Could be a few things.... maybe an air bubble in the system, bad temp sender, water pump, check the electric fans and make sure they are blowing the correct way(if mounted in front of the radiator they should blow through toward the engine, if mounted on the back of radiator they should suck air through..... 

you might have to do a little more research on your car to give us some more information.

Should the air dam be opened or closed when on the highway?

Not the spliter air dam. The  plastic flap door under the radiator.

Plastic flap door?

Yes, I was wondering the same thing. 

Never mind, senior moment. It was my 80 olds starfire that had the plastic hinged flap under the radiator.

In the four years since this thread first appeared I hope the engine has cooled off by now...!!!   🤣