Overheating is an all-too-common occurrence for Jeep XJ Cherokee owners, and running excessively warm most of the time is essentially taken as given.

Perhaps the biggest issue with the XJ cooling system is the XJ itself. The 4.0L inline six engine is nearly three feet long from the bellhousing to the front of the crank, so the boxy little brawler ends up with an undersized radiator and not much room for large fans. Less than four inches exists from the shroud mounting points to the front of the engine, to be exact.

As such, even though the stock thermostat is 195 degrees F, most Cherokees are rarely under 210 degrees F, unless aided by very cool ambient air. If you love the low speed (but high engine stress) fun of off-roading, it can be a recipe for overheating or cooling system failure. This is the type of stuff you really don’t want to worry about when you’re far off the grid or even just out for some fun.

(Image/Christopher Campbell)

Since the last big road trip we took in our XJ ended with a steady drip from the corner of the radiator where the fins meet the tank, we knew we had stressed the system and failure was imminent. That coupled with the fact that the rest of the cooling system was obviously 23-year-old original factory equipment led us to the conclusion that it was time to dive in and do some upgrades.

There’s a full parts list with part numbers and links at the bottom of this story.

Since we know the factory stuff is marginal, we’re going to use the best bolt-in upgrades available. Our goal is to create a reliable cooling system that will actually keep our XJ in the 200 degree F range, even with the A/C on in while in traffic in the summertime. Or while running trails in Utah and the southwest. To do that, we went straight for a few of the best cooling options at SummitRacing.com, including a Mishimoto aluminum radiator and triple electric fan, and a Flowkooler water pump. All of which are designed to help 4.0L sixes run cooler even while crawling at low speeds.

We’ll also show you a few common, but not very effective cooling mods that our XJ had installed and the problems they can create.

If you’ve got a warm-running XJ, follow along as we show you how to get those temps under control!

