NGK spark plugs, installed on my B58 series engine, have served for 25 .. 30.000 km. During the last week, three “crashes” have happened in idle. The first one is described here. After the first case of misfire, I performed the cleaning of the spark plugs from the carbon residues (service procedure intended in ISTA). The situation with random misfires got better, but after a moment, the following case of group misfires in idle happened; several days later – another one. It was clear – something was not okay with the engine’s performance. There are several (even a lot) options possible, but the simplest one is that some of the spark plugs have problems. Carbon residues may have formed a “bridge,” and the spark tends to “jump” incorrectly. The decision was simple. Spark plugs need to be replaced immediately.
The engine had BMW/NGK spark plugs installed. The spark plugs were purchased from a BMW AG dealer, priced around EUR 26/piece.
A perfectly smooth performance of the engine during transition modes could be felt immediately after the replacement of the spark plugs. The improvement is impressive/can be felt. In idle thou, a slight irregular vibration can be felt.
Let’s check Expert mode; what does the mechanical efficiency of cylinders show:
In the images, we can see that the problem dominates for the 2nd cylinder (a curve in green color). Zoom in:
As we can see, the mechanical efficiency of the 2nd cylinder sometimes drops to around +6 .. 8 units, which means at least 8 .. 12% reduction of energy.
On the go, with low required torque:
On the go, everything looks perfect! Cylinder No.2 feels excellent; you can actually blame it for slightly increased efficiency.
Much more complicated is to notice this problem in the Call-up live data menu:
To catch this value, I had to observe data for 30+ seconds carefully. In addition, I accurately observed exactly the data of the 2nd cylinder. The conclusion is simple – to identify such sporadic, irregular problems, the Expert mode should be used!
I observed the Combustion time in idle, too:
Data of the 2nd cylinder: dark orange. Nothing interesting: the Combustion time of this cylinder is the same as the performance of the other cylinders. Actually, the black curve (data of the 6th cylinder) looks much more suspicious.
What could be the causes of this problem of the 2nd cylinder? Most likely:
a. partial misfire due to problems with the spark plug/injection coils. The fuel mixture is ignited not on the 1st try but on 2nd or even 3rd time;
b. partial misfire due to injector problems. The injector either partially “floods” the spark plug, or it has an incorrect beam (the fuel is ignited too far from the spark plug or at the beginning of the ignition cycle – bad atomization, etc.);
c. injector problems. The fuel is ignited correctly, but the amount of it (fuel) or its atomization quality is inappropriate;
d. the cylinder has some mechanical problems – either with the Valvetronic (the opening of the inlet valve reduces) or with the closing of the valves (there is a gas leakage via incompletely closed valves);
e. thanks to the misfires before replacing the spark plugs, DME has created incorrect injector adaptations for the 2nd cylinder;
f. problems of this cylinder are the consequences of incorrect performance of some other cylinder.
I will shortly explain my thoughts regarding each of the possible causes of the problem (judging by the symptoms of the problem):
a. yes, possible and plausible scenario. The spark plug is new, so – only the problem with the ignition coils is left;
b. possible and plausible scenario;
c. not very plausible scenario. Cylinder efficiency drops with quite stable/repeatable “step”. It is not plausible that the injector could “jitter” so stable. In addition, the total fuel mixture (including STFT via OBD) stays stable;
d. not a very plausible scenario. Such mechanical defects would not be so stable and sporadic at the same time;
e. possible and plausible scenario. The cheapest possible scenario. Scenario, which should be checked first when starting to solve the problem!
f. continue to read this entry – there will be a lot of interesting things in the 2nd part of it!
Deviation from the topic: I have observed problems with creating individual adaptations of cylinders since the times of N43/N53 series engines. DME, when creating the adaptation (upgrading them), did not ignore combustion cycles that were misfires. As a result, these “defective” cycles degraded the injector (and other cylinder individuals) adaptations. Could it be that even for B series engines, this problem is not solved? Yes, it could, because Bosch DME is not “allowed” to identify misfires in idle!
If the problem is with incorrect adaptations of the 2nd cylinder, the simplest should be tried out. DME should be allowed to restore correct adaptation. I drove 50 km in different conditions, deliberately allowing the DME to run idle several times. Unfortunately, zero progress.
As stage two, I chose re-adaptation of the engine. I deleted all adaptations. I adapted the flywheel. I have created initial common LTFT adaptations (several cycles of 2 .. 3 + 2 .. 3 minutes in idle and on the go).
I readapted Valvetronic. I repeated “on the go + idle” cycles.
Yes, now the progress can be seen! The problem of the “jumps” of the second cylinder has become rare and is minimized.
When zooming in, we can see that now the “teeth” of the 2nd cylinder can be compared to several other cylinders.
Yes, the situation still is not ideal:
But definitely – the progress is significant! Excluding a slight problem in the 3rd and 14th seconds, cylinder No.2 behaves well!
In addition, here we can see one more interesting nuance:
With red, the “teeth” with the “opposite” direction are marked. Regularly, the performance of cylinder No.2 turned out “better” than planned! Yes, theoretically, it can happen that the injector “dripped”, but – just now, such a problem was not to be seen. Most probably – this, too, is a result of incorrect adaptations of the injector. Anyway, the mechanical problems of the cylinder can be ruled out as the possible culprit. And in this “defect”- the failure of the ignition too.
One more interesting parameter is knock data. Here is how they looked before the repair:
We can see that the knock parameter of the 2nd cylinder is higher than for others. At the same time – it is enough to increase RPM, and the problem disappears! By 2000 RPM, knock drops even lower than for other cylinders!
It is clear that these data are connected with the problem described above. What could they mean? Possible options:
a. slag residues on the piston surface of the 2nd cylinder;
b. bad atomization of the injector;
c. combustion chamber defects due to injector problems (melted piston);
d. lean fuel mixture.
Brief reflections on each reason:
a. unlikely cause. The oil consumption is below 1l/15’000 km, and the slag residue cannot appear exactly in one cylinder;
b. unlikely cause. On the go (checking the mechanical performance of the cylinders, too), everything is fine with the efficiency of the cylinder;
c. very unlikely cause. Significant problems with the performance of some cylinders aren’t observed. In addition, if this defect were present, knock data would not reduce so nicely when the RPM increases (when RPM increases, the time during which the fuel is mixing with the air reduces – the atomization problem would only grow);
d. yes, this is a plausible cause. Even more – reduced (sporadic problem) mechanical efficiency could be (and by activities described above, confirmed by 99%) exactly due to lean fuel mixture.
Here, knock data after re-adaptation of the engine:
The problem with increased knock in the 2nd cylinder disappeared, too! Yes, sometimes, for a short moment, we see 0.02V in the 2nd cylinder. Still, the situation looks much better! So, the residues on the cylinder surface or the atomization of the injector were not ones to blame – such defects don’t “repair” themselves!
It looks like the problem was in the performance of the 2nd cylinder, and now it is reducing. But is it really so? Live data indicate increased unevenness of the performance of the 6th cylinder, too. At the first moment, this problem does not seem as pronounced as with cylinder No.2. But if we check the firing order: 1/5/3/6/2/4, we see that cylinder No.2 “works” exactly after cylinder No.6. Could there be any connection?
About this – in continuation of this entry.