So my 04 TL has been overheating lately. When I drive normally its fine. When I go uphill it's also fine, but then I go downhill right afterwards and the temp gauge goes up(when I'm not accelerating). It goes up to maybe up bit more than halfway, around ~65%. The car gets worse when I put the ac on. As I'm driving around for a bit with the ac on, the temp gauge reaches a bit more than 3/4 full, maybe around ~80%. The driver's side fan doesn't work when ac is on, but I'm not sure if it's supposed to.
I've changed everything I needed to for my 105k service including the water pump. I don't drive my car too hard and it's currently at 130,XXX miles. I'm not a mechanic or anything but I am guessing the thermostat is messed up and it delays. Any help or suggestions would help. Thanks.

 

I have a suggestion:
-make sure the temperature sensor isn't faulty and sending a bad signal to the gauge on the dash before you start changing parts like the thermostat. My old TL that my brother has was doing this and just cleaning the sensor contact fixed the issue.

 

My vote is bad temperature sensor, too.

 

When was your last coolant flush? If it's recent, there might be an air pocket trapped in the system.

 

hows your engine oil? milky? white smoke from exhaust? maybe leaking head gasket. if so you better take care of it asap otherwise you will need a new engine

 

if you suspect a stuck thermostat, would it be a good way to check by checking the temperature of the upper radiator hose? If it's stuck closed i'd suspect it to be relatively cool.

 

Coolant in radiator filled to top and also in overflow container when cold?

 

hmm It seems like to me though the car overheats when run hard like going uphill and while using ac. I'll keep that in mind though.

So your saying burp the cooling system? hmm I did change it like a year ago but don't think I burped it.

I just changed the oil few weeks ago. No smoke. Don't think it's a head gasket.

It's not stuck for sure, I just think it's faulty because a mechanic checked it out with infared temp gun. His thoughts were that the thermostats delays.

Overflow container is below minimum, coolant in radiator filled to the top. Should I fill the overflow up more? Last time I did that there was coolant leaking around the cap area.