Coolant leak at thermostat
 

1998 Accord 4-cyl auto trans.

Over a year ago, there was a leak at the thermostat.
1) I replaced the thermostat and gasket. It still leaked.
2) redid it again- new thermostat and gasket, but added a thin bead of gasket maker. this didn't help at all. In fact it made it a little worse.

3) finally I went back to step 1, with new thermostat and gasket, but this time I sanded down (with 800 grit) some of the white thin stuff that I saw on the mating surface.
This helped a lot- like 99% better.

NOW one year later it is leaking again at the same place. I don't look forward to taking it apart again. I may have to see if the gasket failed, or whether I need to sand it down a bit more?
I have 2 choices.
1) Just do what I did the 3rd time and see how it goes.
OR 2) just drop "Stop Leak" into the radiator.

What do you guys think?

Clean the surface good. You never want to use stop leak in your system.

OK thanks!... let the fun begin!

Just so I don't take anything for granted...
You know the gasket has a slot around the inside, so the gasket stretches over the edge of the thermostat to seal both sides...?

And if I remember, it's pretty difficult to get the thermostat installed backwards or upside-down. The little rattle-pin has to fit into a little bump-out in the housing.

Yea, I am aware of that. Learned the hard way several years ago~!

I plan to redo the work on it tomorrow. I just don't understand why it was fine for 1 year, then all of a sudden a leak. Not bad, but maybe 1 shot glass pooling on the frame below the gasket.

Are you sure it's coming from the housing and not the hose? If you it's going to happen again just change the whole housing.

I'll clean it 1 more time and see how it goes.

When you say whole housing, do you mean both sides? and not just the front that the hose attaches to?
When I look up the parts at auto stores, just the front part shows up.

The other part bolts to the engine & I think it would be a dealer part. Then you'll need the gasket(s) where it bolts up against the engine. I don't think I'd replace that other part of the thermostat housing unless I actually find a leak related to it.

Try a pressure-tester (loaner tool from auto store?) to pressurize the system when it's cold. Then you can have a good look to see exactly where the leak is located. There's a couple smaller hoses that connect around that same area, in addition to the big lower-radiator-hose. I've occasionally seen hoses deteriorate from the inside then leak out where they're clamped onto their nozzles. Besides that, it might need the crud cleaned off of the nozzles for any of those hoses.

Well, the leak is definitely coming from the mating surface where the thermostat goes. I looked everywhere and no coolant anywhere else.

When I removed the housing cover to get to the thermostat, I could tell there was a bit of gunk by feeling the surface (it is supposed to be smooth).
I used a razor blade to shave the thin film of gunk ( I was careful not to nick the metal. After I removed the gunk, I could tell there were several pits because of dark spots.

So I ordered a new thermostat housing cover. I can pick it up in 2 hours. But I am perplexed about the picture.
It shows another gasket that is supposed to go between the mating surfaces. This means I am supposed to use TWO gaskets? ( 1 for the thermostat, and the other for the housing),
I was taught never to use 2 gaskets on top of each other.
https://cimg3.ibsrv.net/gimg/www.hon...48d3756efd.jpg
I am thinking just to use the gasket the came with the thermostat ONLY, and see what happens.
Any 2nd opinions?

Your thermo gasket should go around the thermo. Then thermo slides into a cubby. The matting gasket should not touch the thermo only the matting surface so it's not using 2 gaskets on top of each other.