00 Running Warm Instantly

My buddy just got an 00 XJ 4.0, auto, 242, etc. I've had a 99 for a couple of years.

He is having issues with it running too warm. What happens when I checked it out is that the temperature gauge is low for a minute or two, then it shoots right up to the 220 area instantly. It stays above this area and goes up to the next notch. He said it even went up to the red area when he turned the heat on. When this occurs the electric fan instantly goes on.

The reason this is so strange is that it's been 20 degrees around here for a while and there is no way that it should be running that hot that fast. Mine takes a good amount of time to warm up to the 210 area and his is above it instantly.

He just bought it the other day and I replaced the serpentine belt because it was running very squeaky and the power accessories were straining the engine a lot. I figured this would help the water pump run better but it had the issue before i replaced the belt and still has it now.

What should I look at next? Thermostat? Coolant temperature sensor? ???

Thanks Guys!

verify that the engine is actually running that hot with some kind of themometer (IR thermometers are getting pretty cheap at parts stores). If it is indeed running that hot then I would replace the thermostat. Take it from me, a T-stat should only be bought from the dealership. I have yet to see a waterpump that is bad that doesn't leak from the weep hole or make some kind of noise, so I would hold off on that.

The temp sensor or wires gotta be bad. Feel the engine temperature vs. yours. Does his feel hotter after 2 minutes? Feel the radiator hoses too. Compare with IR is even better, but you should be able to tell by feel.

Thanks!

Is there two temperature sensors on the 2000 XJ, one being for the gauge in the dash and one being for the ECU, or is there just one? I saw someone post that in a 95 there are two. Now if the fan is going on than the ECU one thinks it's hot and this coincides with the dash one.

I am going back to school and will see my buddy in a couple weeks so all the information I can get before then will be greatly appreciated.

All I can find is one in the thermostat housing. I'd check the connection and wires there too before I replaced, but it kind of sounds like the sensor itself. I haven't been able to dig up resoistance values for you on that, but they should be available somewhere. Should be a resistance vs. temperature which could be checked with an ohmeter.

I think the older XJs had two temp sensors and the newer ones have only one. Can't remember what year they made the change.

I agree that you gotta determine if the temp is real or not. If the temp jumps INSTANTLY, it probably isn't real. If the temp climbs quickly, it could be! Point an IR thermometer directly at the thermostat housing. It's really important that your buddy doesn't overheat his 2000. Overheating is of course not good on any vehicle but the head on the 2000 is susceptible to cracking when overheated. Gotta get under this...