Cars despawn when you go outside a certain radius of the vehicle UNLESS you remove the ruined radiator before leaving. Maybe they also despawn with ruined spark plugs / batteries too ? I didn't get to ruin those.
You know how precious cars are on DayZ ! They may be murderous and temperamental, but they are end game items.
I lost loads of cars until I worked out what was happening. I even accused an Admin of being a Badmin and deleting one of them.