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1993 Jeep Cherokee


General Cooling System Servicing


The following article is for general information only.

Information may not apply to all years, makes and models. See specific article in the ENGINE COOLING section.


The basic liquid cooling system consists of a radiator, water pump, thermostat, electric or belt-driven cooling fan, pressure cap, heater, and various connecting hoses and cooling passages in the block and cylinder head.



Remove radiator cap and open heater control valve to maximum heat position. Open drain cocks or remove plugs in bottom of radiator and engine block. In-line engines usually have one plug or drain cock, while "V" type engines will have 2, one in each bank of cylinders.


A good cleaning compound removes most rust and scale. Follow manufacturers instructions in the use of cleaner. If considerable rust and scale has to be removed, cooling system should be flushed. Clean radiator air passages with compressed air.


CAUTION: Some manufacturers use an aluminum and plastic radiator. Flushing solution must be compatible with aluminum.

Back Flushing

Back flushing is an effective means of removing cooling system rust and scale. The radiator, engine and heater core should be flushed separately.


To flush radiator, connect flushing gun to water outlet of radiator and disconnect water inlet hose. To prevent flooding engine, use a hose connected to radiator inlet. Use air in short bursts to prevent damage to radiator. Continue flushing until water runs clear.


To flush engine, remove thermostat and replace housing. Connect flushing gun to water outlet of engine. Flush using short air bursts until water runs clean.

Heater Core

Flush heater core as described for radiator. Ensure heater control valve is set to maximum heat position before flushing heater.


To prevent air from being trapped in engine block, engine should be running when refilling cooling system. After system is full, continue running engine until thermostat is open, then recheck fill level. Do not overfill system.



  1. Visually inspect thermostat for corrosion and proper
    sealing of valve and seat. If okay, suspend thermostat and thermometer
    in a 50/50 mixture of coolant and water. See Fig. 1. Do not allow
    thermostat or thermometer to touch bottom of container. Heat water
    until thermostat just begins to open.

  2. Read temperature on thermometer. This is the initial
    opening temperature and should be within specification. Continue
    heating water until thermostat is fully open and note temperature.
    This is the fully opened temperature. If either reading is not to
    specification, replace thermostat.

Fig. 1: Testing Thermostat in Anti-Freeze/Water Solution


A pressure tester is used to check both radiator cap and

complete cooling system. Test components as follows, following tool manufacturers instructions.

Radiator Cap

Visually inspect radiator cap, then dip cap into water and connect to tester. Pump tester to bring pressure to upper limit of cap specification. If cap fails to hold pressure, replace cap.

Fig. 2: Testing Radiator Pressure Cap

Cooling System

  1. With engine off, wipe radiator filler neck seat clean.
    Fill radiator to correct level. Attach tester to radiator and pump
    until pressure is at upper level of radiator rating.

  2. If pressure drops, inspect for external leaks. If no leaks
    are apparent, detach tester and run engine until normal operating
    temperature is reached. Reattach tester and observe. If pressure
    builds up immediately, a possible leak exists from a faulty head
    gasket or crack in head or block.

NOTE: Pressure may build up quickly. Release any excess pressure or cooling system damage may result.

3) If there is no immediate pressure build up, pump tester to
within system pressure range (on radiator cap). Vibration of gauge
pointer indicates compression or combustion leak into cooling system.
Isolate leak by shorting each spark plug wire to cylinder block. Gauge
pointer should stop or decrease vibration when leaking cylinder is