I have a 2009 JK with 171k miles and recently replaced the thermostat. Prior to this, the temp gauge always stayed at high noon once up to operating temperature. Now, it dips down to about the 10 o'clock position and keeps cycling between these two positions as the vehicle is driven, even on a 2 hour drive. I already suspect a bad blend door actuator as well, but that's a separate issue. Is it possible the fluctuation of the gauge is due to air in the system? The heat coming out of the vents also fluctuates with the temp gauge fluctuation. Thanks.

 

Yes, certainly possible that there's air in the system. Also, have you replaced your radiator cap anywhere in those 171k miles?

 

I replaced that as well, at the same time as the thermostat.

 

Have you checked if the thermostat is stuck in position open? Low coolant, a clogged radiator, bad water pump, and other things could all be a potential issue.

 

I suspect the air, because prior to the cap and thermostat change, the gauge never moved from the 12 o'clock position. It only started the fluctuations afterwards.

 

Air in the system or it's also possible you got a bad thermostat.

I had the exact same symptoms when my thermostat was stuck open and needed to be replaced.

 

I've had thermostats stick open before, but I recall the symptom being it just took the car longer to come up to temp. Once up to temp I don't recall the needle fluctuating normal to low. Seems most likely air in there to me.

 

I burped the system by parking with the nose up, and running it with the cap off for at least a half hour. ALOT of air came out, and now the gauge seems to be steady. The heat out of the vents got better, but not 100% yet. Will still flush out heater core. As it turns out, my JK doesn't use the actuators on the HVAC system. It is all cable driven. Found that out after tearing out the ductwork anticipating replacing the blend door actuator, only to find it's cable driven.

 

I noticed it in colder weather climates when my thermostat looked like this.


I'm assuming with it broken like that, it was in the stuck open position. Anyways it was a cold day, I warmed up my JK for about 10min and drove to the airport which is about an hour drive, all freeway. 1st part of the drive I had good heat and temp gauge was at 12'Oclock. Then 15 minutes later temp gauge dropped between 1/4 to 1/2 and my heat also dropped where I could see my breath. As I reached the airport, the temp went back to the 12 O'clock and my heat came back. Miserable ride in the cold but I was happy no over heating. I noticed this back and forth temp reading from 12 O'clock to below 12 O'clock until I pulled that thermostat out and replaced it.

 

The temperature gauge on my 2010 Wrangler JK rises to 1/2 or the noon position upon warming up and the heater works great. Upon driving on the highway, the temperature gauge comes down to 1/4 and cooler air starts to blow from the heater. I have a brand new radiator, new cap and a new thermostat. This is driving me nuts. Any advice would be greatly appreciated.