SPN 1328 FMI 31: Meaning and Fix
SPN 1328 FMI 31 indicates that the Engine Control Module (ECM) has detected a misfire rate on Cylinder 6 exceeding a calibrated threshold over a defined number of engine cycles (SPN 6394). This fault commonly appears after a forced DPF regeneration when low injection pressure or poor fuel quality causes incomplete combustion. In workshops, technicians often encounter this code after replacing the ECM without performing a proper injector coding procedure, leading to false misfire detection.
Common Symptoms
- Rough Idle: Engine vibrates unevenly at idle due to missing combustion events in cylinder 6.
- Power Loss: Noticeable reduction in torque output, especially under load or acceleration.
- Excessive Smoke: White or grey smoke from exhaust indicates unburned fuel from cylinder 6.
- Check Engine Light: MIL or CEL illuminated on dashboard with fault code stored in ECM.
Probable Causes
- Injector Failure: Stuck open, closed, or worn injector on cylinder 6 causing poor atomization.
- Low Compression: Worn piston rings, valves, or head gasket on cylinder 6 reduces combustion pressure.
- Fuel Supply Issue: Restricted fuel line or low rail pressure starves cylinder 6 of fuel.
- ECM Calibration: Incorrect injector coding or outdated software leads to false misfire detection.
Advanced Technical Analysis
The ECM monitors cylinder 6 misfire rate by analyzing crankshaft acceleration variations via the crankshaft position sensor. A threshold of 2-5% misfire over 200 engine cycles typically triggers FMI 31. The algorithm uses a 720-degree window per cylinder to isolate combustion events, filtering out mechanical noise from gear train backlash.
Electrical faults such as open circuits in the injector wiring harness or high resistance at the connector pins cause intermittent misfire readings. The ECM applies a debouncing timer of 10 seconds before logging the fault to avoid transient noise. Real-world cases show this code after water ingress in the 6-pin injector connector.
Upon detecting persistent misfire, the ECM activates cylinder cutout for cylinder 6 and derates engine torque by up to 40%. The fuel injection quantity is reduced by 50% for that cylinder to prevent catalytic converter damage. This fallback mode is documented in MAN D2866 and Deutz TCD 2013 service manuals.
Long-term prevention requires verifying injector return flow rates and performing a compression test. Technicians frequently resolve this after replacing a leaking injector on cylinder 6 and updating ECM firmware per Mercedes-Benz SI document 2018-04. A cylinder contribution test using a scan tool confirms balanced power output across all cylinders.
Step-by-Step Troubleshooting Guide
- Read Fault Codes: Use J1939 scan tool to retrieve SPN 1328 FMI 31 and related freeze frame data.
- Inspect Injector: Perform injector pop test and measure return flow; replace if exceeds 20 ml/min.
- Check Compression: Cylinder 6 compression must be within 15% of other cylinders; repair if low.
- Update ECM: Flash latest OEM calibration and recode injectors using manufacturer diagnostic software.
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