Fuel Injector Cleaning

This procedure covers cleaning fuel injectors on the 5.9L Cummins diesel engine using in-tank fuel system cleaner or manual cleaning methods to restore proper spray pattern and fuel delivery.

Warnings

Tools required

Parts

• Fuel injector copper crush washers × 6 — Cummins 3936020 or equivalent
• Fuel injector o-rings (if removing injectors) × 6 — Use OEM specification
• Diesel fuel system cleaner × 1 — Heavy-duty diesel injector cleaner rated for common rail systems
• Shop towels × 1 — Use OEM specification

Preparation

1. Park vehicle on level surface and engage parking brake
2. Disconnect negative battery cable, then positive battery cable
3. Allow engine to cool completely (minimum 2 hours) to reduce fuel system pressure
4. Clean engine bay around valve cover and fuel lines to prevent contamination
5. Have fire extinguisher readily available
6. Cover fender and engine accessories with protective covers to prevent fuel damage

Procedure

1. 1

   Relieve fuel system pressure
   Even with the engine off, residual pressure remains in the common rail system. Locate the fuel pressure relief valve on the fuel filter housing (driver side of engine). Place drain pan underneath. Relieve fuel system pressure per the factory service manual procedure for this common rail system. Do not attempt to loosen the rail pressure relief/limiting valve as a bleed screw — verify the correct depressurization method before proceeding. Allow fuel to drain until flow stops. Retighten relief valve.
2. 2

   Remove engine cover and access components
   Remove plastic engine cover by pulling upward on front clips and lifting off. Set aside. This provides access to valve cover and fuel injector hold-down hardware.
3. 3

   Disconnect fuel injector electrical connectors
   The injector electrical connections on this engine are made through the cylinder head cover; the individual injector solenoid connections are located beneath the valve cover on the injector bodies. Disconnect the main injector wiring connector at the valve cover before proceeding. Press down on the connector lock tab and pull connector straight up off each injector. Label connectors 1-6 (front to rear) using tape to ensure correct reinstallation order.
4. 4

   Decision point: Chemical or manual cleaning
   For chemical cleaning (less invasive): Add diesel fuel system cleaner to fuel tank per product instructions, reconnect battery, and run engine through recommended cleaning cycle. For manual cleaning (more thorough): Continue to next step to remove injectors. Manual cleaning is recommended if chemical treatment has not resolved performance issues or if injectors show external carbon buildup.
5. 5

   Remove valve cover (if performing manual cleaning)
   Disconnect any wiring harness connectors attached to the valve cover as needed (note: this engine uses an intake grid heater, not glow plugs). Remove valve cover bolts in reverse of tightening sequence (outside to inside pattern). Carefully lift valve cover off, being careful not to damage gasket. Inspect rocker arms and pushrods while cover is removed.
6. 6

   Disconnect high-pressure fuel lines
   Starting from the rear injector, use two wrenches to loosen fuel line connections at each injector. Hold injector fitting stationary with one wrench while loosening line nut with second wrench. Cap or plug open fuel lines immediately to prevent contamination. Repeat for all six injectors.
7. 7

   Remove fuel injector hold-down hardware
   Each injector is secured by a hold-down bolt or clamp. Remove the hold-down bolt from each injector using appropriate socket. Keep hardware organized by cylinder position. Note that injectors are NOT threaded into cylinder head.
8. 8

   Extract fuel injectors from cylinder head
   Injectors are held in place by o-rings and may be difficult to remove. Attach slide hammer-type fuel injector puller to injector body using appropriate adapter. Apply steady pulling force to extract injector. Do NOT pry or use excessive force as this can damage injector bore. If injector is severely stuck, apply penetrating oil around injector body and allow to soak for 30 minutes before attempting removal again.
9. 9

   Clean injector bores and inspect injectors
   Once removed, cover injector bores with clean shop towels to prevent debris entry. Use brass wire brush to clean carbon deposits from injector bores in cylinder head. Inspect each injector for cracks, damaged o-ring grooves, or worn tips. Check copper crush washer seating surfaces. Replace any damaged injectors rather than attempting to clean.
10. 10

    Clean fuel injectors externally and internally
    Use brass wire brush to remove external carbon deposits from injector bodies. For internal cleaning, connect injector to diesel fuel system cleaning kit following kit instructions. Do not attempt to bench-activate common rail injectors with a 12V supply. Internal cleaning of common rail injectors should be performed on proper injector test/cleaning equipment or by a qualified diesel injection shop. Observe spray pattern - should be fine, even cone. Repeat cleaning cycles until spray pattern is consistent. Alternatively, use ultrasonic cleaner with diesel-safe solution for 20-30 minutes.
11. 11

    Install new seals and prepare for installation
    Remove old o-rings from injector bodies using pick tool. Install new o-rings into grooves on injector body, lubricating lightly with clean diesel fuel. Install new copper crush washers on injector tips. Ensure all sealing surfaces are completely clean and free of debris.
12. 12

    Install fuel injectors into cylinder head
    Lubricate new o-rings with clean diesel fuel. Carefully insert each injector into its bore, ensuring proper alignment. Press injector fully into bore until seated - you should feel it bottom out against copper washer. Install hold-down hardware and tighten hold-down bolts. Torque the injector hold-down bolt to the manufacturer specification (verify exact value in the factory service manual for the 2007 5.9L common rail Cummins before final tightening). Do not guess — over-torquing can damage the injector or seat.
13. 13

    Reconnect high-pressure fuel lines
    Remove caps from fuel lines. Reconnect high-pressure fuel lines to each injector, starting from front cylinder. Use two-wrench method: hold injector fitting stationary while tightening line nut. Tighten the high-pressure fuel line nuts to the manufacturer-specified torque (verify exact value in the factory service manual before tightening). These are precision high-pressure fittings — do not use estimated values.
14. 14

    Prime fuel system and check for leaks
    Reconnect battery cables. Turn ignition to ON position without starting (this activates lift pump). Allow fuel pump to run for 30 seconds, then turn off. Repeat 3-4 times to prime system and purge air. Inspect all fuel line connections for leaks. Tighten any weeping connections before proceeding.

Reassembly

1. Reinstall valve cover with new gasket if removed, torquing bolts in crisscross pattern from center outward
2. Reconnect fuel injector electrical connectors in correct cylinder order (1-6, front to rear)
3. Reconnect glow plug harness connector
4. Reinstall engine cover, ensuring all clips are properly seated
5. Perform final inspection of all fuel connections and electrical connectors

Verification

• Start engine and allow to idle - listen for smooth operation without misfires or unusual noises
• Check for fuel leaks at all injector connections while engine is running - shut down immediately if any leaks detected
• Allow engine to reach operating temperature and verify no fuel odors or visible leaks
• Test drive vehicle and confirm improved throttle response and reduced smoke (if cleaning was performed due to performance issues)
• Monitor engine performance over next 50-100 miles for consistent improvement
• If chemical cleaning was performed, verify improved fuel economy and reduced exhaust smoke after running one full tank of treated fuel

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