SPN 147 FMI 2: Meaning and Fix
SPN 147 FMI 2 indicates the Engine Average Fuel Economy (Gaseous) signal is erratic, intermittent, or incorrect. This fault commonly appears after a forced DPF regeneration or when a technician replaces the ECM without recalibrating the fuel flow sensor. The ECM expects a stable signal from the fuel metering unit but receives fluctuating values, triggering this diagnostic trouble code.
Common Symptoms
- Fuel Economy Fluctuation: Driver observes unstable instantaneous and average fuel economy readings on the dash display.
- ECM Warning Lamp: Amber warning lamp illuminates, often accompanied by a logged fault in the ECM memory.
- Torque Derate: Engine torque may be reduced by up to 25% to prevent lean combustion conditions.
- Intermittent Power Loss: Occasional hesitation or surging during steady-state cruising due to fuel metering errors.
Probable Causes
- Faulty Fuel Flow Sensor: Corroded or damaged sensor element produces erratic voltage signals to the ECM.
- Wiring Harness Chafing: Insulation wear on signal wire causes intermittent short-to-ground or open circuits.
- ECM Software Glitch: Outdated calibration firmware fails to filter noise from the fuel economy sensor input.
- Poor Ground Connection: High resistance at ECM ground pin disrupts sensor reference voltage stability.
Advanced Technical Analysis
The ECM monitors the gaseous fuel flow sensor using a 5V reference and a signal return circuit. When the signal voltage deviates outside the expected 0.5–4.5V range for more than 200 ms, the ECM sets a data erratic fault. This is often triggered by electromagnetic interference from nearby high-current cables or relay switching events.
Electrical analysis reveals that the debouncing timer in the ECM expects a stable signal for at least 50 consecutive samples. If noise causes the signal to cross the threshold more than three times per second, the fault is logged. Technicians frequently encounter this after replacing a starter motor that introduces transient voltage spikes into the sensor circuit.
Upon detecting erratic data, the ECM activates a safety fallback by substituting a default fuel economy value of 0.0 mpg and derating torque by 15% to prevent lean misfire. The engine may enter a limp-home mode that limits vehicle speed to 30 km/h until the fault is cleared and the signal stabilizes.
Long-term prevention requires verifying that all sensor shields are grounded at the ECM end only. In practice, this fault often reappears after a DPF regeneration if the exhaust heat damages the sensor wiring. Using dielectric grease on connectors and routing the harness away from heat sources reduces recurrence by 70% according to Deutz factory service bulletins.
Step-by-Step Troubleshooting Guide
- Scan for Related Codes: Use a J1939 tool to check for SPN 94 or 102 which indicate fuel pressure issues.
- Inspect Sensor Connector: Look for bent pins, corrosion, or moisture at the fuel flow sensor plug.
- Measure Signal Voltage: With ignition on, probe signal wire for erratic fluctuations between 0.5–4.5V.
- Check Ground Integrity: Measure resistance between ECM ground pin and battery negative; should be below 0.1 ohms.
Have more questions about SPN 147 FMI 2?