A brief retrospective overview of the problem.
N53 series engine. As if without any reason, DME switches off cylinder No.6. EML (Check Engine) lights up in KOMBI. Inside the car, the smell of unburned fuel can be felt. Directly before and after the event, all injector and adaptation data are perfect. Live data – perfect. The problem disappears after restarting the engine. Fuel leakage problem (more precisely: live data, which confirms incorrect Lambda) also disappears without restarting the engine, after 5 .. 10 seconds. The problem is progressing – last time, the defect happened already after each 10 .. 20 .. 30 km.
Live data indicate that the problem happens when the engine uses a Stratified charge. Typically – in a range of low/very low torque. Lambda in the 2-nd bank from 2.5 .. 3 drops to 1.2 .. 1.6; after 2 .. 3 seconds, DME confirms the problem and switches to Homogeneous mode. Lambda in the 2-nd bank drops to 0.7 (minimal value, reported by DME); NOXEM indications 0.78 (minimal value).
After stable symptoms, I was convinced that the DME was to blame. But the installation of another DME did not solve the problem. At that moment, I got over my laziness and connected the oscilloscope to check the management of the injectors. Analysis of the signal indicated that DME is managing the injector correctly. I even replaced the plug of the injector to completely exclude the option that the connection is one to blame for this problem!
The injector was replaced with a new one – the problem did not disappear! The main question – what could be the reason for the injector to be stuck in the half-open condition in exact injection strategy. I hypothesized – it could be rust, which builds up inside the Rail. Now it is time to tell you how I succeeded in solving this problem.
So this is an image of the Rail from the outside:

When unscrewing the Pressure sensor, the following picture reveals:

The signs of rust are clearly visible on the thread. And the signs of the rust are also visible on the inside wall of the Rail fuel line (pay attention to the “circle”!). Here I should add that since the purchase of the car, this sensor was never taken off. Obviously, the rust has been building for a long time already!

And here – the rust build-ups on the sensor itself! In addition, this “circle” corresponds to the place filled with fuel. So – the rust is inside the Rail!
And finally, as an example – the seating of one of the Rail drive of one of the injectors:

Here are two types of dirt. Black ones are of soft texture and could be the paint of the fuel tank or leftovers of the coating of plastic spare parts. These are not the main culprits of the problem because they are not forming in the Rail itself. The second “dirt”, instead, is the rust! Obviously, the Rail is rusting from the outside (where the oxygen is available for oxidation) and from the inside too! It is visible here also.
Considering the previously mentioned, the decision was made to purchase a new Rail.

Situation after installing new Rail. 3000km driven, the engine uses Stratified charge regularly. The defect (jamming of the injector) has disappeared. Unfortunately, it is not possible to access the inside of the Rail – the diameter is only around 3 mm. Obviously, the Rail inside was rusting intensively by the drive of the 6th injector.

Here are some additions, shy the defect was precisely like that.
First – this is the management signal that DME uses in this/problematic mode:

DME used three very short injection impulses. During such short impulses, the injector cannot open up completely; its opening is not only short but also very narrow! The length of these impulses is close to the theoretical minimum (100us; fall/rise time around 150us).

Obviously, the microparticles of the rust, created during the oxidation process, are so tiny that they quickly “slip” thru the injector’s nozzle if the injector works in the mode of longer openings stuck if the opening is narrow.

The objection – yes, but there were also successful driving of around 1000km, without any manifestations of the defect directly after installing the replacement DME (but before tidying up the FA and recording of correct FR) – is in place. How so?

Replacement DME has a “historic” FR 75XXXXX recorded directly after installation. The next FR group’s remark was 76XXXXX, and the last – 86XXXXX, is in place. And only the last part of the previous FR group is “usable”. I detected that DME regularly records misfired due to the uneven road and sub optional injection strategy when performing the test drive. Here is the explanation – DME did not use the injection strategies of very short openings, so the defect did not manifest! DME sometimes used Homogeneous lean mode, but it refused the Stratified charge mode during the first kilometers after starting the engine.

Now you see how capricious is the injection system of these engines. I hope that this entry will strengthen your belief that using any fuel additives and/or cleaning aids is a “death sentence” for these engines.