Why is my 1998 Jeep Cherokee overheating?

Answer 1

First you have to understand the components of the cooling system.

• 1) Coolant in the engine water jacket
• a) must be full, fresh and correctly mixed
• 2) Coolant in the radiator
• a) must be full, fresh and correctly mixed
• 3) Thermostatic valve
• a) must correctly open at the appropriate temperature
• 4) Mechanical clutch driven fan
• a) must actuate fan at the appropriate temperature (190)
• 5) Electric cooling fan
• a) must engage at operating temperature (210)
• 6) Water pump
• a) must properly circulate water
• 7) Radiator
• a) must be free of clogs and leaks
• b) must maintain pressure to prevent coolants from evaporating rapidly (boiling)

Next you must understand the operation of this system.

• 1) On cold start, the Thermostatic valve (3) is shut, forcing (1) coolant in water jacket to recirculate.
• 2) Once engine temp reaches the point where the thermostatic valve (3) actuates (~190 f.), the coolant in the radiator (3) is circulated into the engine water jacket (1)
• 3) About the same temperature, the thermostatic clutch on the mechanical fan actuates, blowing forced air onto the radiator (7)
• 4) If the car reaches 210 degrees or more, the secondary electrical fan should engage which will increase the forced air circulation to the radiator.

Overheating typically follows one of a few different patterns, and their most common causes

• 1) Overheats while sitting at idle or stuck in traffic, does not overheat while driving on highway.
• a) improper thermostatic valve operation
• b) low coolant or bad coolant mixture
• c ) improper water pump operation
• 2) Overheats while under load.
• a) improper cooling fan operation,
• b) improper thermostatic valve operation
• c ) improper water pump operation
• 3) Overheats quickly, under any circumstances.
• a) head gaskets (also look for telltales exhaust in radiator fluid - there is a test for this)
• b) lack of proper coolant in engine water jacket

Tertiary issues of no heat in cabin :

• a) low coolant
• b) bad thermostatic valve
• c) blocked heater core in cabin

Be aware that curing one problem may not be the only problem that your cooling system has, so approach the problem with an open mind and a willingness to make clear observations and consider all factors.

The simplest diagnostic procedures, from easiest to less easier are :

• 1) Check coolant level and mixture.
• a) never remove the cap of a hot cooling system, ever. For any reason. The pressure can be explosively high, and the coolant can spontaneously evaporate causing a steam/boiling coolant fountain when given a chance to evaporate.
• b) a cool coolant system should have coolant visible in the filler of the radiator and you should not be able to add more water without it spilling out.
• c) coolant should be of a proper mixture for many reasons including corrosion prevention and thermal conductivity. There are testers available which will test the coolant condition. Coolant should be a mild fluorescent green-yellow color. It should not be brown, muddy or grey.
• d) if you must add water/coolant, and mix it yourself it's recommended to use distilled water. Minerals that go into your engine remain there.

• 2) Check for air bubbles in cooling system and basic circulation
• a) jack up front of car (taking all safety precautions as applicable)
• b) loosen radiator cap to half-lock
• c) knead upper radiator hose, pumping it to draw fluid in and push air out of overflow valve
• d) start engine and allow to run to warm (with car jacked up, and radiator cap in half-lock
• e) test upper hose, it should be hot and full of fluid once car warms up
• f) observe radiator fluid, it should appear to be circulating

after verifying all of the above if condition not found and overheating persists

• 3) Test thermostatic valve operation
• a) start engine and run to warm
• b) turn on heater, hot air should come out. if it does - your thermostat is likely good

• 3b) Observe thermostatic valve operation
• a) remove thermostatic valve from engine
• b) place in boiling water
• c) observe that thermostatic valve opens rapidly and smoothly and closes quickly once cooled
• d) if this works, your thermostatic valve works properly

• 4) Check for exhaust leak into water jacket
• a) test water jacket coolant (you must specifically obtain water jacket coolant) for exhaust residue
• b) If you find exhaust residue in coolant, you most likely have either a cracked head, or blown head gasket

Answer 2

not the right mixture of coolant Was the serpentine belt put back on correctly?

Answer 3

Low coolant level or possibly air trapped in the system. Bleed the system and check the coolant level in the radiator not the reservoir.

Answer 4

Could be a number of reasons. Thermostat, water pump, blown fan relay, bad fan motor.