Nissan YD25 (with the YS23, 2.3 and 2.5)

Nissan's 2.5-litre common-rail diesel — the engine of the Navara, Pathfinder and X-Trail, and a workhorse in markets where those vehicles do heavy duty.

Its known and expensive failure is the timing chain, which on this engine drives from the front but wears fast enough on some build ranges to be a defining characteristic. The failure sequence is worth knowing: a stretched chain lets the tensioner extend, the chain contacts the front cover, and in the worst case it wears through into the coolant passage — so a chain problem on a YD25 can present as coolant loss or oil-and-coolant mixing rather than as a timing complaint. That symptom is routinely misread as a head gasket.

Injector and EGR faults are the other theme, aggravated where fuel quality is poor, and heavy carbon in the intake manifold is normal at mileage rather than exceptional.