Tits up?

How do you know when to throw in the towel? After getting an estimated cost of $1400 for car repairs, I started weighing my options for beloved Sherona. She's been a good girl and is in pretty good shape for being 11 years old. Other than preventative maintenance, she's never really required much attention. The aging process is hard on all of us, but especially my pocket book. So do I trade her in for a newer model? Do I invest a bit in her to keep her on the road a bit longer and avoid a car payment for as long as possible? Hopefully I'll have an easy answer to these questions once I receive word on the second estimate....