1998 Cherokee 4.0 sport. 187k. Jeep was running fine and temp steady right before 210 mark. (1/2 way mark). Could hear water pump making noise so I had it changed. Ever since then it has run warmer steady a bit after 210 mark. Then jeep didn't start for me a couple of times after running warm and got a crankshaft pos. sensor code. Not sure if sensor problem connected with overheating so continued to address overheating. Flushed radiator and found heater core to be clogged with rust colored gook. Bypassed heater coil and changed thermostat. Burped system, flushed and new coolant. Now jeep warms up very fast and stops at white line before 210 mark for a bit and then slowly starts rising to first line after 210 mark. No boil on fluid. Is this problem caused by sensor problem or is it unrelated? If unrelated what would cause it to warm up so fast and then run hot? Thanks for any advice. BTW have not changed sensor yet as it is on order.

 

Have u verified the dash gauge reading and yes, it could be a sensor or bad electrical somewhere giving a false reading.....or bad stat, or fan clutch, or a number of other things.

If the heater core was nasty, the bottom half of the radiator could be plugged as well leaving only half a radiator open and flowing. No amount of flushing will remove a solid plug from the rad core tubes. The rad must be removed, pulled apart and "rodded" out (by a rad shop), or replaced.

 

Well, to answer your question regarding fast engine warm up and then overheating: stuck thermostat could do that

Besides the waterpump, any other maintenance done/known of the cooling system?

 

I just changed the stat with no change. Iput a new radiator from autozone in a couple of years ago. Something odd happened yesterday as I was checking the coolant level. Crank the car and started to add fluid. It took 3/4 of a bottle of coolant depsite topping it off several times over the past few days. No visibe leaks at all and I have checked thouroughly while car was hot, running, not runnng and even laid coardboard over night under engine. Despite adding fluid it still doing the same thing.Gonna change radiator and thermo today.

 

How much have you driven it since you did work to the cooling system? It will suck in more coolant after a few warmup/cool down cycles as it works air out of the system.

If you've been driving a lot and still have to add coolant, it's possible that your head gasket has a slight leak. Also did you get a new radiator cap? Very important & cheap maintenance item.

 

New Rad cap and been driving for about 2 months. Also burped the system several times. Gonna do a pressure test today.