Parts List for the Jeep Cherokee XJ Cooling System Swap & Upgrade

Hot summers are a particular challenge for XJ Cherokees with aged stock cooling systems. In Southern California, and the southwest where we often go off-road, we regularly see temperatures that crest 100 degrees Fahrenheit. Pair that with low speeds, A/C, and traffic, and engine temps climb quickly. (Image/Christopher Campbell)
In conditions like these, our XJ hovers in the 210 to 220-ish degree range. Not abnormal for a 4.0L I6, but certainly not ideal for efficiency or oil life and it is stressing components. This pic was snapped at a stop light on a warm day in Southern California (hence the oil pressure) and is pretty average for our XJ. It’s been as high as 225 F, which makes us pretty uncomfortable when we’re far away from cell phone range. (Image/Christopher Campbell)
While we like the way the custom hood louvers on our XJ look, and they do provide a vent for hot air to radiate outward, they are small and were not very effective in reducing our Cherokee’s engine temperature. To extract heat in any meaningful way, hood vents need to be thoughtfully angled and placed to take advantage of the airflow over the hood—and be designed to create a low resistance path for air to flow. (Image/Christopher Campbell)
Our Cherokee also had this aluminum panel installed on the bottom side of the hood when we purchased it. This highlights another issue with hood vents; they allow water to drip through and onto the engine and harness. Not usually a big problem in light rain, but it can be a very large issue when sitting for extended periods in a rainstorm. The panel kept the engine mostly dry, but it also rendered the hood vents completely useless. (Image/Christopher Campbell)
Have you seen a Cherokee running around with the rear of the hood popped up? This is another quick and easy mod in an attempt to extract hot air from the engine bay. These 1.5 inch aluminum spacers and some longer hood bolts are all it takes. (Image/Christopher Campbell)
The result is a hood that’s popped up significantly at the rear. This method is typically more effective than hood vents, but it’s not very aesthetically pleasing, and it also allows water to drip in around sides and rear of the hood. Thanks to the aluminum panel under our hood, the actual gap between the bottom of the hood and cowl is only a bit over 3/8 of an inch, which doesn’t create much of a path for airflow. We’re also fairly sure it won’t clear underhood components without the hood lift blocks, so we’ll be removing it. (Image/Christopher Campbell)
Here’s the stock XJ Cherokee cooling system layout. On the passenger side, there is a mechanical clutch fan that does the majority of the cooling, which means it’s almost always fully engaged. On the driver side is an electric fan that is controlled by the ECU to turn with the A/C, or when the thermostat hits about 220 degrees F with the A/C off. Although in practice, XJ owners report it kicking on anywhere from 208 to 230 degrees F. (Image/Christopher Campbell)
Time to pull the stock fans. The shroud on the mechanical side is only held by a couple of bolts at the top on the core support and tabs on the bottom. Unfortunately, there isn’t enough room to slide it out with the fan attached. (Image/Christopher Campbell)
A ratcheting wrench is your friend to get the clutch fan off the water pump pulley. (Image/Christopher Campbell)
On the driver side, the factory electric fan also only has two bolts that hold it to the core support with tabs on the bottom. The factory harness is behind the headlight. (Image/Christopher Campbell)
Beside a crack in the electric fan shroud, our stock fans are in good shape. We were not able to determine how many cfm the electric fan is rated for, but we did note that it doesn’t spin very quickly when hooked directly to power. Either it’s a fairly low cfm fan, or our fan is tired. We’re also hoping removing the drag of the clutch fan will free up a little power for the 4.0L six. (Image/Christopher Campbell)
While there is a drain on the passenger side of the radiator, it’s a pain to get to, and it won’t make less of a mess than just pulling the lower radiator hose. We went for the fast drain option. Note, we removed the factory airbox for easier access and photos. (Image/Christopher Campbell)
A five-gallon bucket will suffice for draining the radiator since the entire XJ cooling system holds about four gallons with about two gallons in the radiator and water pump. If you position it in roughly this spot, you’ll catch nearly all of the coolant. (Image/Christopher Campbell)
Removing the radiator requires removing the top of the radiator core support. Fortunately, this can be done without removing the grille by simply removing the four nuts that connect to the top of the grille. There are also three bolts per side and two studs that connect to the radiator. We don’t know why one of them is a Phillips head screw. Typical Jeep things. (Image/Christopher Campbell)
With all of the bolts loose, the top of the core support will slide out. (Image/Christopher Campbell)
The upper line for the trans cooler is a simple flare fitting that unbolts, but the lower is a push-on style fitting that will require a 3/8 fuel line quick disconnect tool for removal. This hose will probably require significant force and wiggling to release. (Image/Christopher Campbell)
The last thing holding the radiator in place is the two bolts that mount the A/C condenser. We’ll also be saving those rubber mounted studs to transfer to the new radiator. (Image/Christopher Campbell)
With the serpentine belt is still holding the pulley tight, it’s a good time to break the water pump pulley bolts loose. Leave them in place for now. (Image/Christopher Campbell)
