4.6 Windsor thermostat housing will not seal

[shadow460]

This is actually my wife's F-150 but it's a very similar engine to the one in my Expedition.
I replaced the thermostat due to it throwing P0125, indicating the stat stuck open. I got a stat and o ring from O'Reilly and installed them. It leaked like no other, so, absolutely fed up with the crap parts I've been getting from O'Reilly recently, I went to Napa. They set me up with an o ring and a gasket, which leaked just as bad. I removed the gasket and reduced the leak to a slow seep.
This truck has *never* leaked coolant in this area, although I replaced both the radiator and water pump a year ago. There were Zero coolant leaks up til I replaced the stat a couple weeks ago.
How on this green Earth do you seal the thing? Everything looks absolutely perfect in there, it's a one piece manifold, torque on the bolts is 22 ft lbs... everything is right but it simply will not seal.
Any advice before I drop a bill and a half off with my mechanic?

 

[1955moose]

I always use a thin layer of high heat red silicone around the housing. Make sure you clean both sides of the mounting surface. It's possible theirs a small chunk of something left over. Is the o ring the right size? I like to use Emery cloth after using a razor blade to clean all old crud off. The other thing to check is that either the thermostat housing, or intake is not cracked. I'm betting its a simple fix, once you figure it out.

Sent from my N9131 using Tapatalk