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New-Style Radiator Cools Older XJ Cherokee

Posted in How To: Engine on July 25, 2017
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Jeep XJ Cherokees are known to run hot, especially when taxed for long periods of time by hill climbs or slow-speed rock crawling. However, despite the fact that our ’89 XJ’s 4.0L I-6 was a great-running engine, some hot summer days the coolant system was happy as could be; on other days when the weather was cooler, it might boil over. We knew the radiator was in good shape and not terribly old. It had a fairly fresh water pump, fan clutch, thermostat, coolant recovery tank and cap; and there seemed to be no discernable pattern between driving at highway speed, commuting around town, or slowly idling down a trail.

It can be a frustrating task to purge all the air from the cooling system on these models, and the high and long upper radiator hose does not help. We tried all the tricks we had learned: added small bleed holes in the thermostat plate, bled the system using the rear coolant sensor port with the XJ parked nose down, and even installed a bleed petcock in a newer water outlet. We could successfully purge the system, yet our erratic overheating issues persisted.

On the ’84 to ’90 Cherokees, Jeep chose to use a "closed" cooling system. There is no traditional radiator cap, only a fill cap on the coolant recovery tank near the firewall. Despite getting a good system purge, we would see the coolant level in the pressurized recovery tank change radically. Sometimes this happened under varying circumstances, while other times coolant levels changed during periods of perfectly normal operating conditions. The tightly sealed cap on the recovery tank would sometimes dislodge itself under heat and pressure, allowing hot coolant and steam to spew out. Replacement of the tank and cap did not improve the behavior significantly.

On ’91-and-newer XJ models, Jeep changed to a more traditional "open" cooling system, returning the fill cap to the radiator. The cap is a pressure-sensitive cap that allows coolant to pass to a coolant reservoir that is not pressurized and does not use a tightly sealed cap. We made the decision to swap our system to this newer style using OEM replacement parts from Omix-ADA. We ordered the newer radiator and hoses to make the conversion.

Another part of this plan to solve our overheating problems addresses the heater hose configuration, and we made some changes there as well. We deleted the troublesome heater control valve and plumbed the heater hoses directly to the heater core. This made the plumbing simpler and is how many modern vehicles are plumbed today. We installed hoses from a newer XJ. Jeep uses 5/8- and 3/4-inch hose connections at the heater core. Depending on the exact hoses you use, you may find the need to adapt the hoses by splicing in hose adapters to mate your water pump and thermostat-housing outlet to the heater core.

With the conversion complete in a handful of hours, we refilled the cooling system and ran the engine to temperature to purge any air and fully fill the system with coolant. As luck would have it, it was 114 degrees F that summer afternoon so the Jeep was in for a good test run. The temperature stayed well below hot while driving it under a number of conditions, and we had no boil over or other coolant system quirks. It’s comforting to know we have a more reliable cooling system, and it looks like we’ll be running cool for a while.

We had seen intermittent overheating with this Jeep a number of times. The coolant level in the recovery tank would vary drastically. At times, the pressure would overcome the top cap and start to steam or dislodge the cap from the bottle threads. In these cases, we would lose coolant completely from the system and risk introducing air back into it.
Shopping at helped us find replacement parts for the project. We got a ’91 to ’01 Cherokee radiator, plus a new cap, radiator and heater hoses, and upper mounts.
Our new traditional radiator cap has a pressure rating of 16 psi.
After unbolting the shroud for the mechanical fan and the electric fan, we removed the top cowl plate that sits over the radiator.
Here’s our old radiator. It wasn’t an ancient, crusty unit. In fact, it was only three or four years old. It had a built-in automatic transmission cooler with input and output ports on the driver-side tank. There was also a plugged port on this tank that accepts a temperature-controlled fan switch. We don’t use such a switch on this Jeep, but some do.
The original pressurized coolant recovery tank and its mounting bracket were removed along with the original heater hoses, heater valve, and some steel tubing. The vacuum line to the heater control valve was plugged.
We had previously swapped to a newer ’91 to ’01 water outlet that has an additional threaded outlet. A radiator petcock was also installed in the outlet as another option for cooling system bleeding.
Here is the old radiator (front) compared to the new Omix-ADA radiator (rear). They’re both the same size with the same mounting features.
As expected, the new radiator dropped into place without any issue. All locations and plumbing sizes are the same for the water ports, transmission fluid ports, and for the fan switch sensor port.
We did make one modification to the top cowl plate near the radiator cap. We cut a semicircle in the plate to make working with the cap easier.
We swapped the upper radiator hose to one for a ’91 to ’98 XJ. It’s more direct and travels over the air conditioning compressor rather than around behind it. We backwashed the heater core using water from a garden hose to clean it out, and then installed all new heater hoses. Heater hoses were for a ’97 to ’01 4.0L Cherokee.
We ran the hose from the thermostat housing to the bottom of the heater core and the hose from the water pump to the top of the heater core. This new setup really cleaned up the engine compartment.
We purchased a traditional radiator coolant reservoir (Dorman 603305) for a ’91 to ’01 XJ Cherokee. Depending on space and preference, one could use a generic bottle instead. We fabricated a couple of metal brackets to secure it to the firewall and used rubber hose to plumb it to the overflow nipple below the radiator cap.
For added cooling efficiency under the extreme heat situations, we added Royal Purple Ice to our coolant mix. It’s formulated to further reduce corrosion and contaminant buildup in the cooling system. It’s a wetting agent that helps keep engine coolant temperatures lower.
With the Jeep warmed up and idling, we watched the coolant settle in the radiator and kept topping off the radiator as the thermostat opened and the system purged. We did the final addition of coolant to the reservoir. With this setup we can now directly check the coolant level in the radiator.

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Royal Purple
Porter, TX 77365
Suwanee, GA 30024

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