when should it be replaced , ive looked in the book and found nothing
and is there a better free flowing on out there ,or would that do anygood?

 

You only need to replace it when it goes bad. And only replace it with the factory part. The thermostat helps the engine reach operating temp as quickly as possible as well as keeping it cool.

 

You don't have to replace it with a stealership part. I have a Murray 180-degree thermostat in mine, no problems several months later.

In fact there are better designs in the aftermarket, factory does not supply a failsafe design and when I pulled my factory unit out, it was stuck 3/4 closed. Which placed the needle on the gauge little past halfway when the truck was warmed up. With a 180, which is only 12 degrees colder than the factory unit's opening point and it now sits just above the first hash mark on the gauge. A thermostat alone will not change how cool one's vehicle runs, it just raises or lowers the bare minimum operating temperature. The other components of the cooling system (Radiator, fan, etc) and external factors (Temperature outside, etc) determine the maximum operating temperature.

Just going off memory here, but the Murray I put in also had more of the metal cut away from the outlet side than the Motorcraft unit, which smooths out coolant flow.