In this entry – I will skip simple situations, for example – the injector is not connected or it has some fundamental defect: it does not inject fuel. In such a situation it will be very simple: the error messages regarding the current injector, regarding misfires of the cylinder. You need just fix the problems and the engine will tun again. I will describe a bit more complicated and specific cases.
I write this entry because several readers of the blog have asked me: how can I trust the Misfire counters if the cause of them could be a defect of the injector of another cylinder? Yes, it is true – not always the misfires in the current cylinder indicate its defects, so the circumstances, when the misfires are recorded, are very significant.
First scenario. For granted – if the injector is leaking strongly (injects too much fuel), the fuel mixture in the cylinder is very rich, and misfires will start. This is the simplest scenario, typical for Homogeneous mixture (mode), especially – in the first 10 .. 15 seconds of the start of cold engine (open-loop mode). Till the moment, while Lambda probes start to work.
The second scenario (continuation of the first scenario) – injector of any other cylinder of this bank is leaking. If the Lambda probe is turned on (warmed up, it works – after first 15+ seconds after the start of the engine), DME sees the fuel left-over in the bank and reduces fuel supply to all cylinders of this bank. As a result – in the cylinder of damaged injector the fuel mixture gets less rich, in others (correctly working) cylinders of this bank – lean. In case of lean fuel mixture the misfires start sooner (lean mixture ignites worse than rich mixture) – accordingly, the cylinder of correctly working injector is misfiring (or even both other cylinders of the bank in case of 6 cylinder engine). The problem is typical for Homogeneous, closed-loop mode.
The third scenario. Typical for Stratified charge mode of BMW Direct injection petrol engines (in some extreme cases can be observed in Homogeneous mode too).
Here, the educational material of BMW AG:
The structure of the BMW Direct injection petrol engines has certain peculiarities. To execute a Stratified charge (just like the BMW does it), the fuel beam, injected by the injector, has to be very close to the spark plug (spark). The reason – the average fuel mixture of the Stratified charge in the combustion chamber is lean (Lambda 2 .. 3.5). Such a lean fuel mixture is not burning if tried to ignite with the spark of the spark plug. To ignite the fuel mixture, it is injected directly next to the spark and ignited directly at the moment, when it is injected in the combustion chamber. In this part of the combustion chamber (directly by the spark plug) directly in that time, the fuel mixture is rich enough to ignite – problem solved.
But a new problem appears – the fuel beam, injected by the injector, has to be targeted very well. In the image BMW indicates, that the beam difference (distance from the spark plug) of 0.5 .. 1.0 mm from the ideal one is yet acceptable, but larger – is not. If the fuel beam injected by the injector hits the spark plug, it floods the spark plug and the fuel ignition is not possible. This is one of the reasons for the misfires.
This problem is especially topical exactly for the Stratified charge. In Homogenous mode – even if the injector pours the fuel on the spark plug, the spark plug has a certain time to dry (the fuel is able to evaporate). The reason is simple: in Homogeneous mode, the fuel is injected a long time before it’s ignited. Whereas in Stratified charge mode the fuel is injected and ignited at the same time – if the fuel hits the spark plug, it has no time to evaporate, and the spark plug can not dry. A wet spark plug is not able to ignite the fuel and misfire is guaranteed!
The fourth scenario. Typical only for Stratified charge. The fuel beam, injected by the injector, is too far from the spark plug or is injected incorrectly (not as a fine mist, but poured). In this case, the fuel does not ignite because in the place where the spark is created the fuel mixture is lean. In this case, even the reason is not important – the fuel beam is misdirected (too far from the spark plug), the fuel is poured or injected too less – in all cases misfire will happen.
In the last two scenarios (topical for N43/N53 series engines) the engine can work more or less correctly in Homogeneous mode, but problems will start exactly in Stratified charge mode.
For Homogeneous mode, correct results of the chemical tests of the injectors (../F5/Shift+F6/F3) and correct live data (../F5/Shift+F6/F1) – both opening times in idle and flowrate for partial load – will be typical.
Stratified charge – could be correct (typically for the third scenario), or – increased correction data of the injector (both idle and partial load: typical for the 4-th scenario; ../F5/Shift+F6/F5).
Note: in the last two scenarios (in the 3-rd scenario – if the engine uses Stratified charge) – in the DME error message memory the error message 3104 will be recorded.
Finally: we need to understand – Misfire counters indicate the problems of the exact cylinder. What causes it – lean or rich fuel mixture of the cylinder itself, or – problems of any other cylinder of this bank, or even – problems common to the whole bank or the engine: it has to be clarified during diagnostics.
