CANNOT Seal Thermostat Housing on 5.0

[bmdubya1198]

I just finished up for the 6th (yes, you're reading that right) time... and it STILL leaks. Using a new thermostat housing, gasket, a generous but not overkill layer of Right Stuff on both sides of the gasket, and super glue to hold the thermostat in place. I was confident this time, but within probably 30 second of filling the coolant with water, it began to slowly drip right out of the bottom of the housing. Same spot as before.

Maybe there's RTV clogging the threads and it isn't torqued down enough? This just makes no sense to me. I just want to get this job done already. Never had such an irritating job on any car, and I've manual swapped two Volvos without the electronics going nuts.

 

[sandsprayer]

WHAT? Big time suck
Are we sure the mating surface to the stat housing is true?

 

[NigonKouk]

I just picked up a 2001 Mountaineer 5.0 last week, and I made the mistake of replacing the thermostat for preventive maintenance. I figured this was a basic thing to change before flushing the cooling system and changing the leaking upper radiator hose. Nope!
First time I put the gasket on with a little bit of ultra black RTV to hold it in place. Didn't work too well, because when I went to fill the cooling system, it poured right out from the thermostat housing. Took it back apart, and it turned out that the thermostat had slipped... common problem on here from what I can tell. No biggie... new gasket, re-positioned it, and tried again. Water poured right out again. So next time, I got some Permatex water pump and thermostat gasket maker, went with others' recommendations and used just RTV. Waited overnight, filled this morning... poured right out AGAIN. Tried Permatex Right Stuff... dumped out again. ANOTHER gasket, I was extremely careful this time to prep both surfaces, used a couple drops of super glue to hold the thermostat in place, and let the RTV tack up on the housing before putting it on. Little bit of RTV on the other side of the gasket, then put it on. I was confident this time that it was good... I was wrong. It just drips right out from the bottom of the thermostat housing when I fill the system enough to reach that high.

What am I doing wrong? There's no reason I should have to do this 5 times. I've been working on BMWs and Volvos for a few years now, and I've never run into such an irritating issue that makes me doubt my mechanical abilities. I've always liked these Explorers/Mountaineers, but my first American car is proving why I've always had European cars. They are SO much easier to work on!

Maybe it's not the right thermostat, the thermostats that I replaced I think they had a rubber bushing that the upper part pushed down on to help seal it. And I used high temp high pressure silicone and let it set overnight. That's just from my experience.

 

[sandsprayer]

It almost sounds like your bolts may be too long keeping the housing from being clamped down.

use bolts that are not too long

clean the holes real good. debris will cause the bolts to bottom out before clamping

Measure the depth of those holes against the bolt lenghts

 

[Ken2012limited]

Hose tight at thermostat housing?

 

[bmdubya1198]

Yup, all hoses are nice and tight. I'll have to see if I can take a picture of where it's leaking. I also saw it seeping from the top when I squeezed the rad hose, around the 1 o'clock position.
I'll take a look at the bolts and threads. I made sure that they were nice and snug, and they certainly are.

 

[koda2000]

Have you tried checking the intake manifold side for flatness? Maybe that side isn't true.

 

[CDW6212R]

The short bypass hose is made too long, OEM and all others.

I trim both ends of that, so when it's on the bend is not pinched, it's 90* matching the two nipples. Any top brand is worth using, none have blown on me except old high mileage originals, twice in the 80's on 351 Crown Vics(I had two). Always replace the coolant hoses when they are old or you don't know how old they are.

 

[CDW6212R]

If it leaks the common reason is the thermostat fell during installation, and the lip is slightly between the housing and intake. When you put it in place, you should be able to wiggle it around and verify by feel that it's seated properly. I hope that's not it, I have done that about three times and it is maddening/embarrassing.

 

[97MarkVIII]

Stop putting RTV aka GASKET MAKER on gaskets,damn people. it's not a gasket sealer. Make sure the bolt holes are clean and clear and the bolts aren't too long if not OEM. Check housing for cracks and flatness. Pick up some Permatex High Tack gasket sealant (NOT a sponsor) and put a thin coat on the intake,gasket (both sides) and housing. Did that with mine and not a drop. BTW RTV can act like a lube a cause parts to move out of place. Plus people tend to use too much then it breaks off and clogs passages etc. Eric the car guy just redid a rear end and the pinion bearings had RTV in them because some moron put RTV on the pinion seal. Hell my transmission pan gasket had RTV on it from some idiot. Anyways the bad with the high tack is trying to clean it off. And always try to use OEM parts. I rather go to the junkyard for another T-stat housing and bolts then buy some made in China Autozone pos. Good luck

 

[donalds]

Sometimes rtv is necessary. Just saying

 

[bmdubya1198]

RTV has its place. I usually prefer to avoid it, and only use it to hold gaskets in place during installation or when it's called for around valve covers and such. I was mad when I noticed how much RTV the last guy who did the thermostat and water pump used, then I realized after I messed up the first of 6 installations maybe I should use a thin layer. The high tack gasket sealant would probably be a good idea in this situation to help hold the gasket and thermostat in place.

