Isuzu 4J-series (4JA1 / 4JH1 / 4JK1)

Isuzu's mid-size diesel four — in the D-Max, Trooper, Rodeo and the pickups and light trucks built on those platforms, and under other badges wherever Isuzu supplied an engine, which it did widely.

Its reputation is durability in commercial use, and the family runs from the mechanically injected 4JA1 and 4JB1 of the late 1980s to the common-rail 4JK1 and 4JJ1 of the 2000s. Those are quite different engines under one family name, and the dividing line is the PUMP rather than the combustion chamber — the 4JA1 and 4JB1 are direct injection driven by a VE rotary pump, so there is nothing electronic worth interrogating, while a 4JK1 is common-rail with everything that follows. The 4JG2 is the family's one indirect-injection member, in the Bighorn and Trooper II, where a pre-chamber and its glow plugs decide cold starting in a way they do not on the others.

On the mechanically injected ones the recurring items are the injection pump and the glow plugs, and hard starting when cold is almost always one of the two.

On the common-rail ones the story is modern: injectors, the EGR, and a DPF where fitted — and, as on every working diesel in this catalogue, short journeys and towing decide how quickly all three become a problem.

The 4JJ1 has its own page here, and it is the one most current D-Max owners meet.