You are here

Troubleshooting Cooling System

XJ's tend to have issues overheating.    XJs changed from Closed to Open Cooling systems in 1990.  

Here are some troubleshooting steps to determine the cause.

Check for Air in the system

Sometimes if you overheat, have a bad radiator cap or general leak, you can get air in the system.     The easiest way to eliminate all air is to have a Tefba Coolant filter mounted at the highest point of the radiator hose.    The Tefba Coolant filter provides an easy entry to fill the cooling system and burps air since its at the highest point.

  1. With ignition off and a cooled engine, remove the upper radiator hose at the thermostat housing.
  2. fill both the thermostat neck and the upper radiator hose with water until it overflows and quickly put the upper radiator hose back on and tighten.
  3. Remove radiator cap and fill to the brim.   Leave the cap off.
  4. Start the engine and watch the water level.     If the water level drops, keep filling.    
  5. squeeze the upper radiator hoses to get rid of any air that might have shifted to the highest point. 
  6. If the water or coolant starts to overflow quite a bit, then put cap back on.

Check Electric Auxiliary Fan

XJ MJ Auxiliary Cooling Fan

The fan is controlled by a coolant temp sensor and relay.    The coolant temp sensor enables at 190*F.    The relay also kicks in when the A/C is used.

Depending on the year of the vehicle:

  • the relay could be mounted on the driver's side inner fender panel (86-??) or in the PDC (Power Distribution Center)
  • Coolant Temp sensor is located either on the radiator outlet tank (above the lower radiator hose) (86-??) or on the thermostat housing.   My 99 has it on the thermostat housing.

When the temperature gauge reaches 210, does the electric fan kick in?  If NO

  1. Disconnect the fan connector and try hard wiring to the battery to make sure the fan motor is still good.    If not, replace the electric fan.
  2. Check or replace the relay in the PDC (Power Distribution Center)
  3. Check or replace Coolant Temp Sensor

Other Sources:

Check Radiator

  1. Start with a cold engine.     Start the jeep up and let it run for a few minutes to warm up
  2. Make sure the upper hose (return) is warm/hot. 
  3. Then shut the engine off and feel/take temps of the core.  If it's cold at the bottom and gets warmer towards the top then the radiator is plugged.    Core of the radiator should be same temperature as the tanks.

If it is clogged, consider upgrading to a 3-core aluminum radiator with aluminum tanks vs the plastic tanks.

Fan Shroud

If you have a broken fan shroud, then the air will not flow properly.   A cheap source on fan shrouds (thanks to Andy Steiner):