SPN 655 FMI 18: Meaning and Fix
SPN 655 FMI 18 indicates the ECM detects the fuel injector for cylinder 5 is operating below its normal current or voltage range, but data is still valid. This fault commonly appears after a forced DPF regeneration or ECM software update, where injector calibration values are offset. Technicians frequently encounter this after replacing the injector without performing the required J1939 calibration routine, leading to a persistent low-drive signal.
Common Symptoms
- Rough Idle: Cylinder 5 misfire causes uneven engine rotation at idle, noticeable as vibration.
- Power Loss: Torque derate of 25-40% under load due to ECM safety intervention.
- Increased Smoke: White or gray exhaust from unburned fuel in cylinder 5 during acceleration.
- Hard Starting: Extended crank time when cold, as cylinder 5 contributes insufficient fuel.
Probable Causes
- Injector Drift: Fuel injector internal wear causing lower than expected solenoid current draw.
- Wiring Resistance: High resistance in harness between ECM pin and injector connector, reducing current.
- ECM Calibration: Injector fuel quantity calibration not updated after replacement, per SAE J1939-73.
- Low Battery Voltage: System voltage below 11.5V during cranking causes insufficient injector drive current.
Advanced Technical Analysis
The ECM monitors injector current via a low-side driver and a sense resistor. FMI 18 triggers when the measured peak current falls below the calibrated threshold (typically 8-10A for a 12V system) for more than 2 engine cycles. The ECM’s microcontroller compares actual current to a reference map stored in flash memory; a deviation of >15% sets the fault. This ensures early detection of injector degradation before complete failure.
Electrically, the fault can stem from increased series resistance in the injector circuit. The ECM applies a debouncing timer of 500ms to avoid false triggers from transient noise. A resistance check between pin A (supply) and pin B (return) at the injector connector should read 0.5-1.2 ohms. Values above 2.0 ohms indicate corroded terminals or a failing injector coil, common after moisture ingress in off-highway equipment.
Upon activation, the ECM enters a safety fallback: it disables cylinder 5 injection and enforces a torque derate of 30% to protect the aftertreatment system from raw fuel. The engine control module also logs the event in non-volatile memory with a freeze frame of RPM, load, and coolant temperature. This allows technicians to pinpoint the exact operating conditions when the fault occurred, aiding in root cause analysis.
Long-term prevention requires verifying injector calibration codes via J1939 diagnostic messages (DM14) after any injector service. Technicians should also inspect the injector harness for chafing near the valve cover, a known issue on MAN D2676 engines. In practice, this fault often recurs if only the injector is replaced without updating the ECM’s trim tables, leading to a return visit within 100 operating hours.
Step-by-Step Troubleshooting Guide
- Scan Freeze Frame: Use J1939 tool to read DM14 freeze frame; note RPM and load at fault onset.
- Measure Injector Resistance: Disconnect harness, measure pin A to B: 0.5-1.2 ohms acceptable; >2 ohms replace injector.
- Check Wiring Continuity: From ECM connector to injector, verify resistance <0.5 ohm with no shorts to ground.
- Perform Injector Calibration: Enter injector trim code via diagnostic tool per SAE J1939-73 procedure.