Just finished timing belt Et Al and I've been chasing a slow coolant leak. I pulled it all apart and wound up replacing the thermostat housing because it looked like crap. I've looked many times and am sure the leak is coming from the thermostat housing and not the water pump. Today I was actually able to see the coolant leaking from the outside of the thermostat housing at the lowest stud. I can watch it leak down, dey it with a towel and see it come from the stud/nut.
how does that happen?
Is the thermostat gasket just leaking and catching a ride to the end of the stud or does the stud actually enter the cooling system?
I definitely have a Toyota thermostat gasket and the housing was torqued to spec.
Thanks

 

It could be a few things, the t-stat gasket could have a nick in it allowing coolant through. The housing could have a casting defect allowing coolant through. The block could have some scale built up allowing coolant to seep through. You get the idea.

 

My t-stat housing was corroded by the Toyota Long life rad fluid and leaking past the O-ring, JB weld, smoothed it out and reinstalled, re-filled using universal yellow rad fluid, that was about 6 years ago.

 

I replaced the housing,, confident its not the thermostat now.
waiting on a new new water pump gasket...

 

Ok, the t-stat housing is part of the water pump,, the water inlet pipe is a separate part, either being corroded can cause a leak.

 

The water inlet (thermostat housing) and the water pump are brand new and pristine.
I didn't do a good enough job cleaning the block and whomever did the last water pump destroyed the thermostat stud threads. They also used a bunch of sealant. Not sure if I was getting an appropriate clamping force @ 14ftlb because the threads were so chowdered up.
Hopefully a cleaner block, chased threads and a new gasket will solve my problem.

 

Well, after days of maddening attempts to stop the "leaking stud" I found a pin hole casting defect in my new water inlet. This was allowing coolant to leak right under where the lower nut on the inlet tightens down.
lesson learned...

 

Mass production. It happens all the time. I've gotter all kinds of bas parts brand new from the dealer. Bummer, but most aren't that expensive. I just get another one, and toss the bad one.

Pat☺