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I have a '55 plate BMW 320d wagon. I bought it in 2009 at 83k and its just gone through the magic 200k mark.
Now from 6' away the old girl still looks pretty damn good, still does 600+ to a tank, doesn't use oil and while its never been "fast" in the accelerative sense, it carries speed well and is far from a slow car through the twisties. I've generally kept on top of maintenance, keep it reasonably clean and I periodically get random people comment on what a nice car it is. Which is kinda nice.
...but...
The grot is starting to creep in - obvious bit is the NS front wheel arch is starting to blister from stonechips. A used wing would be £70-£100 + a morning of fitting it (already done the OS). Some of the underside - assorted brackets and braces mostly - is also rather crusty.
The bonnet gas struts have all but failed. Easy fix
The rear subframe bushes are decrepit - corner to corner I can feel the body shift and settle laterally on the subframe. This would be a major thing to sort and involve having the car off the road for a good period of time.
Its started leaving a very James Bond-esque smoke screen behind it when properly given the beans, which is not something I'm very comfortable with. I'm guessing its tired injectors as it doesn't burn any oil. (easy solution is to NOT properly give it the beans!). Again that's not going to be cheap as I doubt its a DIY job.
It doesn't owe me any money, but equally to get similar but reasonably up-to-date is a BIG wedge of cash - so at what point do you give up on "keeping it nice" and just let it dissolve into the ground with minimum expenditure? It's something I'll hate doing as I like keeping nice things nice, but equally sinking a couple of £k into it feels quite unwise.
Definitely sounds like it's into or past bangernomics.
I liked to keep mine nice but generally that only extended as far as thoroughly cleaning the interior, decent stereo and seat covers so it was a slightly nicer place to waste my life in traffic.
If I were you I'd chuck a can of expensive snake oil in it to see if that cleaned out the injectors.
If that failed I'd take the EGR and inlet manifold off and clean or replace it. It may just be either coked up and restricting airflow, or not closing properly and bleeding too much exhaust gas back under acceleration.
Beyond that you're into injectors and ££££, although my neighbor used to make a decent 2nd income buying Sprinter vans with the known injector issue and fixing them. But it depends on the engine, some need the injectors and ECU coding together, so even if they come out without a fight it's not a plug and play job.
Subframe sounds like it might be an MOT failure. Although after doing similar jobs and after watching KwikFit spend all day on a supposedly 2 hour suspension bushing change for a fixed price of £200 I'd be tempted to somewhere like that and let them waste their time on it because it'll still take 3x longer at home without a ramp.
Isn't your case more of keeping an old car going rather than Bangernomics?
Bangernomics to me is the art of buying something dirt cheap to get you from one MOT to the other. If it fails you scrap it and go again.
In your case I'd get that smoking checked out first and not worry about the cosmetics!
Subframe bushes should be a fairly simple fix.
A good indy garage is your friend.
This is the bonnet of my '07 Civic TypeR - it needs a front end respray really, but it's not a show car and I'd rather spend the money keeping it mechanically sound...

My 08 diesel was getting increasingly smokey, especially noticeable at night on dial carriageway accelerating with headlights behind. Eventually failed MOT on emissions, took to a local garage that does the Terraclean service. A bit sceptical and set me back around £500 but it passed (same garage retested it so sceptical still). Since then I've used Wynn's diesel system cleaner and the pre MOT emissions reducer and it's doing fine. Not noticing haze in rear view and just passed another MOT with 0.37 smoke opacity on a limit of 1.5
So unless you are feeling flush and can afford a change, I'd say keep it going. Even if you pay a garage to do the suspension it's still probably going to be a lot cheaper than changing
Should have said - I have the EGR valve off once a year or so for a clean and the inlet manifold came off last year for other reasons and got a full clear out while I had it in bits.
Subframe bushes. Its project creep. It'll need a bunch of new brake hardlines to get the subframe off, while it's dropped do you do a diff service, do you do swap out all the (hideously complex) multilink suspension arms. Do you spend ages cleaning it up back to bare metal, repainting and rustproofing. Then while you've got clear access to the underneath of the shell do you sort all the little issues with that out.
None of that is cost-effective to get done by a garage, but all of a sudden the car has been in bits in the front yard for 6 months and counting...
Oh and I forgot to add in the initial post, the winter wheels need a(nother) refurb as the previous attempt at powdercoating them was utter rubbish and its flaking off in big lumps. (and one of the wheels is leaking as a result).
Basically I could easily end up plowing a couple of £k into the thing just to do things "right", but then the other 80% of teh car would also need "doing right" to make that worthwhile.
@the_muffin_man. That's hideous. Sorry, I really couldn't live with that, it would give me pain every time I walked past it.
Sounds like a keeper; you know the history and the faults (which all cars have in one way or another); with a bit of maintenance (which all cars need), you should be able to squeeze a good few more years from old faithful... you may even find if you keep it going long enough, its value increases 😉
I know - it's shit! But car is worth £2k/£2.5k tops - a front end respray is close to £800. Maths don't make sense. And I know it will need a rear subframe at some stage.
I've got a search saved in eBay to see if a used bonnet shows up close by. And if you stand back and squint it's not too bad!!... 🤣🤣

If you're replacing it with an older car, you're replacing known issues for possibly worse unknown issues - I'd be tempted to get it done, put not to A1 standard.
FYIW - buying an equivalent 4-year old 320d now and you'll be spending £15-19k.
Paging RNP to the forum!
To me the term 'beyond economic repair' doesn't exist! If a vehicle does the job I ask of it - why would I change it? Bangernomics is buying a snotter and driving it till it dies without spending on it. I like to own vehicles that are maintained beyond their value but that would quite happily jump into to cross continent's at the drop of a hat.
I rebuilt MrsRNP Volvo '04 200k V70 during lockdown and spent ~£2.5 in parts and paint.
I was gifted a '10 215k Xc90 for £500 which I'm looking to spend £3.5 - £4k on in parts.
My trusty '04 Berlingo has spoilt it's copybook and shat it's clutch but it's getting a full clutch/gearbox seals/timing belt/all pulleys and idlers/wheel bearings rebuild.
And I've recently got sidetracked with a £500 '55 Porsche Cayenne that is going to have a full 'overland' spec rebuild once the Volvo is done.
The quickest way to do subframes is to buy a complete one from a breakers and refurb it at your leisure. It's then just a case of swopping over. You can then rebuild the removed one and make money selling it on.
The underside of the bodywork is the biggest issue - you need to keep tinworm away at all costs. Waxoyl / slapping wax type stuff over the top does bugger all. Lanolin based products are utter crap.
You need to be removing all rust, treating what's left with converters then applying epoxy over the top. It's not quick but it's the only way of stopping rust in its tracks.
If you have workshop space/tools/tolerant wife/no children then go for it.
If not buy something newer.
Also look at the amount of energy embedded into the manufacturer of car components - all that is lost when the car is scrapped.
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Checked for a boost leak for the smoke issue?
Bushes can you do in situ with a puller?
If not as above 2nd frame fettle it swap it flog your old one.
Wings are always shite on them. I'd check the sills n back floor pans too....