Have been chasing a mystery coolant leak for months. Pressure tested twice, can’t find it.

The leak is slow but still needs to be resolved. Then I noticed some coolant on the water inlet manifold by the o ring. Replaced the o ring. The old one just crumbled off. Then I figured I should replace the thermostat while I had everything apart. Few days later I noticed that the thermostat housing was leaking on the bottom. I had to use a mirror to find the leak.

I figured it was just a crummy aftermarket part that I got from Autozone. Picked up an OEM thermostat from the Toyota dealer. It did look more heavy duty and fit better. But it leaked even worse!

Did some research and discovered that I was putting the thermostat o ring on incorrectly. I was placing the o ring on the manifold. Then I put the thermostat over it. Then I put the thermostat cover on and tightened the bolts. However, the o ring for the thermostat has a slit on the inside of it the sits on the outer lip of the thermostat. You are supposed to put the o ring on the thermostat first, then install it. This way the thermostat is sealed from both sides.

I messed up here, but I learned a lot and still saved money doing the work myself. Evert thing is back together and there are zero leaks related to the two o rings that I replaced. I am hoping that solves my mystery coolant loss issues but I am skeptical. There could be another unrelated issue that I have not discovered yet. The timing belt and water pump have already been replaced. Also replaced the radiator cap (OEM). If the leak is still there, I am suspecting that the radiator may need to be replaced. I don’t see any wet spots or leaks on it, but it looks original. 260K.