Faulty injection nozzles at Mercedes

D as those responsible for Daimler had imagined very differently: They presented A year ago a completely new four-cylinder diesel in the Mercedes C-Class, with which the more than 70 years of diesel expertise should appear in a new light.

The OM 651 should far trump its predecessor

OM 651 is the name of the hopeful, the one with a displacement of almost 2.2 liters and a maximum of 204 hp up to the Mercedes S-Class should prove that it is better than its predecessor in everything: more power and torque, less emissions and consumption. With a redesigned combustion chamber, dual charging and exhaust gas recirculation, the new one manages the strict EU 5 exhaust gas limits even without active nitrogen oxide exhaust aftertreatment. A prerequisite for this is a modern high-tech injection system that provides the appropriate spray quality for efficient combustion with an even more precise injection process and pressures of up to 2,000 bar.

The piezo injector failed in rows

During the development period, the most modern concept did not come from the usual suspects Bosch or Conti, but from the US Supplier Delphi. A common rail system with a directly acting piezo injector had been developed there, in which the nozzle needle is moved directly by the actuator for the first time and without the previously usual servo-hydraulic actuation. But it was precisely this piezo injector that was used in the first 220 CDI and 250 CDI engines - especially in Mercedes C-Class -Models - straight out in rows. Angry customers complained of a sudden drop in performance, the engines went into the emergency program.

At first only one injection nozzle was replaced

After the next stop, they often could not be started anymore, which meant the towing into the Workshop made necessary. According to Mercedes , 4,500 of around 30,000 engines delivered are affected . Because of the accumulation of injector damage, initially only the defective copy was often replaced without considering any other previous damage. In the meantime, however, the technical problems have been resolved and the workshops instructed, all fourReplace injectors in the event of damage.

What happened to the injectors?

The Delphi injectors undoubtedly offer theoretical advantages which lie in their construction principle. This is because the 80 millimeter long piezo stack, which is controlled with 255 V, is located in the center of the injector body and the fuel flows around it under high pressure. On the one hand, this provides - advantageously - cooling, but on the other hand requires reliable electrical insulation.

Allegedly no callback necessary

This is supposed to guarantee a plastic coating that, according to the Delphi developers, even billions of times as much Withstands stack movement. An overly optimistic assessment, especially since the stack contact was also affected by sealing problems. Comment by the Mercedes managers: These were regrettable start-up problems in series production, which have now been eliminated by modifications at Delphi. Cars currently being produced have the modified injectors installed. They still stand by the technical principle. Incidentally, a recall was not necessary because operational and road safety were not impaired: the cars remained drivable, even if in the emergency program.