Hi Tony,

Both BMW’s N47 and Mercedes’ OM651 four-cylinder common-rail turbodiesel engines have timing chains that are located at the back of the unit (i.e. near the bulkhead of the car), so replacing them is a very time-consuming job; mechanics normally take the engine out entirely and budget two days’ work to get it done. There were issues with tensioners on both and the chains stretching, requiring replacement, but cursory investigation reveals both companies replaced the chains free of charge to some customers as a goodwill gesture, depending on how soon the chain wore out.

The truth of the matter, though, is that while there are fuel pump, injector and turbo issues also reported with both these engines, they have been made in vast numbers and seen service in hundreds of thousands of cars – and there are no major issues that have ever made the headlines with either. You’ll find so many good five- to eight-year-old examples because both the BMW 520d and the Mercedes C/E 220 CDI/E 220 d are immensely popular as company cars. Competitive PCP deals on these machines, which are possible due to each brand’s perceived reputation and quality, mean they are cheap to run on a monthly basis. There’s therefore a very good chance all the 520d and C/E 220 d models you’re looking at have, at one time, been company cars. Check their history carefully – you’ll find they should have been well-maintained during their lives ‘on fleet’, meaning you should be safe to pick up a used example.