Although troubled with problems from the earlier 2.0 and 2.2 AD family of diesel engines, Toyota succeeded in manufacturing one of the best diesel engines made in recent years. The 1.4 D-4D engine is rated at 450,000 km and features a common rail injection with a Denso or Bosch injector. Installed in the Toyota iQ, Yaris, Toyota Auris, Corolla, Urban Cruiser, Verso-S, and Mini One D, the 1.4 D-4D is Toyota's smallest diesel engine. It features a variable geometry turbocharger, intercooler, common rail injection, EGR valve, and an aluminum block (the first Japanese-made diesel engine with an aluminum block).
The 1.4 D-4D produced only 75 hp and 125 lb-ft of torque but achieved excellent fuel efficiency at 47 mph or better depending on the car. In 2004, Toyota introduced a new variant, increasing the power to 90 hp and the torque to 150 lb-ft, and in 2008 upgraded the engine to meet the Euro 5 emissions standard. The automaker installed piezo injectors, an EGR valve, and a soot particle filter. Toyota equipped the Yaris, QK, Aurisie, Verso, and Urban Cruiser with the new engine.
The 1.4 D-4D engine has some problems that are not atypical of other diesel engines, such as a noisy drive chain, excessive oil consumption, or problems with the engine control unit (ECU). However, its overall reliability is among the best in the industry. Per Mlfree, Toyota engineers have developed a simplistic engine, which provides good stability, low failure rates, and easy maintenance. Failures are rare, even in the oldest cars with 1.4 D-4D engines, despite already high mileage.
Read More: https://www.slashgear.com/1368651/most-reliable-diesel-engines-ever-built/