What are the typical common rail diesel engine problems? [manual]

Relatively often in articles about Common Rail diesel engines, the term "typical malfunctions" is used. What does this mean and what does it entail? What should I pay attention to when buying any common rail diesel engine?

At the beginning, very briefly about the design of the Common Rail fuel system. Traditional diesel has two fuel pumps - low pressure and so-called. injection, i.e. high pressure. Only in TDI (PD) engines was the injection pump replaced by the so-called. injector pump. However, Common Rail is something completely different, simpler. There is only a high pressure pump, which accumulates the fuel sucked from the tank into the fuel line / distribution rail (Common Rail), from which it enters the injectors. Since these injectors have only one task - to open at a certain moment and for a certain time, they are very simple (theoretically, because in practice they are extremely accurate), so they work accurately and quickly, which makes Common Rail diesel engines very economical.

What can go wrong with a common rail diesel engine?

Fuel tank - already in long-term diesel engines with high mileage (frequent refueling) there are a lot of contaminants in the tank that can get into the injection pump and nozzles, and thereby disable them. When the fuel pump is jammed, sawdust remains in the system, which act like impurities, but are even more destructive. Sometimes the fuel cooler is also removed (cheap repair) because it is leaking.

Fuel filter - an incorrectly selected, contaminated or poor-quality one can cause problems with starting, as well as “abnormal” pressure drops in the fuel rail, leading to the engine going into emergency mode.

Fuel pump (high pressure) - it often just wears out, poor materials were used in early Common Rail engines due to the lack of experience of manufacturers. An abnormally early failure of the pump after replacement may be due to the presence of impurities in the fuel system.

Nozzles – are the most accurate devices in the Common Rail system and therefore the most vulnerable to damage, for example, as a result of the use of low-quality fuel or contamination already in the system. Early common rail systems were equipped with more unreliable, but simple and cheap to regenerate electromagnetic injectors. Newer, piezoelectric ones are much more accurate, more durable, less accidental, but more expensive to regenerate, and this is not always possible.

injection rail - contrary to appearances, it can also generate problems, although it is difficult to call it an executive element. Together with a pressure sensor and a valve, it acts more like a storage. Unfortunately, in the case of, for example, a jammed pump, dirt also accumulates and is so dangerous that it is right in front of the delicate nozzles. Therefore, in case of some breakdowns, the rail and injection lines must be replaced with new ones. If certain problems occur, only the replacement of the sensor or valve helps.

intake flaps - Many Common Rail diesel engines have been equipped with so-called swirl flaps that regulate the length of the intake ports, which should promote the combustion of the mixture depending on engine speed and load. Rather, in most of these systems there is a problem of contamination of the carbon dampers, their blocking, and in some engines it also breaks off and enters the intake manifold right in front of the valves. In some cases, such as the Fiat 1.9 JTD or BMW 2.0di 3.0d units, this ended in engine destruction.

Turbocharger - this is of course one of the mandatory elements, although not related to the Common Rail system. However, there is no diesel engine with CR without a supercharger, so the turbocharger and its shortcomings are also classic when we talk about such diesel engines.

Intercooler - The charge air cooler as part of the boost system mainly creates leakage problems. In the event of a turbocharger failure, it is recommended to replace the intercooler with a new one, although few people do this.

Dual mass wheel - Only small and relatively weak Common Rail diesel engines have a clutch without a dual-mass wheel. The vast majority have a solution that occasionally creates problems such as vibration or noise.

Exhaust gas cleaning systems – Early Common Rail diesels only used EGR valves. Then came the diesel particulate filters DPF or FAP, and finally, to comply with the Euro 6 emission standard, also NOx catalysts, i.e. SCR systems. Each of them is struggling with clogging of substances from which it is supposed to clean the exhaust gases, as well as with the management of cleaning processes. In the case of the DPF filter, this can lead to excessive dilution of the engine oil with fuel, and eventually to jamming of the power unit.