At the first moment, it seems that everything is simple – the fuel mixture either is or is not ignited. In the second case, there is a misfire. Yes, actually it is. At the first moment, it seems – if there is a spark, everything is fine; if there is no spark – misfire. Really? No – there are many reasons for the misfires. In this entry – about possible reasons.
For a misfire to happen, there should be problems:
a. with fuel, or;
b. the spark itself, or;
c. logically – both problems at the same time.
Possible problems with the fuel: a very rich fuel mixture; lean fuel mixture (in this case – a much more capricious scenario). Taking into account that all last generations of petrol engines are DI (Direct Injection), the fuel mixture in the close vicinity of the spark plug is important, not the average fuel mixture in the whole combustion chamber. Accordingly – exactly the fuel mixture in the close vicinity of the spark plug should be suitable for burning.
Possible options for the fuel mixture problems (in the near vicinity of the spark plug):
a. the injector does not inject enough fuel;
b. the injector injects too much fuel;
c. the injector pours, not injects the fuel as mist (poor atomization);
d. beam of the injector is incorrect – the fuel is injected too far from the spark plug;
e. beam of the injector is incorrect – the fuel is injected directly on the spark plug, flooding it.
Note: this time, I am not looking at the mechanical problems of the engine, nor problems that affect the whole engine of the whole bank (if there are several banks) – problems with the fuel mixture creation, VANOS, EGR, and other hubs. This entry – is about a situation relating directly to problems of one/individual cylinder. Problems of the individual cylinder can be caused by Valvetronic, too, but in this entry – let’s assume that the diagnostics specialist has excluded this version (misfires continue with turned-off Valvetronic).
More about injector problems and misfires read here. At the first moment, it seems that scenarios No.3 and No.4 are related to N43/N53 series engines only, but – no, these scenarios are relevant to other BMW DI engines, too. Also, the new DI engines (N54/N55/BX8, etc.) are working in the specific “layer” mode, not in the Homogenous (with Lambda 1.00) mode. What does it mean? Although the average fuel mixture is close to Stoichiometric (air/fuel ratio around 1:14), the fuel distribution in the combustion chamber is not Homogenous. Close to the spark plug, the fuel mixture is rich; further away from the spark plug – lean. Rich fuel mixture ignites very well, but lean fuel mixture – does not ignite (and the knocking is not created despite the high compression ratio). To create such a fuel mixture, the fuel is injected into the combustion chamber directly before ignition. So – fuel atomization (because fuel does not have time to mix with the air) and the injector beam (because the injector injects the fuel in the direct vicinity of the spark plug) is very important.
One more significant nuance in scenarios: the injected fuel amount is too much/too little. In such scenarios, the injector itself could be the culprit; it is self-explanatory. But other reasons should be considered too:
a. for N43/N53 series engines, the Stratified charge mode is very important. In this mode, the injector atomization quality and leaking are measured. The injectors should be measured, and their adaptations should be applied. The injector adaptations should be correct;
b. for N54/N63 series engines, the injector adaptation problem is slightly less pronounced (cause: in idle, the Rail pressure is reduced till 50 bar), but for these engines, too – the individual adaptations of the cylinders/injectors are very important;
c. for N20/N26 series engines, Bosch performed the upgrade some time ago, during which the MSD was fundamentally upgraded. Both injectors (new ones – trimmed with the laser) and the DME itself. The individual injector fly-time adaptations were introduced;
d. For BX8 series engines, the software upgrades still appear (Years 2023/2024). For my car, I observed that the upgrade of the Year 2022 quite significantly (in a positive way) changed the behaviors of cylinders in idle.
More about the injector troubles read here.
Why did I mention the injector problem in detail? Very often, exactly the fuel mixture is to blame for misfires in the cylinder. It is worth remembering. Otherwise, the desire for endless “tunings” of the ignition system can appear – quite often, it finishes with the fire.
