Hi. I have a 1994 f-150 5.8 with the thermostat housing leaking. I redid it with a new gasket and Permatex Thermostat Gasket sealer and it started leaking again. Does anyone know if they make the o-ring type of seal for this truck or do I have to only use the sealer and gasket? Any tips on how to seal it better?

 

Gasket and a thin coat of sealer should work. My guess is that the housing is warped.

 

If it is a warped housing tape a piece of 100 grit sandpaper to a flat surface, and lap the mounting flange of the housing. Once it is flat reinstall it.

 

Take the thermostat housing and put a good straight edge to it, see if it's truly flat in all directions. I fought for a long time with mine trying to get it to seal, turns out it was warped. Replaced with a used Ford one that was flat and that fixed my problem.