speaking of their common rail problems, I had the diagnosis equipment of the delphi HPCR diesel control system, and have recalibrated some unmolested ones back to 100% performance.
They are very robust and simplified compared to the bosch and siemens units of the germans. They can go out of sync if part of the CAN-BUS system goes out of sync due to some fault, usual culprit that I saw was cut/twist of the wiring behind the aftermarket stereo unit that causes the cabin CANBUS network to go faulty. Can bus is like an advanced muxer with less no. of wires doing many jobs at the same time.
The problems in engine hunting, limp mode etc was always due to some mechanic using oversized tools or incorrect techniques to remove assys or even wiring plugs, The fuel filtration system on these cars are 3 stage like MB, one in tank strainer, one pre filter and one main filter before it goes into the high pressure pump. You are supposed to drain the fuel sediment filter at regular intervals.
Another fault I saw was pressure washing a hot engine, the high pressure iron body pump is located upfront the engine and gets very hot while in operation - it has very minute clearances that enable it to generate 27000 psi of fuel pressure in the common rail. If you spray cold water on it when its hot - it will damage itself and you will lose pressure (I dont think that is a design fault - because this engine is mounted in a road going vehicle)
