خزان التمدد متشقق أو مسرّب

The expansion tank cracked or leaking at a seam or fitting. The tank is a pressure vessel, not a reservoir — it holds the system's pressure and lets coolant expand — so a crack means the system cannot hold pressure at all.

Losing pressure lowers the boiling point, so the engine can boil at a temperature it would otherwise tolerate, and the loss is gradual enough to be missed. These are moulded plastic and embrittle with heat cycling, cracking at seams and around the neck. A hairline crack often only opens when hot and pressurised, so inspection cold finds nothing — pressure-test instead. Check the cap at the same time, since a cap that no longer holds its rated pressure produces identical symptoms and is a fraction of the cost.