Here are some guidelines:
* You can’t depend on it anymore –
Is it realistic to expect that the car will be reliable again after whatever’s wrong is fixed? Or will you merely have fixed the latest thing to go wrong?
Some cars start out better-built than others and so last longer than other cars. But eventually, all cars reach a point where problems due to age and the wear and tear associated with long use become general in the same way that as we age, isolated, individual problems that resolve on their own or which can be treated effectively become chronic and systemic and there’s nothing you can do about them except crutch them with pain meds or other palliatives. The aging body can’t really be fixed anymore.
An aging car can be fixed, of course – if money (and time) is no object. Even a complete basket case car can be restored to as-new condition if your wallet’s deep enough. But that’s neither here nor there when we’re talking about cars as appliances – as a means of getting from A to B. And when money – and time and hassle – are the deciding factors.
Like our physical bodies, cars are really multiple different systems operating in concert. When it’s just one thing that’s gone wrong and once that one thing’s fixed the car can be counted on to run reliably for some time to come, doing the repairs makes sense. But when the entire car is getting obviously tired and several systems are on the verge of collapse, it’s probably time to say your goodbyes.
* It’s no longer safe –
Rust is the major factor here. Not cosmetic rust – structural rust. The frame (and critical mounting points on the frame). Structural damage to the frame/mounting points due to corrosion takes longer to manifest nowadays because cars built since the late ’80s are much better protected (and have better body integrity) than vehicles built before that time – when it was common to see cars only a few years old with significant rust problems. But that doesn’t mean your later model car is impervious to rust. If you keep it long enough – especially if you live near the sea or drive in an area where road salt is used in the winter – you’ll have to deal with rust problems.
Any car that is more than 10 years old that has ever been driven in an area where it was subjected to road salt or sea spray should be inspected on the underside very thoroughly at least once a year by someone who knows how to spot potential safety issues related to structural rust-through. The inspection should also include a close look at (usually) steel brake, clutch master cylinder and fuel lines. Rust-weakened lines can easily break – and can be expensive to replace.
If structural rust is found, it’s time for Last Rites. The only way to fix this sort of problem is by cutting out the bad section(s) with a torch and welding in new metal – which is neither easy nor inexpensive. And then you’ve only fixed one cancerous area. If you’ve got one, odds are you’ve got more.
* It’s becoming hard to find essential parts –
My father-in-law had (until recently) an otherwise nice early ’90s-era Cadillac. The car still ran well but when the AC stopped working he found out that the part he needed is no longer being made by GM. Eventually he found a good condition used part from a salvage yard – but scrounging junkyards is not something many people want to deal with. Even if you succeed in finding the part you need, it’ll be a used part that comes with no guarantee it will work any better than the one you’ve got. Or it might only last for a few weeks/months. There’s no way to know – and no alternative.
As a general rule, if the car is more than 20 years old – especially if it’s a “modern” car with electronics, such as a digital dashboard or electronic climate-controlled AC – some critical parts may be unavailable new – and hard to find used. They will probably also be expensive, too – which leads to the biggest consideration of all:
* You are putting more money into the car than the value of the car –
Here’s the Catch-22 you don’t want to find yourself facing: The transmission in your 16-year-old car fails and a new/rebuilt replacement will cost you $2,000. But the car itself is only worth about that much much. If you spend the $2k on the new transmission, the car will not be worth $2k more. It will be worth about the same as it was worth before the old transmission failed.
On the other hand, if you don’t put the $2k new transmission in, the car (not-drivable and needing a major repairs) will be worth… nothing. Or almost nothing. You might get a few hundred bucks for it as a parts/scrap car. Maybe.
You can’t win.
Even if you’re thinking: Well, I’ll spend the $2k on the new transmission and then just drive the car for another couple of years. The problem is that if something else goes wrong – and it probably will go wrong – you’ll be throwing more money down the well … and facing the same Catch-22 all over again.