Coolant Bypass Pipe

Replace the coolant bypass pipe on the 5.9L Cummins diesel engine, which routes coolant from the water pump to the thermostat housing and commonly fails due to corrosion.

Warnings

Tools required

Parts

• Coolant bypass pipe × 1 — OEM 3925353 or aftermarket equivalent
• Bypass pipe O-rings × 2 — Usually included with pipe
• Thermostat housing gasket × 1 — Use OEM specification
• Water pump gasket (if disturbed) × 1 — Use OEM specification
• Hose clamps (if damaged) × 2 — Worm-drive style, assorted sizes

Fluids

• Ethylene glycol 50/50 coolant — 4 qt

Preparation

1. Park vehicle on level ground and allow engine to cool completely (minimum 2 hours)
2. Disconnect negative battery cable
3. Remove engine appearance cover if equipped
4. Place drain pan under radiator
5. Verify replacement bypass pipe matches original configuration (angles and fittings)

Procedure

1. 1

   Drain cooling system
   Open the radiator petcock at the bottom of the radiator and drain coolant into a suitable container. If the system has a drain plug in the engine block, remove it as well to ensure complete drainage. Collect and properly dispose of old coolant according to local regulations.
   Torque spec

   Drain Petcock7 Nm (5 lb-ft)
2. 2

   Remove upper radiator hose
   Loosen the hose clamp at the thermostat housing connection and slide it back on the hose. Twist and pull the upper radiator hose off the thermostat housing. Be prepared for residual coolant to drain out.
   Torque spec

   Hose Clamps3 Nm (2 lb-ft)
3. 3

   Remove thermostat housing
   Remove the bolts securing the thermostat housing to the engine block; verify the exact number and locations against the service manual for the 5.9L Cummins. Carefully pull the housing away from the engine. The bypass pipe connects to the back of this housing. Note the orientation of the thermostat for reinstallation. Carefully pull the housing away from the engine. The bypass pipe connects to the back of this housing. Note the orientation of the thermostat for reinstallation.
   ⚠The thermostat housing is aluminum and can be easily damaged. Do not pry excessively if stuck; use penetrating oil and gentle tapping.
   Torque spec

   Thermostat Housing Bolt15 Nm (11 lb-ft)
4. 4

   Access coolant bypass pipe
   The bypass pipe runs from the water pump outlet to the thermostat housing, positioned behind the injection pump. Identify the pipe and trace both connection points. You may need to remove accessory brackets or move wiring harnesses aside for better access. Take photos for reference during reassembly.
   ⚠Work carefully around the injection pump and high-pressure fuel lines. Do not place tools or apply force to these components.
5. 5

   Disconnect bypass pipe from water pump
   The bypass pipe is secured to the water pump outlet with either a hose clamp or O-ring seal. If clamped, loosen the clamp and slide it back. Twist the pipe gently to break any corrosion seal, then pull it away from the water pump. If O-ring sealed, you may need to remove bolts securing the pipe flange.
   ⚠The pipe is often severely corroded and may break during removal. Be prepared for this and have extra parts available.
   Torque spec

   Hose Clamps3 Nm (2 lb-ft)
6. 6

   Remove old bypass pipe
   Once disconnected from both the water pump and thermostat housing locations, carefully maneuver the old bypass pipe out of the engine bay. It may need to be rotated or angled to clear surrounding components. Note the routing path for installing the new pipe.
   ⚠Sharp edges from corroded metal can cause cuts. Wear appropriate gloves during removal.
7. 7

   Clean mating surfaces
   Use a wire brush and shop rags to thoroughly clean the bypass pipe connection points on both the water pump and thermostat housing. Remove all old gasket material, corrosion, and debris. Inspect for damage or excessive corrosion that might prevent proper sealing.
   ℹ️If water pump or housing surfaces are heavily pitted, they may need replacement to ensure proper sealing.
8. 8

   Prepare new bypass pipe
   Unpack the new bypass pipe and verify it matches the original. Install new O-rings onto the pipe ends if equipped. Lightly lubricate O-rings with clean coolant to ease installation. Check that all mounting tabs and connection points align with the original configuration.
   ℹ️Some aftermarket pipes may have slightly different angles. Test fit before applying sealant or torquing fasteners.
9. 9

   Install new bypass pipe
   Carefully route the new bypass pipe into position, following the same path as the original. Connect it to the water pump outlet first, ensuring the O-ring or gasket is properly seated. Then align and connect the opposite end to the thermostat housing location. Secure with clamps or bolts as applicable.
   Torque spec

   Hose Clamps3 Nm (2 lb-ft)
10. 10

    Reinstall thermostat housing
    Install a new gasket on the thermostat housing mating surface. Position the thermostat correctly per the service manual orientation for the 5.9L Cummins, then install the housing over the bypass pipe connection. Thread in the housing bolts and tighten evenly to specification in a criss-cross pattern.
    Torque spec

    Thermostat Housing Bolt15 Nm (11 lb-ft)
11. 11

    Reconnect upper radiator hose
    Push the upper radiator hose firmly onto the thermostat housing outlet. Slide the hose clamp into position approximately 1/2 inch from the end of the hose and tighten to specification. Ensure the hose is fully seated and the clamp is positioned over the bead on the fitting.
    Torque spec

    Hose Clamps3 Nm (2 lb-ft)
12. 12

    Close drain points
    Close the radiator petcock by tightening it hand-tight. If you removed an engine block drain plug, reinstall it with a new sealing washer if applicable and torque to specification. Verify all drain points are closed before refilling.
    Torque spec

    Drain Petcock7 Nm (5 lb-ft)

    Coolant Drain Plug15 Nm (11 lb-ft)
13. 13

    Refill cooling system
    Fill the radiator slowly with the correct 50/50 mixture of the coolant type specified for your model year (conventional green for early models, HOAT/G-05 for later models) through the radiator cap opening. If equipped with a coolant bleed screw on the thermostat housing or water pump, open it while filling until coolant flows out without air bubbles, then close it to specification. Fill the overflow reservoir to the COLD mark. Run engine at idle with the radiator cap off until the thermostat opens and coolant begins to circulate. Add coolant as the level drops.
    ⚠Do not run the engine at high RPM before bleeding all air from the system. Air pockets can cause localized overheating.
    Torque spec

    Bleed Screw7 Nm (5 lb-ft)
14. 14

    Bleed air and pressure test
    With the engine at operating temperature, carefully install the radiator cap. Increase engine speed to 2000 RPM for 30 seconds, then return to idle. Monitor temperature gauge and check for leaks at all connections. Squeeze the upper radiator hose to help purge any remaining air bubbles. Shut off engine and allow to cool, then recheck coolant level.
    ⚠Do not remove radiator cap when engine is hot. Allow system to cool before rechecking coolant level.

Reassembly

1. Reinstall engine appearance cover if removed
2. Reconnect negative battery cable
3. Check coolant level again after the first drive cycle and top off if necessary
4. Inspect all connections for leaks during the first 100 miles of operation

Verification

• Start engine and verify no coolant leaks at bypass pipe connections or thermostat housing
• Confirm engine reaches normal operating temperature (190-210°F) without overheating
• Check that heater produces hot air, indicating proper coolant circulation
• Verify coolant overflow reservoir level is stable between COLD and HOT marks after several heat cycles
• Squeeze upper radiator hose when engine is warm to confirm it is firm and pressurized, indicating proper system operation

Related trouble codes on this vehicle

Codes that commonly send this job to the bay — tap one for symptoms, causes, and diagnostic steps.

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