Why is my 1998 Jeep Cherokee overheating?
First you have to understand the components of the cooling system.1) Coolant in the engine water jacketa) must be full, fresh and correctly mixed2) Coolant in the radiatora) must be full, fresh and correctly mixed3) Thermostatic valvea) must correctly open at the appropriate temperature4) Mechanical clutch driven fana) must actuate fan at the appropriate temperature (190)5) Electric cooling fana) must engage at operating temperature (210)6) Water pumpa) must properly circulate water7) Radiatora) must be free of clogs and leaksb) 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 causes1) Overheats while sitting at idle or stuck in traffic, does not overheat while driving on highway.a) improper thermostatic valve operationb) low coolant or bad coolant mixturec ) improper water pump operation2) Overheats while under load.a) improper cooling fan operation,b) improper thermostatic valve operationc ) improper water pump operation3) 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 jacketTertiary issues of no heat in cabin :a) low coolantb) bad thermostatic valvec) blocked heater core in cabinBe 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 circulationa) jack up front of car (taking all safety precautions as applicable)b) loosen radiator cap to half-lockc) knead upper radiator hose, pumping it to draw fluid in and push air out of overflow valved) start engine and allow to run to warm (with car jacked up, and radiator cap in half-locke) test upper hose, it should be hot and full of fluid once car warms upf) observe radiator fluid, it should appear to be circulatingafter verifying all of the above if condition not found and overheating persists3) Test thermostatic valve operationa) start engine and run to warmb) turn on heater, hot air should come out. if it does - your thermostat is likely good3b) Observe thermostatic valve operationa) remove thermostatic valve from engineb) place in boiling waterc) observe that thermostatic valve opens rapidly and smoothly and closes quickly once cooledd) if this works, your thermostatic valve works properly4) Check for exhaust leak into water jacketa) test water jacket coolant (you must specifically obtain water jacket coolant) for exhaust residueb) If you find exhaust residue in coolant, you most likely have either a cracked head, or blown head gasket
