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XJ Cooling


Although initially written as a guide to keeping the older 4.0L XJ cool, I have updated this page with items of interest to all years.

The XJ Cherokee was introduced in 1984, and for the first seven years they suffered with a marginal design for a cooling system. Instead of the typical well-known radiator design with a fill cap on the tank, this design used a remote cooling bottle that, well, sucked. There have been a few people that have done the conversion to a more conventional design, but from experience you can use the closed-style and still keep it cool. A couple things to look at:

  • FIRST! Use the right thermostat. The 4.0L engine is designed to use a 195 degree thermostat, and will normally run at an indicated 210.
    Running a 180 degree thermostat doesn't always help lower the temperature, it only lowers the temperature at which the 'stat opens and circulates with the radiator. The motor actually runs more efficiently with the warmer thermostat.
  • Make sure there are no bubbles in the block or any of the hoses.
    • If you have the older style 'closed' system, park the rear tires on a curb or railroad tie or something that lifts them up a good several inches while filling. You may need to refill a few times after running it and 'burping' air out.
    • For the newer style, parking nose-high often works to remove air although you may need to remove the upper radiator hose and fill that to remove the last bits.
  • Have I mentioned it previously? Use a good 195* thermostat. Factory models have a check valve or small hole at the 12 o'clock position that helps prevent trapped air in the block. You can drill a small hole in the thermostat if needed to help pass air bubbles.
  • Use pre-mixed coolant or mix straight coolant with distilled water. Tap water or hose water can lead to mineral buildup that will clog the radiator. If you're working on an older vehicle with unknown history, there are chemical radiator cleaners that will remove a lot of that scale but can also reveal leaks that would otherwise be plugged. I'd recommend the chemical cleansing only in an emergency however. If the stock radiator is still in an older vehicle, replacement may be needed because of blocked passages. Radiator shops can "rod out" many of the problem spots, but may be cost prohibitive especially since the stock 4.0L radiator was marginal in the Cherokee anyway.
  • Make sure you've got a good water pump. After years of usage, the vanes in the pump impeller have been known to be worn to nubs. That isn't as much of a problem with the 4.0L because of the impeller design, but I've had it happen on a CJ. Nubs don't do a great job of moving water, they only stir it.


    This water pump has seen better days. You're looking at the 'weep hole' on the bottom of the pump. As they age, the seal keeping hot water off the bearing will fail, and once it begins leaking, you'll see indicators like the discolored stream above, dripping down onto the lower radiator hose.

    • Make sure you have the right water pump. The 4.0L in the MJ, TJ, XJ, later YJ and ZJ uses a serpentine belt while the earlier YJ with the 4.2L (258ci) engine uses a pump that fits the same hole but is driven by v-belts. Because of this, they rotate different directions. Make sure yours has the correct impeller on it.
      Correct rotation for the impeller.
      This is the correct rotation for the 4.0L waterpump.
  • If you have the earlier cooling system, pad the coolant bottle with a 4" section of radiator hose that has been split in half. Why do I know you're going to have a section? Because you'll be replacing those hoses. The coil spring inside the lower hose is probably half-gone, and can actually collapse under high revs.
  • Although I just mentioned a use for it, replace the radiator hoses, and make sure the lower one has an internal spring or a spring molded into it. A good water pump can pull enough vacuum that it will collapse hot, weak hoses.
  • Get a GDI (Go-Dan Industries) 3-row radiator from www.radiator.com, you won't be sorry with the added capacity.
    Edited 29 July 2004 to recommend against the GDI. While a few years ago it was a quality radiator at a great price, recent reports from many sources are that the quality just isn't there any longer. Radiator.com still has 3-core radiators from other manufacturers, but the current advise is to steer away from the GDI.
  • Make sure the mechanical fan is working correctly. When the engine is hot, turn it off and then try to spin the fan by hand. On a hot engine, the viscous clutch should be pretty solid and the fan will stop moving as soon as you let it go. If it spins, replace it.
  • Think about what gear you're in.
    • If an auto, stay out of Overdrive unless you are on the freeway (generally over 50mph) and drop into 'N' while stopped instead of building up heat while at long stops.
    • If a 5-speed, stay out of 5th except for highway. Basically, keep the RPM at least around 2000 when driving.
  • If you have an automatic, make sure you have a good tranny cooler - although it sits in front and dumps heat into the radiator, it helps prolong the life of the transmission to keep it cooler. The added surface area for cooling the transmission is useful since the radiator then doesn't have to cool that as much.
  • I almost forgot about the electric fan. If you don't have one, grab one from another XJ. As I understood they are not standard with every XJ, just the air-conditioned ones and possibly the ones with trailer packages. There are three styles, and each reportedly flows more air than the earlier one. 87-93 had 6 square blades. 94-96 (pictured below) used the same electrical connector and had 8 S-shaped blades. 97-01 had 10 S-shaped blades, but a different electrical connector that would need to be spliced in.
    94-96 XJ Cooling Fan.  Note the curved blades, the older one had six straight blades.
    Newer Cooling Fan. Note the curved blades, the older one had six straight blades

  • Consider other sources of underhood heat.
    • As mentioned previously, idling an automatic transmission in gear will build up heat. Either drop to 'N' if it will be a short delay, or 'P' if you will be waiting awhile.
    • A cracked exhaust manifold can do amazing things to build up underhood heat as well. After replacing my thrice-cracked manifold I noticed a reduction in engine temps.
    • Consider hood vents to allow air to escape at low speeds. Chrysler Lebaron vents are a popular addition, and while the mounting position at the rear of the hood doesn't evacuate hot air because of positive pressure in that area, they will allow underhood heat to escape while in traffic or in the rocks moving slowly. Until I get mine installed, take a look at http://www.ericsxj.com/vents.htm to see how this is done.
e-mail Jim
created: July 7, 2003

Updated Jan 5, 2012