I have a 98 XJ 4.0l AW4 for context. The water pump, fan clutch, thermostat, thermostat housing and radiator hoses are all less than a year old. All summer my jeep has stayed at or below 210 except when climbing difficult hills while off roading where it would get to maybe 215 or so. When off roading I would have to turn on my av to force my electric fan to come on but that’s been like that for the past year. About two weeks ago I noticed that suddenly was running hotter on the highway than it was the day before. It would climb over 210 a little bit. I went off roading that week and it seemed to be running hotter in all scenarios (idling, on the highway and on the trail). Not overheating but running maybe a half a bar to a bar hotter than normal. Today I started it to leave work and then was talking for a while and when I got in it I noticed that it was idling at around 220, which I’ve never seen it do before. It basically stayed there until I got on the highway where it dropped a little bit but was still higher than the past few days. Once I got off the highway it when up to the bar above 210 (according to my calculations that’s 222.5). I got concerned and was pulling off into a parking lot when it quickly climbed up to 250. I shut the jeep off and looked at it, if definitely felt hot and the hoses were under pressure but the coolant wasn’t boiling. I let it sit with the engine off for 10-15 minutes and the coolant temperature was hanging out at 250 until it suddenly dropped back to 220. I drove the jeep home and it stayed at 220 on the rest of the way home. The electric fan doesn’t seem to be running even at 220 with the ac on so I’m thinking that’s definitely part of my problem. Does this sound like it could all be caused by the e fan? Why would the temp jump from 220 to 250 and back down so quickly? I’m planning on checking compression this weekend but I don’t see any signs of coolant in my oil.

wow, I apologize for this book I wrote lol

picture of my jeep to make this post more interesting lol

 

I believe the efan kicks in at 225F. If its not kicking on after that, then it appears there is an efan problem. If its not that, then start looking for other suspects - stuck tstat, clogged radiator, coolant leak, radiator cap not holding pressure.

 

Do you have a shroud on your mechanical fan?

I long swore against them as mine would consistently break (always old used parts) I replaced mine with a brand new one and its been phenomenal since. Honestly wouldn't run without one.

 

A good fan shroud properly mounted does make a difference in airflow through the radiator. I added foam seals around the edges of my shroud and electric fan shroud so it only pulls air through the radiator. Pair that with a ZJ fan clutch and you got some good piece of mind. My rig hardly ever touches 210 always stays just a touch below that.

 

A few thoughts based on my experiences with my own '98/4.0/AW4 combo. First, since you say it's running hot on the highway, I'm inclined to say it's not a fan problem, though the fan situation may not be helping at low speeds. On the '98s, the e-fan is supposed to kick in around 219* (or 217* perhaps?), so you either have a problem with the fan not coming on when it should or your gauge/sensor is giving faulty readings. Do you have a infrared temp gun you can use to verify actual coolant temps? If so, aim it at the thermostat housing and see what it reads. If memory serves correct, the '97+ Cherokees all used a single coolant temp sensor that's located on the thermostat housing to control both the temp gauge and the e-fan via the ECU. How does the wiring to that sensor look?

What brand of thermostat are you using? You may want to pull it out, drop it in a pan of boiling water, and see if it opens properly.

What kind of radiator are you using, and when did you last flush it out?

 

The 98 will come on at temp...lets just say 220 ish, you can see exact temp reading with an obd2 reader, OR with an ac mode selected...ONLY if the ac compressor engages. Low freon will inhibit this. Best simple test...run truck and pull off the connector on the thermostat sensor. The fan should turn on. If not then separate fan connector and check for 12v on car side between the 2 pins

 

Yes, I broke my shroud doing my water pump last year and replaced it when I read that it can cause issues

 

I’ve not heard about the ZJ fan clutch before. Is it from the 98 4.0l ZJ? Definitely interested in doing that to get some additional cooling

 

Its a common approach to trying to improve cooling performance. The ZJ fan clutch is a fair bit thicker than an XJ fan clutch, and it is too thick for some aftermarket radiators. Its also a bit more of a pain to install because of its thickness.

USMW model 22148 fan clutch labeled as HD replacement for stock 4.0 fan clutch. Its about the same thickness as a stock XJ fan clutch, but definitely provides more air flow than stock for any engine rpm above idle. For an engine speed of 2000 rpm, a new stock XJ NAPA fan clutch flowed 1150 ft/min, and the USMW 22148 flowed 1750 ft/min. Quite an improvement. I did these measurements from under my XJ with a wind speed meter at a set position about 3 inches from the fan blade. Hood was down.

The Autozone Duralast 922625 fan clutch is a relabeled USWM 22148 when I was investigating fan clutches in summer of 2022.

 

I’m going to get a temp gun to check and verify the temperature.

I don’t remember the brand of thermostat, it came from rock auto.

Radiator has been it the jeep since my brother bought it in 22, it’s probably the oem one. I have a CSF 2 row that I’m gonna put in this weekend after I flush it.

 

So today I did the coolant flush and while letting it run to circulate the flush fluid I noticed that the e fan wasn’t coming on an all. The 40A fuse in the engine bay fuse box was blown. I replaced it and the fan started working again but sound loud when running. I’m suspecting that the fan motor is going out and causing the fuse to blow. I don’t know if this is my whole problem but it has to be some of it at least.