Q: When should I sell my car

If you perform your own repairs, or can obtain repairs at a "reasonable" cost, the answer is never if your objective is to minimize your total cost of transportation services. The economically useful life of a motor vehicle only ends when the FRAME of the vehicle is no longer usable, which typically occurs when rust proceeds to a point where the structural integrity of the frame is compromised. Prior to that point every single component of a motor vehicle is eminently repairable or replaceable, endlessly. If you do not perform your own repairs, or cannot obtain competent services at a rational cost (shop repairs are often NEITHER competent nor rational in cost, hence the omnispresent interest of everyone in "new" cars or "new" used cars), then the calculation becomes somewhat more complicated and has to be evaluated on a case-by-case basis but the calculation can still be done, with great potential benefit to the vehicle owner. If you have a particular mechanical problem of interest, or are trying to figure out what to do with a presently owned vehicle, by all means let us know of the particulars.