As part of the CARB/OBD regulations the engine control module must determine if misfire is occurring and also identify the specific cylinder(s) and the severity of the misfire event, and whether it is emissions relevant or catalyst damaging. In order to accomplish these tasks the control module monitors the crankshaft for acceleration losses during firing segments of each cylinder based on firing order.
The process of misfire detection continues well after the diagnostic drive cycle requirements have been completed. Misfire detection is an on-going monitoring process that is only discontinued under certain conditions.
Misfire detection is only disabled under the following conditions:
Misfire Detection Example: M52 (6 Cyl.) with Siemens System
The misfire/engine roughness calculation is derived from the differences in the period duration (T) of individual increment gear segments. Each segment period consist of an angular range of 120° crank angle that starts 78° before Top Dead Center (TDC).
Increment gear wheel segment period measurement:
• If the combustion process in all cylinders is functioning correctly, the period duration of each segment will be identical (i.e. T0 = T1 = T2 = T3 = T4 = T5).
• If a misfire is encountered in a cylinder, the period duration (T) of that cylinder will be extended by a fraction of a millisecond (i.e. T3 > T0, T1, T2, T4, T5).
• All measured values of T are evaluated within the DME, corrected based on sensor adaptation and compared to a set of predetermined values that are dependent on engine speed, load and engine temperature.
If the expected period duration is greater than the permissible value a misfire fault for the particular cylinder is stored in the fault memory of the DME. Depending on the level of misfire rate measured the control unit will illuminate the “Check Engine” light, may cut-off fuel to the particular cylinder and may switch lambda operation to open-loop. All misfire faults are weighted to determine if the misfire is emissions relevant or catalyst damaging.
Emissions Relevant:
During an interval of 1000 crankshaft revolutions the misfire events of all cylinders are added and if the sum is greater than a predetermined value a fault will be set identifying the particular cylinder(s). The Check Engine light will be illuminated during after the second cycle if the fault is again present.
Catalyst Damaging:
During an interval of 200 crankshaft revolutions the misfire events of all cylinders are added and if the sum is greater than a predetermined value a
fault will be set identifying the particular cylinder(s). The “Check Engine” lamp:
• On vehicles with a Siemens Control Module (M52 engines) - the lamp will immediately go to a steady illumination since fuel to the injector(s) is removed. Fuel cut-off to the cylinder will resume after several (≈ 7) periods of decel if crankshaft sensor adaptation is successfully completed or the engine is shut-off and restarted.
• On vehicles with a Bosch Control Module (M44, M62 & M73 engines) - the lamp will blink as long as the vehicle is operated within the specific criteria under which the fault occurred. Fuel to the misfiring cylinder is not cut-off as long as the “Check Engine” light is blinking. In each case the number of misfire events permitted is dependent on engine speed, load and temperature map.
Crankshaft Sensor/Flywheel Adaptation:
In order to eliminate misfire faults that can occur as a result of varying flywheel tolerances (manufacturing process) an internal adaptation of the flywheel is made. The adaptation is made during periods of decel fuel cut-off in order to avoid any rotational irregularities which the engine can cause during combustion. (i.e. Crankshaft sensor adaptation displayed on M73 with DIS 12.0)
In order to establish crankshaft sensor adaptation 7 periods of decel fuel cut-off are required. This adaptation is used to correct segment duration periods (time from TDC1 to TDC2, to TDC3, etc.) prior to evaluating for a misfire event.
If the sensor wheel adaptation has not been completed the misfire thresholds are limited to engine speed dependent values only and misfire detection is less sensitive. The crankshaft sensor adaptation is stored internally and is not displayed via DIS or MODIC. If the adaptation limit is exceeded a fault will be set (M52 only).
ENGINE MISFIRE DIAGNOSIS
(OBD II monitoring criteria considered)
Engine Misfire is the result of inefficient combustion in one or more cylinders. The causes of Engine Misfire are extensive but can be grouped into the following sub-systems. Consider the charts below as an additional diagnostic aid once the DIS is connected, the correct fault symptom has been chosen and the fault memory has been interrogated. Follow the Test Module as displayed by the DIS.
