1998 Accord 4 cyl- 225,000 miles- auto trans

I had 3 small leaks on the coolant. 2 of them were hoses and the 3rd was at the thermostat gasket.

I changed all hoses, heater hoses, and the thermostat (and gasket). Now it is much better, but still a slight leak at the gasket of the thermostat. bolts were tightened to 110 inch pounds.
I am 95 % sure I cleaned the fitting and don't remember feeling or seeing any lumps, etc...

Is there anything else I can do to stop the leak?
Should I put in "stop leak"?
What about taking the thermostat gasket back off and putting some kind of film of silicone on the fitting where the gasket goes on, just to "seal it up"?
I suspect there is a tiny hairline groove somewhere on the fitting and if i could just seal it up, then I am good.

 

How are you checking for leaks?

I'd recommend pressurizing the system when the engine is cold and use the kit you can rent from most parts stores to pressure test the radiator. This method is easier to locate a leak due to having a cold engine.

The thermostat housing can be removed by 2 different methods. You can remove only the top half that uses the rubber gasket on the thermostat. The other way is to remove the entire thermostat housing, where an o-ring is the seal.

Your issue may be with the o-ring seal if it tore when installing. The o-ring could be the original issue even if you didn't disturb.

Did you use an OEM Honda thermostat?

 

I can put my finger under the thermostat cap and it is wet at the bottom. The radiator hose is attached to this. the hose clamp is fine.
When I changed this, the leak is better ( maybe 75%), but I don't want any leaks.

I got a Murray Thermostat and Felpro Gasket. Like the attached picture ( not an O-ring).

 

The o-ring seals the end of the thermostat housing to the metal pipe. Wasn't sure if you removed that part of the housing.

Something is off on the the installation. Your options are to take apart again, feel around all the sealing surfaces for any imperfections. Also check where faces of the thermostat housing. I'd use a slight coating of silicone spray or grease to help the seal seat properly. Put it back together and tighten down the bolts working back and forth.

Option 2 is to try tightening down the bolts more see if that stops the leak.

EDIT: Option 3 is to go with an OEM Honda thermostat and seal if the thermostat or seal size is the issue. Felpro makes good products, so I'd lean towards the thermostat. To be safe, I'd go with the seal and thermostat from Honda if you get to this point.

 

Just tried option #2. Will have to drive it around a while and see how it goes. Top bolt didn't move. Bottom bolt creaked just a little. Afraid to tighten more, I am sure it is about 130 inch pounds.

If that doesn't work, I will take it back apart and try the silicone/grease and see how it goes (after checking the surfaces).

 

Strange... tightened the lower bolt just a creak (option 2)...still leaked for 3 days, then it stopped!
Decided to leave it alone and see how it goes...

Wonder if the heat softened the gasket and it filled up whatever the leak was coming from