BMW M52

The straight-six of the E36 and E39 era, in 2.0, 2.5 and 2.8 litre forms, and the M54's direct predecessor. Early examples used a Nikasil-coated aluminium block that proved intolerant of the high-sulphur fuel then sold in some markets — bore wear followed, and BMW replaced many engines under warranty before moving to iron liners. A surviving car has almost certainly been through that already, but it explains why block material is the first question on an early one.

Diagnostically it behaves like the M54 that followed it, and the same warning applies most strongly: age-hardened intake boots, crankcase ventilation hoses and vacuum lines produce lean codes that read as oxygen-sensor faults. Test for unmetered air before replacing sensors.

The later M52TU added double VANOS, which brings its own rough-running and timing-code failures as the units wear — a rattle at idle from the front of the engine is the usual first sign.