I set out to do a coolant flush but ran into an increasing number of problems. When I went to replace the thermostat I discovered one of the three bolts would not come out due to the inside thread on the thermostat housing being stripped. I purchased a brand new housing and O-ring.

When I reinstalled the thermostat (also Motorcraft brand) with the above O-ring and attached all the hoses I had a severe leak between the thermostat housing and the intake hose connecter. I diagnosed the problem as the O-ring slipping off of the lip or ledge inside the thermostat housing and not sealing all the way around.

The Motorcraft O-ring seemed like the diameter was too big for the interior lip of the thermostat housing because the ring would pop up on one side whenever I tried to fully seat it on the interior lip. I went to AutoZone and purchased an orange Duralast O-ring, and while it seemed to be a little smaller diameter than the Motorcraft it too slipped off when I tightened down the housing.

I have tried everything, and every time I tightened down the housing I did it very slowly and carefully doing an 1/4 turn at a time with the three bolts but still the O-ring would slip off.

Any suggestions?

 

I use Permatex No. 2 and "glue" seals like that in place before installing any bolts.

 

I'll try that, thanks!

 

Bad news, I glued the ring in place and let it set for a day but when I put it together the O-ring again slipped out and did not seal properly. Any other ideas?

 

Huh...

I never let it dry, I assemble immediately after application and *then* let it dry for 24 hours before letting it get wet.

This is also a place where I will sometimes use RTV (something I generally avoid most of the time except on water connections like this).

But I don't think that alone would cause the problem you're experiencing. Does this engine use an indexed thermostat that fits in only one way (and is it installed correctly)? Other than that, don't know what to tell ya - maybe look in a Chilton's or Haynes manual in a parts store and see if you can get any clues on installation procedures from there....