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The only salvageable parts on cars are tires and windshields this is due to a limitation in the way vehicles are damaged in DayZ such that if an engine or fuel tank is removed (therefor taken to 100% damaged) the vehicle will explode killing the player.
The more damaged the part the higher the chance to destroy the part.
That depends on the object, some times you may get lucky and it will drop metal, engine parts, fuel tank, or etc. but little advice like above it could blow up on you also...