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.
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