A real doozy of a problem
 

So, I have a 2012 jku and am having a strange issue. I drive the jeep as it is my daily driver, sometimes very rarely it will climb rapidly in coolant temps. Generally after I stop for a second get out turn it off and come right back. I have changed water pump to OEM new one, I have changed thermostat to a new fail safe one and I have checked to make sure coolant flows freely through it. It seems as though a relay or something is not being set off when it starts back up sometimes. Anyone seen or heard of this little gremlin? I will probably be changing out fuses/relays to ensure it isnt that.

Have you ever changed that coolant sensor? Is there a chance you might be getting a false reading in regards to temp? It's not terrible practice to replace that sensor at same time you replace the thermostat.

Also make sure the fan is running and coming on when it's supposed to

There is something called a fuse link. It is a wire that controls one of the fans. It connects to the battery. If that connection is not good, the fan may not reliably come on. That happened to me. Tightened the connection and all has been fine.

+1 Anytime the coolant gauge is reading high (even erroneously) the fan should be running high speed. Running the A/C should turn on the electric fan as well, which is a good quick check.

:cheers:

I had a similar problem recently. I changed the fan relay, coolant flush, and new thermostat. That all worked temporarily. The final time that the temp rose I tightened the negative battery terminal. Haven't had a problem since. It seems that the negative terminal is very touchy and has been a cause of numerous problems. It never feels loose, but when I give it a crank or two it solves the problem.