Weird Overheating/Temp Sensor Issue on new to me XJ....

[Sleeper Aero]

Just picked up my second XJ over the weekend from Florida. 1400 miles and it performed great. Temp sat at or a hair over 210 the whole way home.
Anyways, get in it today and start driving. Warms up and eventually gets to 210. Cruising at about 40 mph I hear a chime, look at the dash and see my temp is just below the red. Pull over and shut it down and check under the hood. No boiling coolant, no leaking everythings good. So I give it a minute and start it back up. Temp goes right to 210 and all is good. So I keep driving and got home without another incident. So just a bit ago, ran to the gas station and it was fine for about 15 mins. Coming home all the sudden the temp rockets to just the redzone and sits. Keep driving and after about 25 seconds, the temp goes right back to 210. So now I'm stumped. What is going on? Temp gauge broken maybe?

 

[Jess]

year? sounds like air trapped in the closed system. maybe a short in the wire from the temp sender gague.

 

[ParadiseXJ]

What year? Open system or closed?

I'm thinking X2 on the air bubble. There are a couple things you can do to bleed it.

 

[Sleeper Aero]

The XJ is a 99. Just drove it again. Same thing. Drove for about 5 minutes at 210, then it suddenly and rapidly shot to the red zone. About 30 seconds later it suddenly and once again rapidly fell back to 210. Also, this time when shut off the Jeep, there was a loud hissing noise coming from under the hood. When I opened the hood, it sounded like it was coming from under the battery. Any ideas?

 

[Ecomike]

The XJ is a 99. Just drove it again. Same thing. Drove for about 5 minutes at 210, then it suddenly and rapidly shot to the red zone. About 30 seconds later it suddenly and once again rapidly fell back to 210. Also, this time when shut off the Jeep, there was a loud hissing noise coming from under the hood. When I opened the hood, it sounded like it was coming from under the battery. Any ideas?

Wow , lots of possibilities, first you need to check the coolant level when it is cold.

Was there steam with the hissing? If not, the hissing could be a vacuum leak, and may be unrelated. If there was steam then you may have a problem. Might be a loose, corroded or bad temperature sensor in the thermostat housing if the coolant system is full, in good shape and not leaking, but that is a lot of ifs you need to verify!

Thermostat could be sticking, coolant could be low due to a slow leak over time.

 

[SlackerUSC]

Are you running the AC when this happens?

 

[98XJSport]

My 98 had kind of a similar problem. Once in a while when I put it in reverse, the temp would max, stay there for a few seconds, then drop back to normal. But the jeep was showing no other signs of overheating. Since the newer 4.0s have the front sensor for the guage, I traced the wires back from there to the ECU. The wiring harness was pinched between the fuel rail and firewall, it eventually rubbed through the plastic wrap and the return wire for the coolant temperature sensor. Little bit of electrical tape and some zip ties, and the problem has been gone since fall. Worth checking out.

 

[Sleeper Aero]

Alright so here's what I know. Coolant level when it is cold is fine. Same when it is warm. I have not been running the AC when this has been happening. The one thing that gets me is how fast the temp rises and falls. What I mean is that it will be right at 210 and then when it goes up, it just speeds up to the right, no slow creep. Same when it cools back down, it just falls right back to 210.

 

[wcbuyer]

2 thoughts - 1: bad electric fan relay; 2: bad fan clutch

 

[98XJSport]

Alright so here's what I know. Coolant level when it is cold is fine. Same when it is warm. I have not been running the AC when this has been happening. The one thing that gets me is how fast the temp rises and falls. What I mean is that it will be right at 210 and then when it goes up, it just speeds up to the right, no slow creep. Same when it cools back down, it just falls right back to 210.

The fast reaction makes me think more into the wiring, Imagine turning your XJ off and waiting for it to cool 30-50 degrees. Thats what your guage says is happening, but if it's happening in seconds, I doubt it's really reflecting the temperature of your coolant.

 

[Ecomike]

The fast reaction makes me think more into the wiring, Imagine turning your XJ off and waiting for it to cool 30-50 degrees. Thats what your guage says is happening, but if it's happening in seconds, I doubt it's really reflecting the temperature of your coolant.

X2, if its instantaneous or takes just a second or two, sounds like wiring, or the sensor is going bad internally, barring any other signs of overheating.