thermostat housing leak 2000 f150 5.4

It started leaking before you changed the thermostat and gasket or after? Did you torque the thermostat housing bolts?

Look at it closely and be sure that the housing itself is not cracked.

I don't know about the 5.4's but Ford recognized a problem with the intake manifolds of the 4.6's of that era and put in place an unlimited mileage 7 year extended warranty for replacement of the manifold should it fail.

Try going to our home page and clicking on the gray/silve button marked (Bulletins) see if you find anything there.

Make sure the thermostat doesn't shift out of it's inset while the housing bolts are snugged up, by holding it in place through the hose port opening w/ a finger.

Get your hands on a coolant pressure test kit. Then, clean the area of any residual coolant. Hook up the kit, pressurize and look for leaks. Then tighten clamps/replace components as necessary.