مبيت الترموستات مسرّب

The thermostat housing leaking at its gasket or through the housing itself. Many modern housings are plastic, which embrittles with heat cycling and cracks — so this is a wear-out failure rather than a gasket that needs retightening.

The leak is often small and hot-only, sealing as the plastic expands, so it appears as coolant loss with nothing visible when cold. Look for the crusted residue around the housing and down the block beneath it. Overtightening is a common cause of the crack, which matters for the repair: these housings have a specified torque and a plastic one will split if it is treated like a metal one. Replace the thermostat while it is off, since the labour is already spent.