Running your engine without a thermostat will cause it to run hot. An engine will run cooler initially without a thermostat. After that IT WILL NOT! Why you ask? Again, physics. An engine running without the restriction of the thermostat in the system (remember, even when it's wide open there's still some restriction in the system caused by the thermostat's small opening) will not keep the coolant in the radiator long enough for the radiator to 'radiate' the heat. That is, the coolant must stay in the radiator for a certain amount of time to dissipate some of the heat it has absorbed from being in the engine block. The coolant is in the block. It's hot. It moves to the radiator and gets rid of some of the heat. If it moves through the radiator too FAST, without the restriction of the thermostat body, it won't get rid of as much heat. Basic Thermodynamics at play.
Check the fan clutch, it may not be working properly.
Have the water pump checked. My 99 had an overheating problem; guage would go up and sometimes engine would stall. It turned out to be the water pump.
There is no sure way to know until you take the thermostat out and check it. After removing the thermostat, you can test it by submerging it slowly, with a tool in boiling water. You can then see the thermostat open up. If it does, this is not your overheating cause. If it does not, you need to purchase a new one. Install the new one and you will find out if that was the problem.
If it has the 3.5 motor, it may be a matter of bleeding the air out properly. There are several more common causes of overheating.
try doing a block test its possible you might have a blown cylinder head gasket
Check the coolant in the 96 Taurus. Check to insure that the thermostat is working properly, and that the water pump is working.
If it is overheating or taking a long time to reach full operating temperature then it may need a new thermostat. If you even suspect it is not functioning properly just replace it.
Check the 1991 Legacy for a stuck thermostat. Check to make sure that the water pump is working properly.
Could be the water pump is not working properly.
Check the water pump. If the thermostat is working properly then the next common problem would be a bad water pump.
It means the engine is overheating. Possible causes are, thermostat stuck in closed position, low of coolant, cooling fan not functioning, blown head gasket, clogged radiator inside or outside, or water pump not functioning properly.
Check that there is enough engine coolant. Check to insure that the thermostat is not stuck, and that the water pump is circulating. Turn on the heater on high and high fan. If the temp drops even for a bit this is an indication that the coolant isn't circulating properly and the thermostat or pump are bad.