Possible problems with the ignition system.
a. defect of the ignition switch management. The result: the switch is closed, dwell is not happening, or – opened all the time (see option “c”). In theory: possible. In practice: not happened in my experience; the possibility: is VERY close to 0;
b. defect of the ignition switch: the switch does not open (as in “a”, first options: dwell is not happening). In theory: the defect of the IGBT switch Gate is possible. Practically: I have not seen such a defect. The possibility: is VERY close to 0.
c. defect of the ignition switch: short-circuit. In this case, the current flows continuously via the switch and the ignition coil. Since each ignition coil does not have its own FUSE (and the common fuse is at least 20A), it typically does not work, and there are no emergency switch-off solutions. Around 150 .. 200 W of energy is released in the ignition coil, and the wires, connector, and coil heat up. This is a VERY dangerous defect – it can cause fire! This defect can be seen visually: the ignition coil is melting, stinking, and smoking. Such a defect is very rare in the stock ignition systems but is not uncommon in the “tuned” systems;
d. primary circuit break. In this case, the ignition coil’s energy build-up doesn’t happen (the Dwell period’s current is 0) with no spark. Such a defect inside the ignition coil – possibility close to 0. A typical reason is the connection of the plug. For N series engine ignition coils, the plug is made in a “peculiar way” – the connections “work” only in the last 10 .. 15% of the plug contact area. For BX8 engines, however, the plugs and their contacts are super compact, which raises questions regarding their longevity;
e. partial short-circuit of the ignition coil primary circuit. Possibility of the defect – close to 0. In case of such a defect, the current during dwell time increases faster; the saturation occurs during the dwell period. Only part of the energy is accumulated in the ignition coil; part of the accumulated energy is released in the ignition coil itself, in the heat. There is a spark, but it is much weaker than it should be;
f. partial short-circuit of the ignition coil secondary circuit. The possibility of the defect is high. Consequences of the defect as in the previous scenario. Unfortunately, this defect could be both sporadic or manifest itself only in case of increased temperature and/or high voltages (in actual conditions). The last means – when measuring the injector inductance L and its Q, both parameters can be correct, but the defect is present. The defect can not manifest during the Dwell period, but it can manifest as a “spark” in the ignition coil itself during the discharge;
g. defects/energy leakage in the pipe of the ignition coil or in the insulator of the spark plug itself (by pipe). Possibility of the defect: high. Unfortunately, the pipe of the BX8 series engine ignition coils is black in color, and defects are difficult to see. For the N series ignition coils manufactured by Bosch (more relevant for N43/N53 series engines, for them K=100+), the current leakages could be seen very well. Dwell period: correct;
h. spark creation in the “side” of the combustion chamber, but not in the intended place. Energy leakage between the insulator and the body of the spark plug. Very common in cases when the spark plug (its insulator) is covered in carbon. Dwell period: correct;
i. open circuit of the secondary circuit of the ignition coil. Hard to imagine a defect because the rupture (for it to have a significant impact) should be at least 1+cm. It is possible in the “tuned” ignition systems because in these, the most unimaginable “solutions” have been seen. Also, in the case of this defect – the Dwell period is correct.
Defects [a], [b], [d] will be recognized by more or less each modern DME. The error message regarding the ignition coil connection will be recorded in the DME memory.
In the case of [e] and [f] defects, the combustion time will be shorter, and in the case of [g] and [h] defects, it will be correct or longer (compared to the proper value). Combustion time is measured by the most modern DMEs (MSD85/87 and MEVD17+). Unfortunately, a correct Combustion time fluctuates in the range of +/-10 .. 20%, and the error message is recorded only in case of significant deviation of the parameters. Accordingly – these defects will not always be recognized by DME.
In case of defect [i], the Combustion time will be reduced (voltage during Combustion time: increased). Unfortunately, this defect, too, will be recognized by the DME only when the circuit break is impressive (several cm).
