Discussion in '6.9L IH & 7.3L IDI Diesels' started by James Case, Feb 15, 2020.
Look for bubbles in the overflow. That's the weakest point for air to escape.
The only time that I've had an issue with this sealing was when I accidentally had a thermostat slip out of place and part of the metal "flange" was sitting out of it's groove. On things like this, I like to use RTV on both sides of the gasket. It makes it easier to clean up the surfaces the next time you have to replace it. The gaskets really like to stick themselves down.
By any chance is it the bolt hole leaking and not the housing? It’s been a while since I did a t-stat on one of these engines but maybe someone put to long of a bolt in at one time and made the blind hole a through hole into the cooling system? Mite just need to put some thread sealer on that bolt?
I had this problem too. Check the recessed area in the block where the t-stat sits and make sure there isn't any rust or debris in it. It's a snug but not tight fit. It had a bit of corrosion where the outside of the t-stat sits and I didn't clean it out good enough. It must've not let the housing seat fully.
The Right Stuff is great sealant and should work well.
You don’t have a blown headgasket,
if you take the rubber off the thermostat then it will seal ( go buy the oem motorcraft replacement)
but since it’s not motorcraft it will fail eventually and you’ll be swapping it again , (took mine out in the middle of a road trip cuz it failed )
if you use s sealer , you run the risk of getting it in your coolant and then you get to flush it out which is a headache you don’t need
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