In the first part of this entry (here), I got to the idea that the performance of cylinder No.6 should be observed more carefully.
The reasons are:
a. by firing order (1/5/3/6/2/4) cylinder is before “the culprit” No.2 – it is possible that cylinder No.2 fixes problems caused by cylinder No.6;
b. performance of cylinder No.6 (discarding No.2) was less quality than for the other four cylinders.
What indicates the low-quality performance of cylinder No.6? Several examples:
On the go, low load mode, directly after readapting the engine. Light blue color: mechanical efficiency of cylinder No.6. The cylinder No.6 quite often is lazy:
Black color: Combustion timing of cylinder No.6. Shorter than for other cylinders:
Periodically, the cylinder No.6 had increased knocking indications:
In this image (after readaptation), we see that cylinder No.6 is even less stable than No.2!
Now the situation looks a “bit different”, isn’t it?
Possible option: cylinder No.2 tries to compensate for problems caused by cylinder No.6. If during any cycle, cylinder No.6 was too active (for example, its injector was sporadically leaking slightly), in the cylinder No.2, a leaner fuel mixture is created to reduce its activity. If, at this moment (as we know, exactly lean fuel mixture ignites poorly), some sporadic misfires start, DME creates incorrect injector adaptations. If the “set” is completed with quite worn spark plugs, then more serious problems are on their way and will appear soon.
I should admit that NGK spark plugs looked quite ok after worn of 25 .. 30.000 km:
As we can see, the melting of the precious metal on the side electrode can be seen. The air gap from the initial 0.75 mm has increased to 0.9 mm. It is acceptable.
Returning to cylinder No.6. Let’s see how cylinder No.6 behaved when cylinder No.2 was misfired.
For example, during a cold start:
The efficiency of cylinder No.2: green color;
The efficiency of cylinder No.6: light blue color.
With red, the events when the efficiency of cylinder No.2 fluctuates strongly are marked. In ALL cases, it is clearly seen that a short moment before the event, the light blue graph reacts exactly the opposite. Cylinder No.6 has been too active, and the efficiency of cylinder No.6 was reduced so the flywheel does not start to “run”. So, really, cylinder No.6 is the cause, and cylinder No.2 is correcting its defects!
In addition, the last event in the previous image and the event marked in this image are “opposite”: the activity of cylinder No.2 is increased! Why? Because as we can see, during previous cycles, the mechanical efficiency of cylinder No.6 was increased, and now it was a bit lazy.
I think now is the time for a short entry regarding how DME of different generations corrects the idle (read here).
Here, a “desert” – zoomed-in image:
It is not hard to see that the green (cylinder No.2) mechanical efficiency mirrors the mechanical efficiency’s light blue (cylinder No.6) curve. Here is a 100% confirmation that cylinder No.2 corrects the problems of cylinder No.6.
Such behavior of the DME is new – introduced exactly in B series engines!
Is it possible that cylinder No.6 corrects the mistakes of some other cylinder? The firing order for B58 is 1/5/3/6/2/4. So, in this case, we have to check the performance of cylinder No.3. The performance curve of cylinder No.3 is in red.
Conclusion – no, cylinder No.3 behaves stably. In none of the problem cases, cylinder No.3 was to blame!
One more nuance – why the performance “spikes” of (initially) the culprit – cylinder No.2 – were so “stable”? The explanation is simple – the correction amount using the changes in the fuel amount (the air in the combustion chamber stays the same) is limited. You can not increase fuel by 20% by hoping to increase the mechanical efficiency by 20%. There is not enough air in the combustion chamber! Instead, in the “opposite direction” – you can not significantly reduce the fuel amount in the combustion chamber – the fuel mixture will become too lean, and misfires will appear. Obviously, the efficiency correction of 10 .. 15% is the max allowed. And, as we can guess, even such corrections create misfires in the spark plugs, which are not in perfect condition.
What could be the reasons for the problem of cylinder No.6?
a. injector problems – it sometimes drips slightly;
d. due to misfires in the past, DME has created incorrect adaptations for the injector of this cylinder.
On the second day after readapting the engine, the idle looked like this:
Very small “hills” of cylinder No.2 can be seen in the 5th and 18th seconds. The overall situation has radically improved! Obviously, the injector defects are not ones to blame – they can not “disappear”.
Then I decided to see a week-old (after the first crash, but before readaptation of the engine) idle graphs:
What do we see in the old data?
a. the idle was less stable (the vector method was performing significantly worse) – it is obvious;
b. we can see that, time by time, cylinder No.3 (red graph) has been too active!
So – initially, the possible culprit was cylinder No.3? It is possible.
My conclusions are as follows:
a. misfires are degrading both injector and vector method adaptations and their performance quality;
b. as DME uses multi-ignition, very often misfires are not very well pronounced and are not prevented on time (note: using of multi-ignition was NOT confirmed later);
c. misfires in some of the cylinders (in idle) can appear due to unstable performance of other cylinders (previous by firing order);
d. after solving the problem, the engine is not able to “fix” the incorrect adaptations fast enough;
e. after the readaptation of the engine, the optimal result requires several motor hours with regular pauses in idle;
f. during the readaptation, the RPM below 1000/zero load segment has a significant impact.
Now, there are more questions than answers. Largely because even in the Year 2023, DME is not able to identify misfires in idle. And not due to technical limitations but – due to patent wars. It is sad. In addition, even with the dealer diagnostics tools, no injector adaptation data are available. Even not fly-time adaptation information (Bosch had to introduce these adaptations when it turned out that the behavior of the “new” non-piezo injectors was worse than planned).
Actually, it is very interesting what would the BMW AG dealer say in this case. Replace spark plugs, ignition coils, and/or injectors? A little repair in value of EUR 3500 .. 4000?