Some causes of car coolant loss are cracks, splits, corrosion and other forms of damage to the radiator, water pump, reservoir, heater and engine block cores. Many coolant leaks occur in the plugs, hoses, clamps and connectors to these parts.
Vibrations cause leaks around the hose connectors in the radiator. Stone damage to the radiator core and internal corrosion to the seams that connect the radiator core to the end tanks are sources of leakage. A damaged or broken radiator cap seal allows fluid to seep out. If a radiator is unable to sustain pressure, the engine can overheat and lose coolant.
A degraded shaft seal in the water pump allows coolant to escape out of the vent house. In a two-piece water pump with a backing plate, the gasket between the backing and the housing is prone to leaking. A worn or bad gasket that seals the pump to the engine front cover can lose coolant.
A hole or crack in the plastic reservoir permits coolant to escape. Freeze plugs are prone to corrosion and developing leaks, and the intake manifold gasket, particularly if it is plastic, can leak.
Cracks, splits and pinholes in a radiator or heater hose are causes of leaks. Hoses that have hardened, corroded or developed soft spots and blisters may leak as well.