Mazda MZR-CD (2.0–3.0)

Mazda's common-rail diesel four — in the Mazda3, Mazda5, Mazda6 and the CX-7.

⚑ It is not the MZR, despite the name, and this catalogue's own alias search proposed merging the two before a check refused it: the MZR is petrol and this is diesel, and a test exists precisely to stop one name covering both. The -CD is doing all the work in that name.

Its defining problem is the diesel particulate filter, and the reason is specific to Mazda's design. The filter sits close-coupled to the engine and regenerates by injecting extra fuel on the exhaust stroke — so an interrupted regeneration puts diesel into the sump. An oil level that has RISEN on the dipstick is the warning, and continuing to drive dilutes the oil until the bearings and the turbo suffer.

Mazda's own countermeasure was an oil-level warning and a shortened interval, which is the strongest available evidence about how common this is.

Short journeys are what cause it, and a car used for them will do this regardless of condition — which makes the use pattern, not the mileage, the thing to establish about any example.

⚑ Two unrelated engines wear this badge. The 2.0 and 2.2 are Mazda's own design and are what everything above concerns. The BT-50's 2.5 and 3.0 are the Ford-derived WL and WE units it shares with the Ranger of that period — so the particulate-filter and oil-dilution warning above does not describe them, and a bulletin written for a Mazda6 diesel is the wrong document for a BT-50.