The 4.0L doesn’t use a spring-loaded idler pulley to keep tension on the serpentine belt like most modern systems. The idler pulley tension is set by this bolt on the power steering pump bracket. Backing it off will loosen the idler pulley and allow the serpentine belt to be removed. (Image/Christopher Campbell)
The power steering pump needs to be unbolted from the intake manifold. The three mounting bolts are accessed through the holes in the pulley. Once these are loose, the pump can be set aside and the mounting bracket removed from the water pump. If you removed the airbox, the pump will easily lay on the engine bay in that location. (Image/Christopher Campbell)
The thermostat housing is just two bolts. Getting the factory radiator and heater hoses off was the most annoying part. We’ll be replacing the heater hoses as well, and we’d encourage you to do the same. (Image/Christopher Campbell)
We were surprised to find a fairly fresh 195 degree F thermostat and gasket with no gunk or corrosion!  (Image/Christopher Campbell)
As part of our cooling system efficiency upgrade, we decided on a FlowKooler water pump. This pump doubles the water flow rate at low rpm & idle, but at 3,500 rpm the flow rate drops to normal rate to prevent excess water flow at high speed. It also generates 22 percent more water pressure inside the engine block to help suppress hot spots, steam pockets, and pinging. Surprisingly, the pump is also claimed to be about 32 percent more efficient than the factory style pump, meaning less horsepower is required to turn it. (Image/Christopher Campbell)
Our water pump gasket came off pretty cleanly, but we always use a razor blade and bit of green Scotch-Brite to get the surface perfectly smooth and clean. Great time for us to de-grime the front of the engine and maybe figure out where the oil is coming from for a future update. (Image/Christopher Campbell)
We’re going to try out a 180 degree F thermostat instead of the stock 195. It’s worth noting that a lower temp thermostat will NOT help your car run cooler if you are reaching the thermal capacity of your cooling system, as evidenced by the fact that our Cherokee ran in the 205 to 220 degrees F range with a 195 F thermostat. Cooling capacity is mostly governed by the efficiency of the water pump and the ability of the radiator and fan to transfer heat. We’re hoping the lower thermostat will work with the low speed efficiency of the FlowKoooler pump by getting the water circulating earlier. (Image/Christopher Campbell)
Here’s the star of the show! Mishimoto’s all-aluminum 2-row core radiator design is thicker than stock and features a 100 percent increase in fin surface area, and a 90 percent increase in coolant capacity. It’s an upgrade in every way and a direct fit for 1989-2001 Jeep Cherokee XJs with any factory style radiator. It also carries a serial number and a lifetime warranty. (Image/Christopher Campbell)
The filler neck comes with a port and a cap to run either open or closed style cooling systems. We’ll be running an open style with the factory overflow bottle. (Image/Christopher Campbell)
The Mishimoto radiator does make use of every bit of real estate available while still fitting the stock mounts and accessory brackets. Note that we swapped the upper rubber isolator from the old radiator onto the new Mishimoto. (Image/Christopher Campbell)
This is the one spot that may need a little massaging for the Mishimoto to fit properly. The lower transmission cooler line fitting was just barely pressing against the engine bay on our XJ. We lightly tapped the body seam a few times with a hammer to gain clearance. (Image/Christopher Campbell)
Another possible clearance issue are these studs on the A/C condenser. Ours were pressing against the radiator when we tried to mount it, so we opted to grind them down a little to gain clearance. (Image/Christopher Campbell)
It was thoughtful of Mishimoto to include these condenser brackets, because you are definitely going to need them if you still have A/C. That plug is for the water neck if you decide to run a closed style cooling system. (Image/Christopher Campbell)
The Mishimoto relay/controller uses a standard push-in style temperature probe to trigger the fans. The push probe should slide in between the fins of the radiator about three inches down from the top of the radiator and one inch over from the side tank. Note the A/C condenser brackets; you’ll need 2 x 8mm nuts to attach them to the radiator. (Image/Christopher Campbell)
On the driver side, the factory transmission cooler lines will connect directly to the radiator. The top flare fitting should just be snugged tight. The bottom quick release fitting should just click into place, however, if you’re going to have a leak, this will be the spot. Our recommendation is to replace the hose with a new one, or just remove it and run a piece of transmission hose from the hardline on the transmission—which is what we’ll be doing. (Image/Christopher Campbell)
Our upper and lower radiator hoses were the originals from 2000 and feeling pretty puffy, so we went with quality Continental parts. No point in having a nice cooling system fail because of 23-year-old hoses. Mishimoto also makes a nice silicone radiator hose kit if you want the upgrade. Either way, make sure to use the corrugated plastic sleeves to protect the hoses from abrasion. (Image/Christopher Campbell)
To get as much airflow as possible in the tight space between the radiator and the engine, Mishimoto uses multiple small fans in an array to cover the length of the radiator’s core. Each 10 inch fan flows 950 cfm for a total of 2,850 cfm, which a claimed 41 percent airflow increase with all three running, compared to both factory fans with the fan clutch hot. These fans also carry a lifetime warranty. (Image/Christopher Campbell)
