The 2000 3.8L Mustang comes from the factory with a 192º thermostat that keeps the engine at a normal operating temperature of 190º-200º.   But if you live in the Southwest (like I do) you'll occasionally see an indicated water temperature of 210º-215º when idling in traffic on a hot (100º+) summer day.   Those high water temperatures can be reduced by installing a cooler thermostat.   Since I installed a 180º thermostat in my car I'm seeing normal operating temperatures of 180º-190º on those same hot days.   As you'll see in the write-up below, on a 3.8L V6 Mustang it's a simple swap.... no more difficult than an oil change.

Installation

Let the car cool completely.   Then siphon or drain about 1 gallon of coolant from the radiator. That will lower the coolant level below the thermostat and allow you to disconnect the upper radiator hose from the thermostat housing without any spillage.

Remove the two bolts (5/16" socket) that hold the thermostat housing to the block, and pull the thermostat out of the housing.

Note that the stock thermostat (right) has a small hole that helps the system pressure stabilise and the valve open easier. This hole also allows air pockets to move past the thermostat even when the thermostat is closed.   My new aftermarket thermostat (left) did not have this hole, so I made one using a 1/16" drill bit.   Also note that there is a relief area cut into the thermostat housing (center) so that this air hole is indexed at the 12 o'clock position when the stock thermostat is installed.  

When you install the new thermostat, be sure that the side with the spring goes inside the block and that it is rotated so that the hole you drilled is at the top (12 o'clock position).   The arrows on this diagram show the direction of coolant flow, from the block to the radiator.

Some new Mustangs seal the thermostat housing with an o-ring gasket.   My 2000 3.8L engine uses a fiber gasket (Fel-Pro #35390).   It stayed stuck to the block and did not tear, so I decided to reuse it.   Apply a layer of gasket adhesive to the mating surfaces, some anti-sieze compound to the bolt threads, and use a light touch when bolting the thermostat housing back onto the aluminum block.   The specs call for only 89 lb-in (7.4 ft-lbs) of torque.

Refilling the Cooling System

The vent plug must be removed before the radiator is filled or the system may not fill completely.   After filling the radiator, add some more coolant (same 50/50 mix) to the vent until it is full. Then install the vent plug.   Leave the radiator cap off and run the engine until it reaches normal operating temperature.   Stop the engine and add coolant to the coolant overflow bottle until it is between the fill level marks.

UPDATE - 06/20/04:   I've received several emails from people asking where they can get a 180º thermostat for their 3.8L V6 Mustang.   I found mine, a heavy duty Stant # 65358 Superstat, at a local autoparts store.  Stant is a well known brand.   If you can't find it in your area, you should be able to cross-ref that Stant part number to an available substitute.

UPDATE - 08/11/04:   Why not use a 160º thermostat?