Ok Im about to replace the temp switch on the drivers side head........Im just going to unscrew the old one and quickly screw in the new one.
Do I have to bleed the system????
also I bleed it After I turn off the engine right??
Ive done this before but its been a while.

 

You bleed it with the engine running. Unscrew both screws and wait for coolant to start streaming out, then close them.

 

If you do it fast enough, you probably won't have to worry about bleeding it at all. Any air that gets in will be purged in a few heat/cool cycles.