Just to make sure, you list that you have a 6MT, but unless you're one of the few people that swapped in a 6th gear, you most likely have a 5MT. Assuming you do have the manual transmission. The mishimoto radiators don't work on the automatic transmission models without a separate transmission cooler.

That being said, $50 is pretty tempting for an all aluminum radiator. I just bought an OEM radiator for my automatic Element, and had planned on installing an aftermarket transmission cooler anyway, maybe I should think about installing that one...

 

Have you checked the relays to see if their functioning as they should? Also, are you sure there's no air in the cooling system from the refill?

 

Cars overheat over two reasons.

1.- cooling system not disipating the heat from the engine as it should.

2.- engine producing more heat than it should, therefore, overloading the cooling system.

Looks to me that you have checked all or most scenarios on option 1.

It caught my attention that it overheats at highways speeds, which is when the cooling system operates best, regardless of the fans and temperature switch.

It is also when the engine works the hardest for a steady period. I would look at valves & timing as well.

Also. How you know is overheating? Is it loosing coolant? Or is just the needle? How much is it moving outside of "normal"?

 

Get ahold of those Samco silicone hoses!! Ever since I've put them on my E never gets hotter than 1/4 on tge needle. I just hauled 700lbs ovrt 600 miles thru mountains and hills and it stayed true to that rule. Might not totally solve the problem but it sure would help. The E ysed to run close to the half way mark wuth rubber hoses.

 

Sorry, hoses won't make an engine run cooler.

 

What are the condition of the fins on the radiator? You may need to straighten them if they are bad enough to prevent airflow.

And yeah... I've never seen silicone hoses produce a nearly 50 degree drop in temperature on any street or racing vehicle.