An electric fan’s effectiveness is dramatically increased by a good shroud, so Mishimoto did some in-house testing vs. the stock Cherokee shrouds to make sure its shrouds would help the fans keep a high flow rate across the entire radiator core. The real key here is an efficient and tight fitting shroud that makes sure that the fans pull air through the radiator, not from gaps that allow the fans to pull from an easier path. (Image/Mishimoto)
The relay/control module for the Mishimoto fans features a 40/60 amp relay and an adjustment screw that allows the fans to turn on between 150 to 240 degrees F. Each fan pulls 13.08 amps at startup, so if all three start at the same time, it will draw 39.24 amps. Once started, each fan pulls 6.8 amps for a total draw of 20.4 amps. (Image/Christopher Campbell)
The fans come with barrel clips attached, but we’ll be clipping them off to splice the wires together. The blue wires are 12V+, while the black ones are 12V-. (Image/Christopher Campbell)
This is the basic wiring diagram for the fans and the relay/controller. The black 12V- wires from the fans will be connected to the heavy gauge red wire from the relay/controller. The blue wires from the fans will be connected to a loose red wire included in the kit that will run directly to the battery. The yellow wire is 12V+ switched. The green wire won’t be in this application used unless you opt to run a manual override switch. If you need some additional instruction, Mishimoto has created a video to assist with wiring. (Image/Christopher Campbell)
Believe it or not, there isn’t a reliable or easy to reach source for switched 12V+ under the hood. The best option for the XJ is to run a wire through the firewall grommet on the passenger side and tap into the fuse panel in the in the passenger side kick panel. Any 12V+ switched source here will work. You can use an add-a-circuit kit or tap directly into an unused port such as #8 on this generation Cherokee. (Image/Christopher Campbell)
The Mishimoto kit includes a jumper wire for 1997-2001 XJs that will connect the driver side fan directly to the factory wiring to command A/C. We’re going to try to this set up for now, but we may end up wiring all three to run together based on the radiator temps so we get the total 2,850 cfm at all times. (Image/Christopher Campbell)
To prevent metal-on-metal contact, Mishimoto supplies a roll of foam tape for the fan shroud. For the passenger side, measure 1.5 inches over from the edge of the shroud and place a foam strip on the top and bottom. (Image/Christopher Campbell)
On the driver’s side, just run a strip of tape from top to bottom. (Image/Christopher Campbell)
The Mishimoto fan shroud will slide right into the three factory mounting tabs at the bottom of the core support. At the top, the shroud has two large bolt holes that will line up with holes in top of the core support. We removed all other J-clips and even tapped the edge of the core support flat in a few spots to get a nice, flush fit. (Image/Christopher Campbell)
The adjustment screw on the relay/controller has a range of 150 to 240 degrees F. It’s a trial and error process to figure out exactly where you want the fans to switch on. We’re going to start in the 170ish degree range and adjust from there. Be gentle when turning it; the screw has a short sweep and can be damaged if turned too far. (Image/Christopher Campbell)
Here’s the factory electric fan harness with the Mishimoto jumper. If you decide to run all three fans together on the relay/controller, do not splice into this harness, just disconnect it. Removing it will not trigger a code. (Image/Christopher Campbell)
Our initial crank was done with distilled water, but our final fill will be with Zerex G05. Hybrid Organic Acid Technology (HOAT) coolants like this are ideal for the XJ and our upgraded components. To flush out the old coolant, we ran distilled water through the system and drained it a several times. (Image/Christopher Campbell)
All buttoned back up! We love the look over stock, and we were surprised how quiet the fans are. Placing your hand in front of grille, you can feel the air being pulled through. (Image / Christopher Campbell)
After 25 minutes of idling in the driveway while we checked for leaks and verified functionality, this is where our engine temp climbed to on the factory gauge. Nice improvement, we can’t wait to see the result on a hotter day. (Image/Christopher Campbell)
Since the factory gauge makes you guess the actual operating temperature, we plugged into the OB2 to get sensor level data. Our engine never climbed over 185 degrees F, even with no additional airflow across the radiator from driving. This should translate to 195 to 200 degrees F in real world summertime driving conditions. (Image/Christopher Campbell)
And to top it all off, now we can run our hood flush again without worrying about heat! (Image/Christopher Campbell)

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Christopher Campbell has been heavily involved in the automotive world since he began building his first car, a 1967 Ford Ranchero, with his dad at the age of 14. That started a lifelong passion with custom hot rods and muscle cars. After graduating from Cal State Long Beach, he went to work for HOT ROD magazine as Associate Editor. From there he became Technical Editor at Popular Hot Rodding magazine. Currently he creates freelance content for OnAllCylinders as well as many diverse enthusiast magazine titles such as HOT ROD, Muscle Mustangs and Fast Fords, Mopar Muscle, Super Chevy, Mustang Monthly, and 8-Lug.