Many people are asking – why the misfire algorithm tests for different software releases were performed. Why is it so important?
Very short about theory. If one of the cylinders has misfired, the changes in flywheel speed are insignificant. They are small and with fluctuation effect.
As a sample – image from OVERVIEW OF ENGINE MISFIRE DETECTION METHODS USED IN ON BOARD DIAGNOSTICS. Journal of Kones. Combustion Engines, Vol8, No 1-2, 2001.
As we see, the changes of torque for the current test sample on 2000 RPM are even hard to see, not even trying to identify the “guilty” cylinder!
It can be seen by checking the live data via INPA in the situation when some of the cylinders have been turned off.
In this sample, cylinder No.1 has been switched off. As we see, cylinder No.1 also affects values for other cylinders: cylinder No.5 (next according to firing order) has seemingly higher efficiency, No.3 (next after No.5) – with slightly increased, etc.
As can be seen here, the self-diagnostics of MSD80 for many releases (especially for older ones) are making gross mistakes when detecting fluctuation process differences (in case of damage of any cylinder) from ideal performance.
In case when the MSD80 Rough run algorithm functions incorrectly, sometime the truly “guilty” cylinder shows not decreased, but quite the opposite – increased activity (as a result of a mistake in self-diagnostics).
Accordingly, in case of incorrect performance, “guilty” is marked the next cylinder (in firing order) – in case if the polarity of all transition processes has been defined incorrectly, or next after next (in firing order), if the efficiency of the damaged cylinder has been detected incorrectly, but the polarity of the rest of process – correctly.
What does it mean in the case of a real engine?
Let’s suppose that the ignition coil of cylinder No.1 is damaged. But, as the MSD80 has old (for example, 7611396) software release, the algorithm marks “guilty” cylinders:
No.1 – in cases when the algorithm performs correctly;
No.3 – (next after next, in firing order) – in cases when the efficiency of the “guilty” cylinder has been detected incorrectly (close to nominal value);
No.5 – (next, in firing order) – in cases when the polarity of the process has been mixed up.
In a typical case (release No.7611396): if cylinder No.1 during one hour has missed the ignition cycle 10 times, we can get the following (typical) results:
1st cylinder: 5 misfire points;
3rd cylinder: 2 misfire points;
5th cylinder: 3 misfire points.
These data are approximate – they are obtained by provoking a misfire in an exact cylinder. As if everything is OK – cylinder No.1 (really damaged) is dominating. But, in real life, each of the cylinders has been detected as damaged several times, even without real evenness (for example, the effect of the uneven road).
Taking into account the rough road factor (influence), even in the case of an even road, the misfire counters become more similar, for example (the same release 7611396):
1st cylinder: 7 misfire points;
3rd cylinder: 5 misfire points;
5th cylinder: 6 misfire points.
Which cylinder(s) is(are) damaged? And if the changes in counters due to Rough Road are “in favor” of cylinder No.5 (for this scenario, even several additional one-time/random events are enough)? Or – one or even all three counters of cylinders are exceeding the threshold and give the reason for ISTA D to count them as “damaged” during a performance of the test?
The experience shows that the existence of a defect for one cylinder (even not real existence, but, for example, the sensitivity of 1-st cylinder to data logger connection) creates messages for at least 2 “damaged” cylinders!
One more significant reason why the Rough run self-diagnostics system for early releases of MSD not only doesn’t help to identify the damaged cylinder but actually does the opposite: several “healthy” cylinders are identified as damaged!
What to do? Carefully follow the recommendations described here, especially:
1. check the misfire counter with the ELM adapter, don’t rely on values detected by ISTA D;
2. perform short driving sessions on good quality roads, not long ones on bad quality roads;
3. during observation of misfire counters, turn off the diagnostic tools (turn it on only when reading data);
4. if possible, upgrade the MSD80 software release. As the best possible (during tests) are recommended:
8603156 N53B30 UO E60/E61
8603178 N53B30 OO E60/E61
8603182 .. 188 N53B30 OO E63 (additionally carefully check the fuel offset LTFT, their correct creation).
These (newer) releases have a tiny number of falsely detected “damaged” cylinders, and self-diagnostics are performed reliably. If the ELM adapter shows large misfire numbers for any of the cylinders during driving on an even road – check the spark plugs and ignition coils of the “guilty” cylinder.