Performing diagnostics of several N46/N55 series engines (with Bosch management units), the problem, which perfectly fits in the theme of this entry, became clear.
If we check the patents, which are dedicated to detection of misfires, we can see, that Siemens for this cause uses analysis of changes in flywheel spinning speed, but Bosch in turn – using of the accelerometer.
As already mentioned in this blog, for Siemens management systems (notably MSD80/81/87) increased sensitivity to false misfires is characteristic (for example, the error message 3104, when driving on uneven road etc.)
Bosch management units, instead, are very “lazy” with identifying misfire. There is no information, do the Bosch systems indeed use the accelerometer (and in this case identification of misfire in idle and for a vehicle, standing still, is not possible), or yet – data of the spinning of the flywheel, but ME/MEV/MEVD management systems have several specific problems.
1. The decreased threshold for misfire identification and registrations of the error message only in case of essential damages (long group misfires). This problem leads to a situation, that when engine works unevenly (regular individual misfires are present), there will be no error messages in the error message memory;
2. In case of these individual misfires (logically) – the portion of the oxygen is being thrown out in the exhaust, the control probe detects it and it’s voltage drops. ME/MEV/MEVD interprets this drop of the voltage as inadequacy in wide-band probe reference value and slightly enriches the fuel mixture. In case of regular misfires, the value of wide-band probe(s) is overwritten to strongly enriched fuel mixture “side”.
3. Both because of regular changes in control probe values and due to enriched fuel mixture, the performance of CO catalytic converter is strongly decreased, ME/MEV/MEVD records the error message regarding the decreased efficiency of the CO catalytic converter. As a result, the fuel consumption grows (because the engine management unit regularly reinforced heats up the CO catalytic converter, to increase its performance), the lifetime of CO catalytic converter decreases.
If MEV(D)17 series engine management systems are reporting misfire counters via OBD mode 6, ME(V)9 series engine management systems don’t even report, and the task of finding misfiring cylinder is highly difficult.