So I'm pretty new to Jeeps. Bought a used one last year and it needed a little work to begin with, its had the thermostat and waterpump replaced already. The needle will go past the halfway mark just a little when idling every once in a while, and up to or even past 3/4 after a while of driving. The fan always kicks on, and it sounds like a jet engine. It'll start overheating when I'm not accelerating, like going downhill on the highway. But it will start cooling back down when the RPMs start going up, like when hoing up an incline on the highway. Dealership couldnt figure it out or replicate it, probably because they are lazy and didn't take it for a drive like they said they did. So they just replaced the thermostat again. It's still doing it and I can't find any answers or anyone with that same problem.

 

I think your first step has to be making sure that system is 100% bled. Have you double-checked, in the morning when the system has set overnight, that the coolant is filled to the top of the radiator neck? You would think the dealer would ensure it's bled properly......but I think that is often a poor assumption.

 

Yes start there, and also look for any kind of evidence of leaks anywhere.

Also make sure the radiator is not physically blocked somehow and that there is airflow through the fins (ie: not crudded up with bugs and dirt etc).

If all that fails you may have a faulty water pump that's not circulating like it should unless the revs are higher.