What are the symptoms of SPN 655 FMI 7?

Rough engine idle or misfiring on cylinder 5. Loss of power or reduced engine performance. Excessive smoke from exhaust (black or white). Fuel dilution in engine oil. Hard start or extended crank time. Engine knock or unusual combustion noise from cylinder 5. Failed active cylinder cutout test on cylinder 5

What causes SPN 655 FMI 7?

Stuck open or stuck closed injector on cylinder 5. Carbon buildup causing injector needle to stick mechanically. Injector return/leak-off circuit failure causing excessive back-leak. Low fuel rail pressure preventing proper injector actuation. Worn or damaged injector tip or spray holes causing poor atomization. Injector o-ring or copper washer failure causing seat leak. Contaminated fuel causing injector internal wear or seizure. Failed injector solenoid or piezo element (mechanical non-response). High-pressure fuel pump wear reducing adequate rail pressure to injector

How do I diagnose SPN 655 FMI 7?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 655 FMI 7 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 655 FMI 7Fuel SystemHigh

Injector Cylinder 5 - Mechanical System Not Responding

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Overview

Fault Code

SPN 655 / FMI 7

Common Symptoms

Rough engine idle or misfiring on cylinder 5
Loss of power or reduced engine performance
Excessive smoke from exhaust (black or white)
Fuel dilution in engine oil
Hard start or extended crank time
Engine knock or unusual combustion noise from cylinder 5
Failed active cylinder cutout test on cylinder 5

Root Causes

Stuck open or stuck closed injector on cylinder 5
Carbon buildup causing injector needle to stick mechanically
Injector return/leak-off circuit failure causing excessive back-leak
Low fuel rail pressure preventing proper injector actuation
Worn or damaged injector tip or spray holes causing poor atomization
Injector o-ring or copper washer failure causing seat leak
Contaminated fuel causing injector internal wear or seizure
Failed injector solenoid or piezo element (mechanical non-response)
High-pressure fuel pump wear reducing adequate rail pressure to injector

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

AI walks you through every test — one step at a time. Plans start at $12.95/mo.

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Derate Behavior

Power Derate

Yes

Mileage / Regen Notes

DPF regeneration may be inhibited or interrupted due to cylinder 5 misfire or poor combustion contributing to unburned hydrocarbons and unreliable exhaust temperature; active regen may be blocked until fault is resolved.

System Interactions

SPN 655 FMI 7 interacts with the following systems and may trigger cascading faults if not resolved promptly.

Fuel rail pressure — injector timing and quantity affected

Engine power output — derate may be triggered to protect injectors

Aftertreatment regen — low fuel pressure can prevent regen completion

TSB Reference: Injector TSBs

Common Diagnostic Mistakes

Replacing the sensor without verifying wiring continuity and supply voltage first.

Clearing codes without identifying and resolving the root cause — the fault will return.

Ignoring related active faults — cascading faults often share a root cause.

Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.

Condemning the ECM before exhausting all wiring and component tests.

Replacing injectors without verifying fuel supply pressure — low supply pressure causes injector faults.

Ignoring fuel filter restriction — a clogged filter causes low rail pressure that mimics injector failure.

Related Faults in This Category

SPN 157-18Fuel Rail Pressure Low SPN 94-1Fuel Delivery Pressure Low SPN 1189-2Fuel Pressure Regulator Erratic SPN 174-0Fuel Temperature High

OEM System Resources

Cummins Fault Codes Detroit Diesel Fault Codes PACCAR Fault Codes Volvo Fault Codes Freightliner Fault Codes Mack Fault Codes

ASE Training Resources

Why ASE Certification Matters How ASE Tests Work ASE Study Tips ASE vs AI Diesel Tech Certification Paths Get Started with ASE

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Authority References

Diagnostic procedures on this page are informed by OEM service literature and industry standards.

Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.

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