Problem of Ford 1.9 & 2.0 Liter SOHC dropping Intake Valve Seats



Does anybody have any idea as to why the SOHC 1.9 & 2.0 enginess have a tendency to drop intake valve seats? I have a 2000 Focus that lost an engine to that very problem and its second engine appears to have done the same thing, but is still running (poorly and with a lot of noise).