SPN 94 FMI 2 — Fuel Delivery Pressure Data Erratic
Erratic delivery pressure needs sensor-versus-hydraulic classification. Chasing pumps for a bad supply pressure sensor and the opposite error when aeration is real are both documented patterns. The FSM separates sensor noise from true hydraulic instability.
Fault Overview
SPN 94 FMI 2 indicates low-side fuel pressure signal erratic or implausible. Failing sensor, aeration-induced real fluctuation, harness intermittents, failing lift pump, and restriction varying with load are the primary causes. Chasing pumps for a bad supply pressure sensor and the opposite error when aeration is real are documented patterns. Replacing the lift pump without sensor and gauge correlation is the most documented misdiagnosis pattern.
Engine & Platform Coverage
Symptoms
- •Supply pressure sensor intermittent
- •Fuel delivery pressure fluctuating
- •Low-side pressure erratic
- •Hard start from erratic supply pressure
- •Low rail under load from erratic supply
Possible Causes
- 1.Failing sensor — failing supply pressure sensor producing erratic readings
- 2.Aeration-induced real fluctuation — aeration producing real pressure fluctuation
- 3.Harness intermittents — harness intermittents producing erratic sensor signal
- 4.Failing lift pump — failing lift pump producing real pressure fluctuation
- 5.Restriction varying with load — restriction varying with load producing real pressure fluctuation
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
3 steps locked behind Technician Mode- 1Step 1 — Electrical Sensor Proof: Perform electrical sensor proof first when fluctuations are abrupt and unrelated to load. Verify reference voltage, ground, and signal circuit integrity at the sensor connector. Abrupt dropouts are electrical; aeration and restriction are usually load-correlated.
- 2Step 2 — Gauge Verification: Verify with a mechanical gauge. Compare the PID reading to a mechanical gauge installed at the same point. A PID that disagrees with the mechanical gauge confirms a sensor or circuit fault. A PID that agrees with the mechanical gauge confirms a hydraulic fault.
- 3Step 3 — Suction-Side Aeration and Pump Performance Tests: Perform suction-side aeration and pump performance tests if the gauge confirms real pressure fluctuation. Inspect suction-side fittings, filters, and lines for air ingestion. A failing lift pump producing real pressure fluctuation requires pump performance testing.
- 4Step 4 — Repair Based on Evidence: Repair based on confirmed evidence. Replace the sensor if sensor fault is confirmed. Repair the harness if intermittent is confirmed. Address suction-side aeration if aeration is confirmed. Replace the lift pump if pump failure is confirmed after performance testing.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Replace sensor if sensor fault is confirmed after circuit proof.
- 2Repair harness if intermittent is confirmed.
- 3Address suction-side aeration if aeration is confirmed.
- 4Replace lift pump if pump failure is confirmed after performance testing.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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Summary
Erratic delivery pressure needs sensor-versus-hydraulic classification. The AI Diesel Tech FSM separates sensor noise from true hydraulic instability.