Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Misfire Detection: Notes
The Digital Motor Electronics (DME) detect misfiring.
The misfire detection system records the engine speed and can therefore detect poor combustion in a particular cylinder. The operational smoothness values of the individual cylinders are displayed for troubleshooting purposes. The engine must be operated in idle for at least 3 minutes in order to obtain reliable values. The idle smoothness is evaluated exclusively with the engine in idle (cold or warm). If the combustion in one cylinder is poor, this is detected. Random fluctuations of the operational smoothness value of an individual cylinder can be detected by observing it closely. If combustion in the engine is theoretically even, the operational smoothness values will be 0 (averaged over all cylinders).
Increased operational smoothness values may be attributable, for example, to one of the following causes:
- Misfiring
- Extraneous air
- Mixture deviations
- Faults in fuel supply
- Insufficient compression pressure.
It is therefore not possible to define precise control limits. The engine speed is measured at the increment wheel assisted by the crankshaft sensor. The operational smoothness of the engine (misfire detection) is also monitored in addition to engine speed recording. In order to detect misfiring, the increment wheel in the Digital Motor Electronics (DME) is divided into segments based on the firing interval (between 2 ignition processes). The period duration of each segment is measured in the Digital Motor Electronics (DME) and statistically evaluated. The maximum permissible values for irregular operation are stored (as function of engine speed, load and coolant temperature) for each mapped value. If these values are exceeded for a specific number of combustion cycles, a fault entry is stored for each faulty cylinder detected.
The following graphic shows the engine N55 as example.
| Index | Explanation |
|---|---|
| A | General view of crankshaft from above |
| B | General view without starter motor |
| 1 | Three-pin plug connection |
| 2 | Dust seal |
| 3 | Crankshaft sensor |
| 4 | Multi-pole sensor gear |
| 5 | Starter motor |
Notice! Separate functional description!
There is a separate functional description for the engine speed recording.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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