In addition to the possibility of jail time, you could also be ordered to pay damages to the lender. This could include the amount of the loan that you still owe, as well as any costs that the lender incurs in recovering the car or selling it at auction.
If you are considering junking a car with a lien on it, it is important to talk to an attorney first. An attorney can help you understand your rights and options, and can represent you if you are charged with theft.