My newtome 2001 XJ runs consistently at 210 degrees and that temp is clearly being controlled by the T-stat. I can watch the gauge from cold as the engine warms up and the temp rises to about 215 then drops to about 205 as the T-stat opens and lets in cold radiator coolant. It then stabilizes at 210 and stays there. I have been reading here on the forum that normal operating temp should be 195. I just wanted to confirm that 195 is the preferred operating temp for the 4.0. Thanks!

 

No, your XJ (my 1996 also) uses a 195F thermostat. Normal operating temp is around 210-215. Sounds like yours is working fine.

 

The jump up to 215 is odd - you should see the temp gauge sit a hair to the left of 210 at full operating temp. 318 is correct that the spec temp is 210.

Now there is another factor you must be aware of for 00-01 XJ's that makes "odd" temp situations more concerning than normal and that is the 0331 head defect. If you're not familiar, please read up on it - it's basically a casting issue on the heads used on I6 engines from 99 (GC only) - 02.5 (XJ final model year was 01, 00-02 TJ) where the head cracks, typically around 100-130k IIRC.

Your XJ may or may not have the original head, and there were many OEM heads that never failed (I had one on my 00 TJ, 302k miles and engine still runs perfectly - original head), but you need to be aware, check your head to determine if it has already been replaced, and if not, keep a close eye on temp stuff and engine oil.
If caught early, it's a relatively minor head replacement and you're good to go, left uncorrected it will lead to block failure and you'll be in need of a whole new engine.

If it were me, I'd be replacing my t-stat asap with a Mopar brand part (per our resident Renix guru Cruiser54) and getting a status on my head asap.

 

If you are getting near the red / into the red, I'd be concerned.
Questions to ask.
- How accurate is the stock gauge you are going off of?
- Is the coolant temperature sensor in specification?
- OEM / Old / aftermarket. Are they all the "same" or close enough?

Take a look in my profile & follow the link to another website where I kept notes on my build in progress. It describes what I did & how I got the engine to apparently run slightly cooler.

 

The engine fuel management system requires the operating temp to be 195*-215* for proper mixture control to keep emissions within parameters set by the EPA. If lower than that, it will run rich because the PCM thinks it's still warming up. Before fuel injection, the sweet spot was 180*, but we'll never see that again.

 

I just replaced my radiator and thermostat on my 1996 XJ. Murray 195F thermostat tested good (perfect actually). While running straight distilled water and cooling system cleaner, I was sitting below 210F indicated for a full hour of driving. It is noticeably lower than when the failed radiator and factory thermostat were in, BUT it appears to be on the low end of the proper operating range. Nothing I did could get it up to 210 either. Sitting at an idle, heater off, heater on, driving 45 MPH, driving 30, didn't matter.

Seems like it is all working OK.