I have a 99 Jeep Cherokee XJ. It had been sitting for a few months. I drove it today. After about a mile, the temp shot from 210 to 250 almost instantly. Turned engine off. Started a minute later and temp was back at 210. Another couple miles and it shot back to 250. This time I kept driving. After 30 seconds it went right back to 210. Before i got home it went back to 250.
The temp swings were instant and not gradual. I'm thinking that the thermostat must be stuck open or shut I guess. Thanks

 

Could be a fault in the sensor, but the thermostat seems like a great place to start investigating.

 

I've never replaced it myself, so it's probably due anyway. Thanks

 

Do yourself a favor and get a Mopar one. Too many reports of bad T-stats out there.

You might also want to test for exhaust gasses in the coolant. Your symptoms might be a blown head gasket. If it is, you want to know now.

Also, what color is your coolant? If it's rusty brown, you might need a water pump. The brown might be what's left of the fins.

 

The thermostat is for sure in play here. But also in play is the coolant temperature sensor. On a 99, it is located on top of the thermostat housing.

What is the history of cooling system maintenance on this XJ? If it's been so long that you can't remember, it's time. New Mopar 195 Stat, (I agree with BlueRidgeMark here, go Mopar) fresh 50/50 coolant and distilled water, new hoses if they haven't been changed. Test/verify your fan clutch and be sure shroud is intact. And while you're at it, why not swap out that coolant temp sensor with a genuine Mopar sensor. I'm not a big fan of throwing parts at this but when you mention that the temp changes "instantly" on your dash temp gauge, that could be an electrical problem and the #1 culprit here may be the CTS. Not that expensive and super easy to install when you are putting in that new Mopar stat.

Cooling system maintenance, hands down...the BEST money you can spend on your XJ! You do not want an actual overheat, it can lead to a blown head gasket. Or worse.

Good luck and keep us updated!

 

The tstat was replaced in 2012. Just put a new radiator in March 2020. But it has been sitting more or less since for the past 6-7 months. I'm thinking to do the housing, stat and sensor all at once. The immediate spike and decrease have me thinking it could be the sensor.

 

One thing to keep in mind here is with OBDII systems, the gauges do not receive information directly from the sensors, but read whatever the computer tells them to read. Instantaneous swings of the needle generally indicates an electrical problem. The temp sensor is nothing more than a thermistor, or a variable resistor that changes value due to temperature. The hotter it gets, the less resistance it has. SO, if you unplug it with the engine running, it should show full scale hot, I would think. You may just have a loose connection.

This is with systems that only have a temp sensor at the thermostat housing. If you have a temp sensor on the rear of the head, that sends information directly to the gauge.

I've used Stant Superstats in every vehicle I've ever owned and never had one stick closed. I've had several that have stuck open however. The thing I like about them is they "throttle" the flow rather than pop open or closed.