CANNOT Seal Thermostat Housing on 5.0

[bmdubya1198]

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!

 

[Rick]

Gray RTV without a gasket should do the trick. Also, are you using the stock bolts? It almost sounds like your bolts may be too long keeping the housing from being clamped down.

 

[bmdubya1198]

Gray RTV without a gasket should do the trick. Also, are you using the stock bolts? It almost sounds like your bolts may be too long keeping the housing from being clamped down.

The high torque stuff or the glycol resistant stuff? I'm pretty sure they're the stock bolts, the left bolt is a little longer than the right one, but it also passes through more of the housing.

 

[donalds]

Permatex® Water Pump & Thermostat RTV Silicone Gasket Maker

A noncorrosive, sensor-safe RTV silicone gasket material formulated specifically for water pumps and thermostat housings. Highest water-glycol resistance available in an RTV silicone.

www.permatex.com

 

[koda2000]

Unless you've damaged either the t-stat housing or the t-stat, which is very likely, you should never have coolant pouring out from this location. At this point you may have a warped or cracked t-stat housing. I recommend you replace it with a new one. They're not expensive. You should also carefully examine the t-stat. If you pinched it under the housing you may need to replace it as it may now be interfering with the housing's ability to seal to the engine. Install the t-stat into the housing and glue the gasket in place over it with some high-tac or a smear of the RTV of your choice (I use the red RTV for almost everything except oils) and let it sit overnight, or at least a few hours This will prevent the t-stat from slipping out of place during installation. Do not overtighten the t-stat housing bolts and be sure to use bolts that are not too long.

Tip: Make sure to install the t-stat so that the "rattler" valve is at/near the top. This lets any trapped air escape from behind the t-stat.

 

[410Fortune]

spring goes towards engine
weep hole goes up
No rubber O ring on these
Some 5.0 thermostat gaskets are STICKY, you have to peel the backing off, it is designed to hold the thermostat in place while you attach the housing
If you leave the sticky covering wax paper thing on there it will pour out like this. Yes I have done that once too...
You can seal them up with no gasket, but it makes it hard to keep the thermo from moving

Check the housing for "trueness" of make sure its flat on a piece of glass
Any small knicks can be overcome with permatex, but a warped unit will need to be replaced.

Let us know what you find!

 

[koda2000]

To be clear... the pointy end of the t-stat faces the radiator.

 

[donalds]

Check the housing for "trueness" of make sure its flat on a piece of glass

I do this all the time on v twin engines glass and 400 grit sand paper taped to the glass I color the surface with a marker

 

[bmdubya1198]

I'm going to carefully inspect the housing for cracks when I pull it off for the 6th time. It's very possible that there's a hairline crack that I missed, or even that the thermostat could have been warped when it fell down from the recess and I tightened it down. The local O'Reilly has a thermostat housing in stock, so I might go grab one if that turns out to be the problem. Hopefully that will finally solve this... I just want to drive my Mountaineer!

My thermostat actually doesn't have the rattle valve, which I found strange. It's a Carquest part from Advance, 195 degree. I might grab a new one just to be safe.

 

[koda2000]

I'm going to carefully inspect the housing for cracks when I pull it off for the 6th time. It's very possible that there's a hairline crack that I missed, or even that the thermostat could have been warped when it fell down from the recess and I tightened it down. The local O'Reilly has a thermostat housing in stock, so I might go grab one if that turns out to be the problem. Hopefully that will finally solve this... I just want to drive my Mountaineer!

My thermostat actually doesn't have the rattle valve, which I found strange. It's a Carquest part from Advance, 195 degree. I might grab a new one just to be safe.

You should get one with the rattler. If you haven't changed the small W/P bypass hose this would be the perfect time to put on a new one.

 

[Gary Crist]

A dab of yellow 3M on the thermostat in couple of spots
will "weld" it to the housing not letting it slip.

 

[Turdle]

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

 

[CDW6212R]

Those are tricky to make stay in place. Letting the RTV set up slightly works well, and patience.

I learned a new issue recently, some off brands make crappy thermostat housings. Those have one hole that is not located properly, the gasket doesn't line up with the hole, and it will leak every time. So always check the gasket fit carefully, against both the intake and thermostat. I like a gasket and a thin layer of Ultra Black.

Use a well fitting socket and ratchet etc, so it's easy to run the bolts in. If you have to fight a tool to run a bolt in, then that may cause an issue with having the thermostat fall while fighting the bolts.

 

[bmdubya1198]

You should get one with the rattler. If you haven't changed the small W/P bypass hose this would be the perfect time to put on a new one.

Ended up exchanging the thermostat for the same one at Advance. I would have liked to swap it out for one with the valve, but that's all they had and I was tired of running around lol.

A dab of yellow 3M on the thermostat in couple of spots
will "weld" it to the housing not letting it slip.

I've learned after doing this job 6 times in the past couple days that this is necessary. A couple dabs of super glue did the trick for me.

Those are tricky to make stay in place. Letting the RTV set up slightly works well, and patience.

I learned a new issue recently, some off brands make crappy thermostat housings. Those have one hole that is not located properly, the gasket doesn't line up with the hole, and it will leak every time. So always check the gasket fit carefully, against both the intake and thermostat. I like a gasket and a thin layer of Ultra Black.

Use a well fitting socket and ratchet etc, so it's easy to run the bolts in. If you have to fight a tool to run a bolt in, then that may cause an issue with having the thermostat fall while fighting the bolts.

Yup, I let the RTV dry for a little while on the new housing before putting the gasket on. Interestingly enough, the new gasket matched up perfectly with the new housing. The Fel-Pro gaskets I was using before were close, but not quite lined up with the housing. Maybe the housing I removed was replaced at some point and one of the crappy aftermarket ones... it looks pretty nasty and old, but at 211k miles and 19 years old, anything is possible.

That said, I put it on earlier tonight, and now I just need to get a new bypass hose while it's all apart. I noticed the short one from the water pump to the thermostat is pretty cracked at the water pump side after moving it around so much. Is there a good source for the exact fit hose, or does the generic trim-to-fit elbow that the auto parts stores have work? I checked Ford parts sites, but couldn't find the hose. Granted I didn't look too hard.

 

[Gary Crist]

If one can not buy a thermostat w/ a rattle valve, just drill
a small hole in the tstat toward the top of it (old trick and
cheap too).

 

[koda2000]

Try looking on RockAuto for a Motorcraft bypass hose (or at least get the part number). Once you have the part number do a Google search to find where to buy one. I bought a generic one and was not happy with it, so much so that I threw it away and bought a Motorcraft hose. A little more money but worth it for the fit and quality. I found mine on Amazon.

 

[410Fortune]

trim to fit is all I can find anymore, they work just fine, the ones I use are made by Gates
The little bypass hose is a royal PITA when it lets go on you
and a royal PITA to install
Some vaseline on both sides makes it much much easier to get it on there

 

[koda2000]

trim to fit is all I can find anymore, they work just fine, the ones I use are made by Gates
The little bypass hose is a royal PITA when it lets go on you
and a royal PITA to install
Some vaseline on both sides makes it much much easier to get it on there

RockAuto $10.80 MOTORCRAFT part number KM1935

 

[bmdubya1198]

I'll have to call the dealer and see if they have one. Only $14 on Amazon with next day delivery though, so worst case I guess it can wait a day.

 

[Eric S.]

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!

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