Detroit Diesel · Injector Cylinder 5High

Detroit Diesel SPN 655 FMI 6: Injector Cylinder 5 Current Below Normal

SPN 655 FMI 6 (FMI 6) — Injector Cylinder 5 - Current Below Normal

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Quick Answer

SPN 655 FMI 6 means Detroit ECM measured below-normal actuator current on cylinder 5 injector circuit. Misfire, rough idle, and possible cylinder-disable derate can follow until injector resistance and harness integrity meet factory specifications.

Overview: Detroit Diesel SPN 655 FMI 6

Cylinder 5 low-current faults on DD platforms often start as oil leaks into the valve-cover harness, and ProfessionalDieselRepair.com resolves many Detroit SPN 655 cases with gasket and connector service instead of a full injector replacement.

Repair Snapshot

Time

2 - 5 Hours

Difficulty

Advanced

Parts Cost

$380 - $950

Dealer Labor

$780+

Tools Required

  • Detroit DDDL8
  • Noregon JPRO
  • Digital Multimeter
  • Injector Socket Set

What SPN 655 FMI 6 Means

Detroit Diesel SPN 655 FMI 6 signals cylinder 5 injector actuator current below normal on DD13/DD15 common-rail engines. High-resistance coils, oil-fouled harness branches, or weak ECM drivers reduce injection current and cause misfire, rough running, and possible cylinder-cutout derate until current returns to spec.

Symptoms Drivers Notice

  • Cylinder 5 misfire logged as SPN 655 FMI 6 in Detroit DDDL8
  • Rough idle with noticeable cab vibration at 600-800 RPM
  • Exhaust imbalance or faint white haze at idle
  • Cylinder 5 balance rate low in comparative cylinder data
  • Code may follow valve-cover gasket leak or harness disturbance

Most Likely Root Causes

Degraded cylinder 5 injector solenoid

Common

Coil resistance rises with heat and age, pulling actuation current below ECM thresholds.

Oil-contaminated valve-cover harness

Common

Oil increases connector resistance and drops current without fully opening the circuit.

Weak ECM injector driver on bank 2

Rare

Driver output loss mimics injector failure until swap testing proves otherwise.

Common Misdiagnoses (Don't Waste Parts)

Replacing all bank injectors for one FMI 6

Why it happens: Fleets bundle jobs to minimize labor windows.

Do this instead: Swap-test cylinder 5 first; parts-cannon bank replacements hide a harness repair under the cover.

Treating as mechanical FMI 7 failure

Why it happens: Misfire symptoms overlap between electrical and mechanical subclasses.

Do this instead: FMI 6 is low current — verify electrical path before assuming stuck needle mechanical failure.

Master Tech Commentary

Mechanic's Notes

ProfessionalDieselRepair.com logs cylinder 5 peak current in DDDL8 before quoting injectors on Detroit SPN 655 FMI 6. Oil wicking into the valve-cover harness drops current without killing the coil entirely — another parts-cannon injector mistake if you skip ohms and swap tests.

Step-by-Step Diagnostic Procedure

  1. Step 1: Run cylinder balance test in DDDL8; cylinder 5 contribution should match bank average within 5 percent at idle.
  2. Step 2: Measure injector resistance on cylinder 5 connector — expect roughly 0.8-1.2 ohms on many Detroit solenoids at 70°F; readings above 2.5 ohms fail spec.
  3. Step 3: Inspect cylinder 5 branch of valve-cover harness for oil saturation and green pin corrosion at the 2-pin injector connector.
  4. Step 4: Compare peak actuation current on cylinder 5 versus cylinder 6 during crank — below 8-10A peak indicates circuit loss.
  5. Step 5: Swap cylinder 5 and 6 injector connectors temporarily; if FMI 6 follows the connector, repair harness — if it stays on cyl 5, replace injector.

Professional service / OEM-level scan tools are recommended before clearing this fault for good.

Live Data & Freeze-Frame Tips

In Detroit DDDL8, graph cylinder 5 actuation current during idle and snap throttle events. Current pulses should match neighboring cylinders in amplitude; consistently lower peaks confirm FMI 6 focus.

Safety Warnings

  • Relieve high-pressure fuel before injector service.
  • Avoid skin contact with pressurized diesel.
  • Use torque specs on hold-down clamps to prevent gallery leaks.

When to Limp In vs Tow

Tow if cylinder 5 is cut out, idle is unstable enough to stall, or multiple injector faults trigger severe engine protection derates.

Prevention Tips for Fleets & Owner-Operators

  • Fix valve-cover oil leaks at first sign of smoke smell.
  • Clean and dielectric-grease injector connectors at PM.
  • Avoid prying harnesses during cover removal.

Related Fault Codes

SPN 654 FMI 5 — Injector Solenoid Driver Circuit - Cylinder #4

Cylinder 4 open or low current — shared harness routing in valve cover.

SPN 653 FMI 7 — Injector Cylinder 3

Cylinder 3 mechanical not responding — different FMI but similar misfire feel.

Frequently Asked Questions

What does Detroit SPN 655 FMI 6 mean?

Cylinder 5 injector actuator current measured below the ECM's normal range, indicating high resistance or weak driver output.

What resistance should cylinder 5 injector show?

Many Detroit common-rail injectors read roughly 0.8-1.2 ohms at room temperature; significantly higher indicates failure.

Can an oil leak cause low current?

Yes. Oil in the valve-cover harness increases resistance and is a frequent root cause on DD engines.

Will DDDL8 show cylinder balance?

Yes. Use cylinder balance and injector diagnostic tests to confirm cylinder 5 under-contribution.

Is driving safe with this code?

Limited local driving may be possible but continued operation risks aftertreatment damage and further derates.

Harness repair or injector replacement?

If swap testing moves the fault to another cylinder position, repair harness; if fault stays on cylinder 5, replace injector.

More Detroit Diesel codes on this system

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