I am at my wit's end. I have a 2002 Dodge Durango 5.9L, and performed a coolant flush (it was long overdue for one) It took me 16 gallons of distilled water before it finally came out mostly clear.

Anyways, I have everything complete, however every time I install the thermostat housing (which is brand new, as well as the hose) there is a leak right between the housing and manifold.

At first I tried using just the paper gasket. Then after a couple of failed attempts, I tried lightly sanding the manifold, and got some permatex aviation and applied to the housing and manifold (lightly) and tried again. No luck, still leaking.

Then I tried cleaning the area again, got a brand new gasket (with sticky side) new coat of permatex on manifold and bolted back on. This time, I thought I was good, until I realized after some time the leak appeared, so nows a very slow leak, but a leak nonetheless.

I dont know what else I can do. Im going crazy trying to stop the leak. The manifold does appear pitted, so I am sure that has something to do with it. The only other thing I can think of is I got another bottle of permatex, but it is make a gasket meant for water pump/thermostat. Would that be better than using paper/aviation?

Anyways, any help would be greatly appreciated.

 

You sound like me. I've perfected things to death too, lol. I have a 4.7 and have not replaced a thermostat on a 5.9 before, but here is my two cents.

First off, you're using the wrong stuff. You need to use coolant resistant RTV, or it will break down. I looked up the aviation stuff you mentioned and....."Permatex’s Aviation Form-A-Gasket No. 3 Sealant Liquid is a slow-drying, non-hardening brush top sealant approved for use in aviation as well as automotive applications. It’s resistant to gasoline, oil and grease."

So yeah, use a tube of the water pump stuff, like this....
https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

Also, u don't need sixteen gallons of distilled to flush. You really don't have to obsess over the color. One flush with the water would have sufficed, or u could have flushed to your hearts content with tap water, but made the last flush distilled. Jack up rear end when draining. Jack up front end when filling and bleeding air. Heater temp to full hot so core is opened. Anyway, I digress.

I'm not sure how much u sanded and with what grit u used, but you should have probably used steel wool, if anything. Nothing would have been best. Usually a wipe with some brake clean on the mating surfaces is enough for me. If u sanded with the same amount of zeal u used for flushing, you may have changed the flatness of the mating surfaces.

Anyway, here is the key. i don't know if a paper gasket will help at this point. I'd be inclined to exclude it now. I would clean with brake clean, then put on a nice continuous bead of the water pump RTV, going around any holes etc. Now assemble and BARELY FINGER TIGHTEN, and I do mean barely. Now walk away and let it firm up for a half hour, then come back and torque it down evenly, going incrementally, side to side. Don't go gorilla either. Then let the truck sit for 24 hrs before u add back the coolant. It needs time to cure.

Hope this helps.

 

Make sure you clean the manifold surface with emery cloth very well. Then get some good silicone( ) and put a light coat on all the mating surfaces. Put it back together and don't put coolant in it till the next day.

 

Have a look at your new housing as well. "New" does not always imply "good".....

 

Back up and quit trying for perfection. It's been a while since I put a thermostat in a small block Mopar. As such I think I remember how it is set up but I might be wrong. Now, pull it apart and clean it good. I believe the thermostat fits into a groove in the thermostat housing. The gasket goes between it and the block. Get some of the Permatex water pump and thermostat gasket maker. It's a little thinner and will fill any small pits that can let the coolant seep. Loop, but do NOT tie a piece of mechanics wire through the bow on top of the thermostat. Feed the wire through the hose pipe so you can pull it up. Sometimes the thermostat can slip as you move the housing into place. Glue the unit into place with the gasket and sealant and let it sit a few minutes. Now put a little more sealant on the block side of the gasket and put the housing in place. While you are moving it into place, pull the wire snugly so it doesn't shift. Bolt the housing down mildly and make sure it is seated. Grab one end of the wire and pull it out. Now tighten the housing down. If you're having trouble with one of the bolts, get a distributor wrench so you can get under a bracket if one is in the way.