With the engine off and completely cold, wipe the area around the thermostat housing clean, including the hose and hose clamp.
Then start the engine and let it idle until the thermostat opens. The pressure should cause the leak to begin again, and since you have the area clean, it should be relatively easy to determine exactly where it's leaking from.
I suspect that the leak is originating from the hose or hose clamp. If so, you may have not tightened the hose clamp down enough, or you many have placed the clamp directly ontop of the nipple instead of behind it. This will cause a leak.
Also, you may have a small hole of crack in the hose. If you're sure you removed all old gasket material from the housing and used RTV with a new gasket, then it can't be anything other than the hose clamp or hose. Clean the area and follow my directions. Get back with me when you're done. We'll take it from there.
Monday, January 18th, 2010 AT 9:18 PM