I don't think the thermostat slipped down again, that happened the first time, but from how the gasket/RTV was creased, it didn't happen again. The string method of holding the thermostat works well too, but you need to be able to hold it while bolting the housing in place.

Somehow after clearing out the right bolt hole a little bit (definitely a little bit of old RTV in there) and tightening it back up, it seems to have slowed the leak. Not gonna complain!
Just need to figure out how to reach the bleed screw so I can fill the coolant. This system is so clogged up it's ridiculous. Nastiest one I've ever seen.

 

[CDW6212R]

I love high tack for water pump gaskets and other simple easy to work with gaskets. The side mounted thermostats are a whole different animal, whatever gets it on without leaking is the best. Yes the high tack is tough to remove, but not really much worse than RTV such as on timing cover(WP) gaskets.

I don't over apply RTV, the only place RTV should be put on thick is the front and back edge of intakes. A thin skim coat is all it takes to make the thermostat stick well enough to the housing. But it still requires extra patience while placing the parts and installing the bolts. I put it on without bolts, and then do not let go of it until the bolts are snugged. That means using one hand to install each bolt, so the tool you use better work freely unlike a wrench etc. I like a 1/4" ratchet and socket with short extension. It's best to get everything out of the way around the thermostat, spend time to make room, which helps a bunch.

 

[donalds]

Only place I use more than a thin coat is on the rear diff cover

I like the brush on hi tack

 

[Tgerpwz]

I just picked up a 2001 Mountaineer 5.0 last week, and I made the mistake of replacing the thermostat for preventive maintenance. I figured this was a basic thing to change before flushing the cooling system and changing the leaking upper radiator hose. Nope!
First time I put the gasket on with a little bit of ultra black RTV to hold it in place. Didn't work too well, because when I went to fill the cooling system, it poured right out from the thermostat housing. Took it back apart, and it turned out that the thermostat had slipped... common problem on here from what I can tell. No biggie... new gasket, re-positioned it, and tried again. Water poured right out again. So next time, I got some Permatex water pump and thermostat gasket maker, went with others' recommendations and used just RTV. Waited overnight, filled this morning... poured right out AGAIN. Tried Permatex Right Stuff... dumped out again. ANOTHER gasket, I was extremely careful this time to prep both surfaces, used a couple drops of super glue to hold the thermostat in place, and let the RTV tack up on the housing before putting it on. Little bit of RTV on the other side of the gasket, then put it on. I was confident this time that it was good... I was wrong. It just drips right out from the bottom of the thermostat housing when I fill the system enough to reach that high.

What am I doing wrong? There's no reason I should have to do this 5 times. I've been working on BMWs and Volvos for a few years now, and I've never run into such an irritating issue that makes me doubt my mechanical abilities. I've always liked these Explorers/Mountaineers, but my first American car is proving why I've always had European cars. They are SO much easier to work on!

Ii usuly take a string and tie it to thermosat...pull string through hole and put on.

 

[fast_dave]

@bmdubya1198

Just sayin - pictures are worth 1,000 words

Your mileage may vary -

 

[97MarkVIII]

With high tack it actually sticks or to the gasket and metal vs most rtv's just sit on the gasket and will peel off. Back in the late 90's not knowing any better I would put rtv around the coolant ports when doing an intake, smh. And they never would seal right until I stopped using it. Now if there's pitting then I would use high tack or to hold the gasket in place like on SBC otherwise it's dry. RTV is thicker than high tack and can cause the gasket to not lay flat and prevent the gasket from sealing to the metal. And of course rtv for the end rails "China wall" if they were cork. I do use the silicon ones though as that's what rtv is anyways. Other thing is people usually don't do is let rtv cure properly then it leaks. Did the rear end clutches and I had to wait 24 hours for the rtv to cure before adding the gear oil.And only put down about 1/8" bead, max. But of course then the rear main seal decided to get worse and drips right on the cat and smokes now,so it's been sitting all summer.

 

[Or Bicycle]

My original housing eventually leaked where the metal connector for the bypass hose entered the housing. The difference in metal caused corrosion that destroyed the tube. I had a piece of copper tube plumbing at home. I was able to pull out the old connector and with a dab of sealant as lube pounded in the copper tube. It even got a lip on it to insure the hose will not easily pull off. BTW, I did the same thing about 15 years ago on the water pump without removing it. That water pump now has 330,000 miles on it as it is the original. I have a small water leak now that I have not found. I am thinking maybe it is time to dig in and replace that pump. Other things keep getting attention first.

 

[Or Bicycle]

Could the housing be cracked? I have a 2004 4.0 sohc and the thermostat housing is plastic.

See a reply I made to the main post. I should post here more. I fixed a housing leak I had a couple of months ago.

 

[BUXTER]

DITTO to ALL the above - & Yes I cracked my Thermostat Housing when the Vertically Installed Thermostat slipped & I tightened the fasteners, so yes something to glue Thermostat in place is the WTG - There are SEVERAL different angles of Thermostat Housings so get the same one or ur Upper Rad Hose won;t fit - Gurus >>>> WHY do we need the Vented Thermostat when the PITA Bypass Hose is RIGHT THERE to vent it______?