There are two ways to approach this subject. My folks were of the mindset of keeping a car until the "wheels fall off" and that it was always cheaper to keep repairing an older car vs buying a new or newer one. When I was younger I had a similar outlook. As I have studied economics I have also learned of the concept of the "Sunk Cost Fallacy" the mental trap that people fall into believing that once they make a purchase you to get your "money's worth" since you already threw money at it in the past.

you can read more about it here

https://www.logicallyfallacious.com/...k-Cost-Fallacy

The gist of it is that people tend to believe that because they have spent money on something in the past that they have to keep spending more to recoup their "investment" this fallacy follows us in many ways, relationships, purchases, government programs etc.

Unless you have bought a car in hopes that it is a collectors items that you can get a big amount of money for or a hyper car you plan to flip (John Cena and his Ford GT 40 for example) a car is NOT an investment, it is a tool, and like every other tool you have to decide when to cut your losses and let it go.

so the questions you should ask yourself

1. Is there something newer with less miles that will effectively do the same thing as what I am driving now?

2. A engine replacement, can cost $3500-5K for a normal car, would you get that back our of your 2003-if you paid to replace said engine?

3. Are you willing to take the chance that whatever safety features your 2003 has will still function in the event of a serious accident (ABS, Airbags etc) and keep in mind that a car that old will have weakened body integrity?