Convert your old closed system to a modern open system!

By Brian Pawlak

If you have a 4.0 liter 6 Cylinder engine in your 1993 or earlier Cherokee, you most likely still have the pressure bottle for the radiator. There is no conventional radiator cap on the radiator, and if your Cherokee is like mine, you must replace that pressure bottle about every 12-14 months. The bottle costs about $40.00 and the cap is $6.00. The last one I had in mine hadn't started to crack yet, but was leaking at both hoses no matter how tight I made the hose clamps. Most likely the combination of bigger tires, more weight due to accessories and camping gear was too much for the original single core radiator. I have seen one conversion to a radiator with a cap, and the newer Cherokees have a cap on the radiator. This eliminates that pressure bottle and adds a normal overflow bottle. I was told that they switched at about the '94 model year, but the '96's definitely use them. The existing radiator can be re-cored to a double or possibly a triple core, and have an elbow added to the driver side end cap for the radiator cap. The cap should be no more then 3 inches from the radiator cap to miss hitting the air cleaner box. 

I decided to buy a Mopar Maxi Cool radiator for a '96. It came complete with a cap and dropped in just like the original, and is a double core radiator. I removed the air filter box for easier access to the lower hose, automatic transmission lines and the wiring for the fan heat sensor. There are about 18 nuts and or bolts to remove the radiator. Be careful not to lean on the front body parts while the top cross member is removed, they will bend easily while unsupported. There are two rubber mounts that are reused on the top of the radiator. The hoses to the bottle will be rearranged and a '96 heater valve will eliminate all the "T" fittings in this area. I used the hose from the bottom of the pressure bottle to go from the top of the heater core to the top of the heater port of the valve. About 1/2" was trimmed off the hose. The original lower heater core hose went to the lower heater port on the valve. The hose from the water pump goes to the port that is in line with the top heater core hose, and the hose from the thermostat goes to the top 90 degrees port on the heater valve. This is a little confusing in print, but see the pictures and look at the diagram at the dealer when you buy the valve.

 
The heat switch to activate the electric fan on my '90 Cherokee screwed into the radiator just above the lower radiator hose. The fitting for this is on the '96 radiator, but it is plugged and not threaded. The '96 Cherokee used a heat sensor switch in the thermostat housing to turn on the fan based on a hot engine rather than a hot radiator. The thermostat housing is identical to my '90 except for an additional threaded lug for the '96 sensor. It needs an electrical plug that can be purchased at an auto parts store and wired in just like the original sensor. These two wires leading to the sensor can also be wired to a switch on your dash if you wish to be able to run the fan manually. This can be useful on trail runs that are slow moving with a lot of idling. An overflow bottle must be added with the overflow tube leading to the bottom. The hole left by removing the original bottle and its shelf gave me room for the bottle. Depending on what year Cherokee is the layout under the hood may be different than mine. A 1-1/2-quart bottle fits easily and overflowed once, so I bought a 2-1/2-quart bottle that had a nice wire basket to mount it. That overflow might have just been air making its way out of the system. The 2-1/2-quart bottle has more than enough capacity and has not come close to overflowing in moderate driving conditions. If you are going to all this trouble and expense, be sure to replace your thermostat, coolant, and flush the engine. The job took me about eight hours, which included remounting my RS-9000 compressor that was under the pressure bottle. Prior to this conversion, my engine ran close to, or just inside, the red zone under any load. I didn't want to spend $50.00 for the 3rd time in three years for another pressure bottle.