Where is the thermostat on a 91 Buick LeSabre Custom 3.8L?

Answer

havent ever worked on that car,but in most cases the thermostat is located where your main water line from radiator connects to engine,just follow the main radiator line to engine the part that the water hose connects to on the engine usually has two bolts take two bolts off thermostat located underneath.

This guy is mostly right, but these cars only have one (1) bolt holding the thermostat housing on, you have to take the piece of plastic trim off the intake that has "Fuel Injection 3800" embossed in it to get to the thermostat. As you're putting the thermostat housing back on wiggle it a little as you're tightening the bolt to mske sure it goes in all the way.