BMW B37

The three-cylinder diesel of BMW's modular family — 500 cc per cylinder, the same module as the B47 four and the B57 six — and the smallest diesel BMW has built.

Three cylinders do not balance the way four do. The firing interval leaves a rocking couple that a four-cylinder does not have, and the engine carries a balance shaft to cancel it. That matters diagnostically rather than academically: a vibration complaint on this engine has a mechanism behind it that its four-cylinder sibling does not, and the mounts work harder for the same reason.

Almost every car it is fitted to is transversely mounted, on the front-wheel-drive UKL platform shared with MINI — the 2 Series Active Tourer, X1, X2, Countryman and Clubman — with only the 1 Series carrying it in both layouts across its generations. A procedure written for a longitudinal BMW diesel does not describe this installation, and the timing drive is at the gearbox end of the engine either way.

What it carries otherwise is the modern emissions stack — EGR, particulate filter and AdBlue on most markets — and the EGR module on early modular diesels was subject to recall for a cracking and overheating risk. Check open campaigns against the VIN. Beyond that its faults are the diesel duty-cycle faults: a filter that cannot regenerate on short journeys, and the cascade of codes that follows.