Cars - For Car Enthusiasts

My neighbor highly recommended that I don’t touch anything past the 90s, as the electrical systems in modern cars are apparently terrifying even to skilled mechanics, but I feel like the older the car gets the rarer (more expensive) the replacement parts would be.

Your neighbours are only partly right and you're also mostly right.

I mentioned it in another comment, but I could never get my KE-Jetronic fuel system to work properly. Car was a 1992, no OBD-II for diagnostics. I tried playing parts darts, but to no avail. Simple electronic systems can be worse than complex ones because:

Diagnostics. Okay, trouble codes won't tell you the whole story, but they tell you where to get started! And sensor readings, when you know what you're looking at, are even better.

I've had cars built in the 00s from Audi, BMW and Mercedes. Yes, all of the big 3 everyone's so afraid of. From what's considered the worst era for reliability. All chock-full of complex electronics. But you know what? 60€ Aliexpress clone Delphi DS150E (not a tool I'd recommend in this day and age, I got mine years ago) and the car will spill its guts. Or an Aliexpress clone of VCDS if you get into VW/Audi. MB and BMW specific tools also have clones and good generic tools work for most things.

My car current, an absolute piece of shit I paid 1800€ for, has air suspension, kind of an expensive thing to fix. So you can imagine I wasn't very happy when it threw up a light and the car was sitting all wonky. Connected VCDS, saw a fault code saying sensor data inconsistent or calibration wrong. Weird. Deleted the fault code, opened the sensor readings, made note of all four sensors' height readings, raised the air suspension, looked again. One of the four sensors didn't change at all. Get under the car and look, it's mechanically disconnected itself when I had another unrelated suspension issue that caused too much travel. Reconnected it because there's just a ball joint that it popped off of. Did a recalibration of the system (requires tape measure, level ground and VCDS, I paid 19€ for the knockoff ross-tech cable + pirated software off Aliexpress and I run these things in a laptop that doesn't have internet access) and the car's sitting completely level, better than when I got it.

You can also actuate the fuel pump to make sure it's running or to bleed the system if needed. There are self-tests for all kinds of things like VNT actuator, EGR, etc.

I hate that it has an automatic transmission, but hey, it literally shows me the individual clutch adaptation values and also limits that would indicate an issue. Now it won't tell you WHAT the issue is, but if you've got a significantly increased pressure adaptation on clutch E, there's probably a leak somewhere in the clutch E hydraulic circuit. Probably not something you want to fix at home, but I didn't have that issue, my adaptation values were all fine, I just needed a fluid change to get my transmission shifting properly.

Other cool things my current car's electronic systems have told me: Glow plug in cylinder 3 is bad, all four door lock modules had a bad electric connection, but door modules themselves were good. There's an intermittent issue with fuel pressure: sometimes when I let off the fun pedal, the fuel pressure doesn't drop to the requested level immediately. Well, there's a pressure regulator on these that tends to get stuck.

Things my BMW told me over the OBD-II port when I had it: Boost leak (difference between pressure sensors, found it quickly because there was a steady stream of hot air coming from a pipe connection), DPF backpressure high but no recent regen cycles attempted (meaning the DPF itself was fine, the other issues were causing it to avoid regen). And of course my favourite: EGR valve stuck.

The thing is, cars from the 90s can also develop weird electronical issues. But they won't tell you where to start looking, you have to figure all that out with a multimeter and a circuit diagram. Which doesn't mean you don't need a multimeter on a newer car. It's still a great tool to own, particularly because the electronics won't often tell you the root cause of an issue and you don't want to play parts darts. But "VNT actuator open circuit or short to ground" is a MUCH better place to get started than "I feel like the car doesn't have power anymore" and it ended up being a blown fuse. Which by definition IS an open circuit, it just wasn't in the actuator itself like the car suggested.