Okay so here we go. Vrrrm Vrrrrm.
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Okay. This gets a little complicated so bear with me. My note above wasn't quite accurate after I've just done a bit more reading on the issue now.
The problem primarily affected the 150bhp 2.0 litre (320d) diesel engine introduced with the E46 facelift (around 2001 for saloon, around 2003 for the coupé/convertible). Also affected the 3.0 litre (330d) models to a much lesser extent.
Pre-facelift 320ds with the 136bhp engine were not affected by this problem at all.
The problem that manifested was either a less-worrying split inlet manifold, or a more worrying problem of swirl flaps being ingested by the engine and fucking the whole lot up. Or indeed a combination of both the above.
Not sure if you're familiar with the E46, but the easiest way to tell which is facelift and which is pre-facelift is probably by the headlights.
The above is pre-facelift, has the 136bhp diesel engine, and is not affected the above problem. Note the two large headlights on either side.
The above is facelift, the newer one, with the problematic 150bhp diesel engine. Note the smaller outside headlights which taper round the side.
Facelifts also have rear LED lights and various other more minor cosmetic differences.
There is a huge wealth of information on BMW Land, where there is a whopping 110-page thread on exactly this reliability issue. Linky
If you are serious about going for an older 3 series coupé (or indeed saloon), check this link for potential issues before you buy ...
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If however, you are looking at the new E92 coupé, as below, there are no persistent problems with the diesels.
Check the www.bmw.co.uk used car pages, there are usually around 100 diesel E92s of various flavours on there, right from your basic 320d models at around £12k-£13k up to your mind-numbingly quick 335ds which will start at around the £18k mark up to well over £40k.
Remember that BMW are the forerunners of performance diesels in the industry and have pushed the envelope, leaving the rest of the field to play catch up. They are quicker and more powerful than rival diesels, and also much more efficient. The 118d fr'instance is more efficient with higher mpg and lower CO2 emissions than a Toyota Prius hybrid! Plus it's a damn sight cheaper, quicker, and more fun to drive than a poxy Prius. Do I sound like a Beemer advert yet?
It is felt they pushed things a bit too far with the E46 facelift 320d leading to the reliability issues with the inlet manifold and the swirl flaps as described above.
BMW (in the UK at least) acknowledged the issue and compensation and goodwill gestures were forthcoming.
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If you test drive the new Mercedes C Class coupé Lor, let me know. I've not driven one so would love to know how it goes.