The typical, overheating :(
 

I have a 1997 XJ with the 4.0. Still having overhearing problems. So let's down the list lol. Performed compression test so head gasket/head seems good. I replaced the tstat, water pump, temperature sensor. The radiator appears to be good visually. I performed a deep clean twice through the system. I know the normal operating temp is around 210. It's seems to want to stay at around 220, without a upgraded cooling system in this weather that makes some sense and I'm okay with that for now. But intermittently the temp will immediately jump to pegged on the gauge, could be at idle, street, or freeway (doesn't have a Preference ). Then it as fast as it jumped up, it will drop back down to 220 again after a few minutes or so. Made me think tstat could have been defective, so for kicks and giggles I pulled the tstat out (looked okay) then ran it with no tstat. Does the same thing. I noticed just now that with it running while squeezing hoses to ensure I have no air in system that the upper hose and super easy to squeeze and not sure I feel any flow coming through, but it does get hot. At a lost been tinkering around with this for couple weeks, need some outside ideas lol

Fan clutch. Get a t-stat from the local Mopar dealer. Replace the rad or get it tested at a rad shop. It's probably plugged and running at 50%.

Think I am just going to replace radiator. Literally last thing I haven't replaced but it seemed good and didn't think it would make it jump up instantly like it did. I forgot to mention in original post that i did the fan clutch as well

The "immediately jumps" makes me wonder if it's a sender/wiring problem. The lower the resistance (low ohms) the sensor puts out the higher the temp will show on the gauge.


So:


1) the sender could be messed up.


2) there could be a short to ground in the wire to the sensor.


A way to check would be to measure the temp of the housing the sensor is in with an IR thermometer when the temp is 210 and measure again when it's pegged and see if it's really that hot when pegged.