SPN 679 FMI 3: Meaning and Fix
SPN 679 FMI 3 indicates the Engine Fuel Injection Control Pressure Regulator signal voltage is above the normal operating range, typically exceeding 4.8 VDC. This fault commonly appears after a technician accidentally shorts the regulator harness to a 24V source during battery jump-starting, causing immediate ECM detection of an overvoltage condition.
Common Symptoms
- Hard Starting: Engine cranks excessively without firing due to incorrect fuel rail pressure regulation.
- Reduced Power: ECM limits torque to 40% as a safety response to the overvoltage condition.
- Check Engine Light: Red malfunction indicator lamp illuminates on the dashboard within 2 seconds.
- Rough Idle: Intermittent misfire and unstable idle due to erratic regulator duty cycle.
Probable Causes
- Shorted Harness: Regulator signal wire chafed against 24V battery positive or starter cable.
- Failed Regulator: Internal short in the solenoid coil causing resistance below 2 ohms.
- ECM Damage: Internal driver short due to voltage spike from improper welding procedures.
- Connector Corrosion: Moisture ingress in the 2-pin connector creates a partial short to power.
Advanced Technical Analysis
The ECM microcontroller monitors the regulator feedback voltage via a 12-bit ADC with a reference of 5.0 VDC. When the sampled voltage exceeds 4.8 V for more than 100 ms, the diagnostic sets FMI 3. The ECM then disables the regulator driver and logs the event in permanent memory for post-failure analysis.
Electrical analysis reveals that a short to high typically occurs when the regulator signal wire contacts the 24V battery rail. The ECM’s internal clamping diode limits the voltage to 5.5 V, but sustained overvoltage can damage the driver MOSFET. Workshop oscilloscope traces show a flat line at 5.0 V during the fault.
Upon activation, the ECM executes a safety fallback: it sets the regulator duty cycle to 0%, causing fuel rail pressure to drop to base spring pressure (approximately 30 MPa). Torque is derated to 40% and vehicle speed is limited to 20 km/h to prevent engine damage from lean combustion.
Long-term strategy involves verifying the regulator resistance (3.5-5.0 ohms at 20°C) and checking for intermittent shorts using a megohmmeter. Technicians frequently encounter this fault after ECM replacement if the harness was not inspected; always perform a wiggle test on the engine harness before clearing the code.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check regulator connector and harness for chafing, burns, or moisture near the valve cover.
- Measure Voltage: With key on, engine off, measure signal pin voltage to ground. Should be below 0.5 V.
- Resistance Check: Disconnect regulator and measure coil resistance between pins. Acceptable range is 3.5-5.0 ohms.
- ECM Pin Test: Backprobe ECM connector pin for regulator signal. Voltage above 5.0 V indicates short to high.