I have replaced the thermostat, water pump, temperature sensor, radiator, and radiator cap. The truck runs hot in town but doesn't overheat. It only overheats on the highway. Both fans work.

It no longer boils over. If it overheats I can pull off the side of the road and it will almost immediately cool off. The gauge hits around 230, then immediately jumps to 260 when the "check gauges" light comes on. When it cools off it drops from 260 to 230 or less in the space of a second or two. It does not boil over. No fluid comes out of the cap nor does any flow in to the overflow tank.

I have tested the radiator for compression gasses with zero results. I have had the oil changed and it showed no hint of coolant, nor does the coolant show any hint of oil. The coolant does show signs of rust despite being new and the radiator having been replaced. I suspect the heater core will need to be replaced as well but that shouldn't be affecting this.

The lower hose has been checked while the truck was "overheating" and it never collapses. My mechanic was very doubtful about the spring theory anyway. Both hoses were solid while the engine was running.

I have burped the system as well as possible via the temperature sensor on the thermostat housing.

My mechanic checked the temperature at the thermostat housing via infrared sensor and it reports a 30 degree difference from what the gauge shows.

Checking codes shows no results (only a code 12 for the battery being disconnected).

I just had the catalytic converter replaced (it was rattling) and that mechanic said the converter was red hot and suggested I have a mechanic check the fuel/air systems. I'll be having that done this week.

I am also waiting for delivery of a true Mopar temperature sensor (to the tune of $60 ) as I have read that generic sensors can read the temperature incorrectly. That should arrive tomorrow (Monday).

I think I covered everything that has been done so far. If anybody out there can think of something I am missing I would appreciate the help!

 

Did you do a cooling system flush thoroughly before replacing the pump and radiator? With that mileage it's probably about due for one. High combustion temps may also raise the temps a bit too. Not sure if the increase would be significant. If the old cat got melted it's possible. And a dirty water jacket in the block may be cause poor heat transfer at higher rpm. Or when you drained the coolant, changed parts and refilled it may have loosened up some crud inside the block causing partial blockage and not allowing enough flow to keep the temps down at higher speeds.

 

I've had multiple mechanics actually tell me a flush wouldn't do any good, that they have never seen a block need it. Either that, or that they don't have the equipment to flush more than the radiator.

If the sensor replacement fails tomorrow I'll make another attempt at finding a mechanic that can do a power flush of the block.

 

has your mechanic checked the fan clutch? Yes both fans work but the mechanical fan's clutch might not only be holding good enough to spin the fan on local roads but when the engine turns at high rpm's like on the highway, the fan clutch might slip.

 

**** those mechanics haven't seen the rust that comes from a 4.0's cooling system when it is flushed.

 

So the new Mopar sensor reports the same issue as the cheaper model from Autozone. That was $60 wasted.

I could also hear the coolant boiling over into the overflow tank, so it is definitely overheating.

The damned thing runs so damn well right up until it overheats. No hesitation, nothing, just get-up-and-go.

Something I did notice is that once I shut it down and started checking it over, as it was still boiling into the overflow, I could grab the clutch fan and easily spin it. It should have been engaged at that point, and extremely stiff, shouldn't it?

I don't think that will fix my issue as the cool night air being rammed into the radiator at 75 mph should have been enough to cool it, especially once the electric fan kicked in (I clearly heard it kick in when I reached the first off ramp). Still, I shouldn't have been able to spin it so easily with the very hot radiator right next to it, right?

 

Replace the fan clutch. Do you have a fan shroud in place?

 

Yes to the shroud.

I replaced the clutch but it is still overheating. I'm out of options so it is going to a mechanic now.

 

Did the coolant boil over after you swapped out the fan clutch? I beginning to wonder if your gauge is messed up since you mentioned in your original post that the difference in temp between the T stat housing and gauge was 30 degrees. The XJ's tend to run around 205-210 degrees normally so if they gauge is off by 30 degrees it could make you think that the vehicle is overheating.

Which coolant sensor did you replace, the one in the t-stat housing or the one in the back of the cylinder head?

The T-stat sensor is for the PCM and the sensor on the rear of the cylinder head goes to the temp gauge.

this is a YJ but same engine

 

What radiator did you get ? How many rows is your radiator and how thick is the core ? it sounds like your radiator is to small ( not enough rows or core to thin )

 

There is no sensor on the cylinder head on my 97. I think that disappeared after 96? The only sensor I am aware of is the one on the thermostat and I just replaced it for the second time with a Mopar sensor without success.

The gauge reaches between the first and second mark past 210 then the "check gauges" light comes on and the gauge pegs out to 260. Today I actually saw it peg out, then as I pulled onto the off ramp it fell back to the second mark for a few seconds, then pegged out again for a minute, then fell back to the second mark again and stayed there for the remainder of my drive home.

It is really weird.

It is a stock replacement from Quadratec, I believe it is this one.

 

Read the FSM regarding how the temperature gauge works in the cabin. It doesn't work like most people would intuitively figure. They don't even really call it a temperature gauge, because it is not indicating temperature measurements. It has 3 "categories" if you will...normal operating temperature, high temperature, and critical temperature. The sensors feed the ECU/PCM with actual measurements, but the ECU/PCM is responsible for setting the "temperature gauge" via messages (their words). When the operating temperature is normal the gauge will be positioned at its normal operating location (around 210), when the temp is high, the gauge will be positioned in the red, and when the temp is critical the gauge is commanded to go to its fullest extent to the right.

This is why you see the gauge making huge leaps in a 1-2 seconds. Because it is not a measurement of temperature, but the reflection of what could better be called "temperature status" from the computer.

If I recall, the FSM doesn't even specifically state what temperatures the 3 categories are bounded by.

So you may still be running hot or overheating, but really the temperature gauge is not useful in troubleshooting and should be left out of it. Use a IR thermometer on the tstat housing to find out engine coolant temperature. Then you will know for sure.

You can also use your OBD2 system to read temperature data more directly from the ECU (It stays fairly consistent with a IR thermometer measurement being taken at the tstat housing).

 

This shouldn't be dismissed. I wouldn't dig to deep or go throwing any money at the overheat issue until this is addressed.

 

Compression test? Just for funzies?

 

The mechanics couldn't find anything wrong with the cooling system so they looked for other issues. Putting the XJ in drive, keeping their foot on the brake, and pushing the gas resulted in a drop in oil pressure. This indicates bad main bearings and could cause other issues as well, including the overheating. I work overseas otherwise I would repair the Jeep. I'm only home on leave. So tomorrow I will be listing the Jeep on Craigslist for a significant loss. If any of you are near enough to Bozeman, MT, and need an XJ that is in excellent condition except for the engine, come get it. I'm losing about $5K on it.