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Discussion Starter #1
The history on this is that a few years ago, I had the plastic housing replaced, as it had a small crack which leaked as the engine cooled. Now, I have been getting a leak between the thermostat housing and the block.

When I took things apart, I found not only the o-ring, but a lot of some sort of gray sealant, both before the thermostat and o-ring went in, and after the o-ring went in. Not sure why, but the same mechanic had done something similar on the coolant flange (which I discovered when it leaked again a while later). So, I cleaned it up as well as I could, used just an o-ring, put it back together and now it leaks even without pressure. The mating surfaces did look a bit rough, but there is also the chance that I may not have gotten things seated properly.

I'm thinking of augmenting with a bit of sealant, as was done by the mechanic. What type of sealant would I use?

Also, any tips on getting the hose loose from the heater core, so I can burp out the trapped air? It is pretty stuck to the pipe there.

The first time, I worked without removing the alternator (just the wires), but I'm not sure I can apply sealant with such restricted access
 

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Couple things here, typically sealant is NOT used on an O-ring installation. Depending on the application (where used) a vey light rubber grease might be used to lubricate the O-ring for installation. But that is not a sealant.
The reason why it leaks now is because the O-ring has retained memory of it being compressed. A new O-ring should be used.
If you were to remove the plastic housing, clean everything so it is absolutely free of dirt, debris and any corrosion, make sure the housing is flat across the mounting face, install a brand new O-ring, you should be in good shape.

To burp air out of the cooling system, the heater hose on the passengers side of the heater core nipples has a small hole in it (about an 1/8" - 3mm). You slide the hose forward (removal direction) so that little hole is off the heater core nipple. As you add coolant air will come and eventually you'll get a constant squirting stream of coolant. At that point the system should be 'full'. Slide the heater hose back onto heater core nipple to it's original position and your done.

I'm not familiar with the 1.8's if they get air pockets (like the V6 is notorious for) or not, but if does, burping the cooling system might take a few tries.
 

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If he used a cheap Ebay type of housing, it could have warped. I use a bit of 200 sandpaper to clean up all coolant seal areas before assembly. Unless the car is sitting for days with the system open, sealant rarely cures in time.
 

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I'd clean it out and get a new housing and o-ring. I used Dorman 902-991 aftermarket which came with o-ring, I got it from autozone for $10. I noted the installation problems in my review in the product listing. For one, I had to clear the caked on crud out of the hole. I'm pretty sure I used plastic shims and a toothbrush to scrape it away instead of metal so I wouldn't gouge the surface. Also I did not use grease or sealant.

97892


Tighten both bolts equally, bentley says torque there for AUG/AWM bolts is 11 ft-lbs. (FYI the pic above is an Audi B5 pic and their torque spec is less at 7 ft-lbs, so not all B5s have same spec in this area).
 

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Discussion Starter #7
Thanks for the replies. I had already ordered o-rings and the housing. The old housing looked okay, so I tried re-using it, but wow what a leak. I then tried the housing dry fit without the o-ring or thermostat, and it was obviously warped -- the new one is flat. I still haven't installed it, as I think I need to clean up the hole more first --looks a bit rough.

I have a question about the thermostat, for those that remember doing this job. The old one (changed maybe 20k miles ago) fits kind of sloppy against the flange in the hole, maybe 1/8" smaller in diameter than the outer hole, so 1/16" on each side. This makes it hard to exactly center. i.e, slides to bottom. The hole seems to have a bit of a ridge that might limit this, but it doesn't. Do you remember the fit of the thermostat being like that?

Thanks,
Gary
 

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I've had them move around, but I did not notice how much. I would hold them in place and push the O-ring in to hold them. They usually stayed in place, but on a few it would take a couple of times as placing the housing would often cause it to fall out of place again. This was due to installed the housing with the hose attached. Less chance of leaks if you don't have to remove it.
 

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Discussion Starter #9
Okay, seems to be fixed now. I spent a long time with sandpaper, removing the remnants of the sealant which had been previously used. Thanks again for the help.
 
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