Internal coolant leaks can also occur if the cylinder
head or engine block has a crack in a cooling jacket.
A combustion chamber leak in the cylinder head or
block will leak coolant into the cylinder(s).
This dilutes the oil on the cylinder walls and can
damage the piston and rings. If the coolant contains silicates (green antifreeze), it can also foul the
oxygen sensor and catalytic converter.
If enough coolant leaks into the cylinder, as when the
car sits overnight, it may even hydro-lock the engine
and prevent it from turning over when you try to start it.