Issue for you all to contemplate...

Temp has NEVER moved above 1/2 since I bought my beloved TJ1 magna...
Always stays 1/2 notch below half temp....
Today after 40km highway run...she starts to raise temp to 2 notches above half temp?!?!?!?
Ran her at idle for 15 mins...nil coolant leaks, nil leaks of anything!
All fluid levels ok...all at top mark of region...
Large fan running non stop until 2 marks above half...then secondary fan kicks in...back to normal ark of just under half?!?!?
Never had temp gauge move before...very worried...
Recently had200k service done including new genuine water pump...all hoses ok..ie no crushing or vacuuming or pipes...
Has tranny cooler mounted attached to front of radiator...
Wondering if its just tranny cooler heating up radiator???

Your thoughts.....:eek2:

My first thought was the cooling system needs bleeding or 'burping' as trapped air from a refill can cause overheating. You say fluid levels are constant. When hot, massage the top hose. Soft? or hard, like no give at all? Can't get from your description whether you've considered this so mentioning it anyway in case it might help someone.

edit add :

Just to deal with possible confusion of anyone reading this post :

It's good that the hose is hard when squeezed. It means there is only liquid in the cooling system and the cap and hoses are good.

Another way to see if there is air in the system is to park on a steep incline with the nose up and with the cap off and engine on (coolant circulating) gently squeeze the hose. Watch for bubbles.

To 'burp' the system >> after a refill << :

Park on a steep incline or raise the nose so the cap is the highest point of the engine and, with the engine running, cap off. :

Keep adding coolant untill no more bubbles come out. It might take a while. I let it run for 20 mins or so while doing something else and then start adding to replace any air bled out with fluid.

Once that's satisfactory, Fill to top with engine off, replace cap, fill overflow bottle and you're done. (rinse off spilled coolant)

From now on leave the cap on, system sealed.

Only top up at overflow bottle. Check this regularly.

OP, go with MM's advice re thermostat et.c.

My first thought was the cooling system needs bleeding or 'burping' as trapped air from a refill can cause overheating. You say fluid levels are constant. When hot, massage the top hose. Soft? or hard, like no give at all? Can't get from your description whether you've considered this so mentioning it anyway in case it might help someone.

When hot, top hose is unsqeezably hard ie no give...

May be first sign of Thermostat having issues, start there and use genuine

Mr Madmagna...you know your sh!t !!! (this is meant as a compliment)
It appears I had a combo of problems. Thermostat was RS as predicted.
Second issue was new water pump fitted at last major service (3 months ago) appears to pump a lot better than the old one. Every single piece of crap that was hiding in my engine....was now in my radiator.
New radiator and thermostat fitted, all is good in wonderland.

By the way for those who have bothered to read this far....magnas make a really funny sound when cold soaked overnight at about minus 8....sounds like the whole bottom end was about to fall out until the oil got up and around it ! Awful noise ! looks like I might need to go thinner than 10w with my next oil change !

Mr Madmagna...you know your sh!t !!! (this is meant as a compliment)

By the way for those who have bothered to read this far....magnas make a really funny sound when cold soaked overnight at about minus 8....sounds like the whole bottom end was about to fall out until the oil got up and around it ! Awful noise ! looks like I might need to go thinner than 10w with my next oil change !

Yeah, he knows a lot about cars too! lol

That "aweful noise" is more likely to be the hydraulic tensioner, when they get cold they can take some time to extend and take up cam belt play.
My TL does this, sometimes, in winter as well.
Thinner oil won't help.

Had the same in my TJ - it was the thermostat - it for whatever just got lazy - worked fine during summer 40 degree temps and then packed up in autumn... Cheap fix though.