Ls 400 temperature gauge has moves back medium from red line area while driving the last two days. I added antifreeze no visible leaks. Today stayed in red area while I was driving to mechanic. It went from little smoke to heavy smoke. Thermostat??

 

could be thermostat but most likely it is air in the cooling system (hoping it's not anything more major such as a blown head gasket). you will need to bleed the system properly, which your mechanic should be able to do easily.

http://www.lexls.com/tutorials/cooling/coolant.html

 

You most likely blew the head gasket or a seal if it went from light to heavy smoking. Hopefully you didn't warp the head while you were driving it in the red or you're in for a big ticket bill (head job or new engine). Hope you have the best of luck with it though.

 

are you building an overheating car or is your car overheating? lol. Just joshing browski (cuz this is in the build thread). Would love to know what happened though.