Well, what's likely the case is that 'car' is fetched using lazy loading and because the parent entity, person, is not fetched, car will not be found in the db because the PK is not set in person. This means that you'll get a new entity for car, every time you access it.
To overcome this:
PersonEntity person = new PersonEntity();
person.Car = new CarEntity();
person.Car.Name = "ford";