For anyone reading this post, NEVER run your car without a T-stat. If the motor is overheating, removing or installing a lower temperature T-stat will NOT fix the problem.
Removing the T-stat will hurt fuel mileage, delay motor warm up, cause the PCM to remain in "open loop" mode much longer than it should. It's a bad idea.
For the V6, it is vital that all air be removed from the cooling system. If you are sure there is no air trapped inside the motor, then go back to basics.
If after all air has been removed air comes back, then it's coming from somewhere. The first place to look is a leaky water pump. The second place to look is a bad head gasket.
I'm assuming there are no DTC codes (you would have mentioned that right?).
Is the radiator clean inside and out?
Does the fan work? (both speeds for the GT)
Does the heater work? If not, likely there is air in the system.
Are all air dams and shrouds in place?
Does the fan motor itself turn freely (possible bad bearings)?
Is the anti-freeze percentage between 40-60%?
Does the cooling system hold pressure? The top radiator hose should be hard.
Check the water pump bearing for play.
Check for blocked exhaust.
From here on, things get nasty.
In my experience a motor that is in good repair but slowly overheats is a clogged radiator. The best option is to replace.
Check for a head gasket leak. There are kits to test for combustion byproducts in the anti-freeze. A cooling system pressure test is also helpful. A engine compression test is also important.
>>V6 coolant re-fill procedures. Follow exactly for best results.
http://www.allfordmustangs.com/foru...stang-irregular-rpms-showing-hot.html#2604913
