SPN 1328 FMI 15: Meaning and Fix
SPN 1328 FMI 15 indicates the Engine Control Module (ECM) has calculated that Cylinder 6’s misfire rate has exceeded the programmed normal operating threshold, though the condition is not yet severe. This code commonly appears after a forced DPF regeneration where fuel dilution has occurred, or following an injector replacement if the correct calibration data wasn’t flashed. The rate is calculated as a percentage of misfires over a specific number of engine cycles (SPN 6394).
Common Symptoms
- Rough Idle: Engine exhibits noticeable vibration at idle, specifically felt as a rhythmic shake indicating Cylinder 6 is not contributing fully to combustion.
- Power Loss: Reduced engine torque during acceleration or under load, particularly noticeable when climbing grades or during highway merging maneuvers.
- Increased Smoke: White or light blue smoke from exhaust, especially during cold starts or immediate acceleration, indicating unburned fuel from Cylinder 6.
- Fuel Economy Drop: A measurable decrease in miles per gallon, often reported by fleet operators as a sudden change in operational cost per route.
Probable Causes
- Injector Fault: Sticking or worn fuel injector nozzle on Cylinder 6, causing poor atomization, dripping, or a completely failed injection event.
- Compression Loss: Mechanical issue such as a burnt exhaust valve, broken piston ring, or worn cylinder liner leading to low compression in Cylinder 6.
- Wiring Harness Damage: Chafed, shorted, or open circuit in the injector wiring harness for Cylinder 6, often near the valve cover or engine harness connector.
- Crankshaft Sensor: Miscalibrated or failing crankshaft position sensor providing erratic signal, causing the ECM to misinterpret Cylinder 6’s contribution.
Advanced Technical Analysis
The ECM monitors crankshaft acceleration and deceleration patterns during each firing event. For Cylinder 6, a specific angular velocity window is analyzed. If the acceleration is below a calculated threshold for a defined number of cycles, a misfire rate is incremented. This data is transmitted via the Engine Cylinder 6 Combustion Metrics PG, with the rate being a percentage. The debounce timer prevents false triggering, but consistent poor acceleration will set FMI 15.
Electrical integrity is critical. A high-resistance connection in the injector return line for Cylinder 6 can cause a slow current ramp-up. The ECM’s injector drive circuit monitors current and voltage; a deviation of more than 10% from the expected profile can lead to a partial injection, which is interpreted as a misfire. Technicians should measure pin-to-pin resistance and perform a wiggle test on the harness while monitoring live data.
When FMI 15 is active, the ECM may initiate a torque derate strategy, typically limiting engine power by 25% to protect the aftertreatment system from fuel wash. In severe cases, the ECM will disable Cylinder 6’s injector entirely. This is a protective measure to prevent catalytic converter damage. The derate is not immediate but ramps in over a 30-second period if the misfire rate persists above the threshold.
Long-term strategy involves verifying the injector’s actual flow rate against the nominal value on a test bench. A common workshop scenario involves a truck that had a head gasket replaced; after reassembly, the wiring bracket was routed incorrectly, causing a chafed wire. This code also appears frequently after ECM software updates if the injector correction codes were not re-entered, leading to incorrect fuel quantity calculations for Cylinder 6.
Step-by-Step Troubleshooting Guide
- Scan & Record: Use a diagnostic tool to record freeze frame data, engine load, RPM, and coolant temperature at the moment the fault was set.
- Injector Test: Perform a cylinder cutout test or injector electrical test to isolate if Cylinder 6 is mechanically or electrically failing.
- Compression Check: Conduct a relative compression test to compare Cylinder 6’s peak pressure against other cylinders; a variance over 15% is critical.
- Harness Inspection: Inspect the engine harness from the ECM connector to the Cylinder 6 injector for chafing, corrosion, or bent pins. Check for water intrusion.