Coolant Flush

Complete coolant system drain, flush, and refill procedure for the 5.9L Cummins diesel engine, removing old coolant and replacing with fresh 50/50 mixture.

Warnings

Tools required

Parts

• Block drain plug washer × 1 — Use OEM specification
• Radiator drain petcock (if damaged) × 1 — Use OEM specification

Fluids

• Ethylene glycol 50/50 (green conventional early, HOAT later) — 28 qt
• Distilled water (for flushing) — 10 qt

Preparation

1. Park vehicle on level surface and engage parking brake
2. Allow engine to cool completely (minimum 4 hours)
3. Raise front of vehicle and support securely on jack stands
4. Remove engine cover or air box if obstructing access to radiator cap
5. Place drain pan under radiator drain location

Procedure

1. 1

   Drain radiator coolant
   Slowly remove radiator cap by turning counterclockwise. Locate radiator drain petcock at the lower corner of the radiator (typically driver's side on these trucks; verify location on your radiator). Open petcock by turning counterclockwise and allow coolant to drain completely into drain pan. If equipped with block drain plug on driver side of engine block, remove it to drain block coolant.
   Torque spec

   Radiator Cap1 Nm (1 lb-ft)

   Drain Petcock5 Nm (4 lb-ft)

   Block Drain Plug25 Nm (18 lb-ft)
2. 2

   Disconnect overflow reservoir
   Locate coolant overflow reservoir on passenger side fender. Remove hose clamp securing hose to radiator using pliers or screwdriver. Disconnect hose and allow overflow reservoir to drain. Remove reservoir mounting bolts if complete draining is needed.
   Torque spec

   Coolant Hose Clamps4 Nm (3 lb-ft)
3. 3

   Inspect cooling system components
   With system drained, inspect all accessible hoses for cracks, swelling, or deterioration. Check radiator and heater core for external leaks or damage. Inspect water pump weep hole (located on bottom of pump) for coolant residue indicating seal failure. Replace any damaged components before proceeding.
4. 4

   Close drain points
   If block drain plug was removed, install with new washer and torque to specification. Close radiator drain petcock securely. Reconnect overflow reservoir hose and secure with hose clamp. Ensure all drain points are sealed before filling.
   Torque spec

   Block Drain Plug25 Nm (18 lb-ft)

   Drain Petcock5 Nm (4 lb-ft)

   Coolant Hose Clamps4 Nm (3 lb-ft)
5. 5

   Perform flush (if desired)
   For thorough flush, fill system with distilled water through radiator cap opening. Start engine and run until thermostat opens (approximately 180°F operating temperature). Allow to run 5 minutes, then shut off and allow to cool. Drain water completely using same procedure as step 1. Repeat if old coolant was heavily contaminated.
   ⚠Monitor temperature gauge during flush. Do not allow engine to overheat.
6. 6

   Locate and open bleed screw
   The 5.9L Cummins does not use a dedicated cooling system bleed screw; the system is self-bleeding when filled slowly through the radiator filler neck. Fill the system slowly at the highest point (radiator cap) to allow air to escape naturally during the filling process.
   Torque spec

   Bleed Screw7 Nm (5 lb-ft)
7. 7

   Fill cooling system
   Using funnel, slowly fill cooling system through radiator cap opening with fresh 50/50 coolant mixture. Fill slowly to allow air to escape naturally as the system is self-bleeding. System capacity is approximately 22-24 quarts total; verify against the factory service manual for your specific year. Pause periodically to allow coolant to settle and air pockets to burp out. Continue filling until the level stabilizes at the base of the filler neck.
   ℹ️Cummins diesel cooling systems are prone to air pockets. Take time to fill slowly and completely bleed system.
8. 8

   Close bleed screw and top off
   Continue filling slowly until the coolant level stabilizes and air stops burping from the radiator neck. Top off radiator to base of filler neck. Install radiator cap and turn until it locks fully.
   Torque spec

   Bleed Screw7 Nm (5 lb-ft)

   Radiator Cap1 Nm (1 lb-ft)
9. 9

   Fill overflow reservoir
   Fill overflow reservoir to cold fill line marked on side of tank. Do not overfill. This allows for coolant expansion during warm-up cycle.
10. 10

    Burp cooling system
    Start engine and allow to warm up to operating temperature with heater on high. Monitor temperature gauge closely. As thermostat opens, coolant level in radiator may drop. Add coolant as needed. Run engine for 15 minutes after reaching operating temperature. Rev engine to 1500-2000 RPM several times to help purge remaining air pockets.
    ⚠Watch for coolant leaks during warm-up. Shut off immediately if overheating occurs or leaks are detected.
11. 11

    Final check and top off
    Shut off engine and allow to cool for 30 minutes. Carefully remove radiator cap and check coolant level. Top off to base of filler neck if needed. Check overflow reservoir level and adjust to cold line. Reinstall radiator cap and any removed engine covers or air box components.

Reassembly

1. Reinstall engine cover or air box components if removed, ensuring clips and fasteners are secure
2. Lower vehicle from jack stands
3. Wipe down any coolant spills from engine bay and frame components

Verification

• Start engine and verify no coolant leaks at drain points, hoses, or connections
• Confirm heater produces hot air at idle and under load
• Monitor temperature gauge stays in normal range (190-210°F) during test drive
• After first drive cycle (50-100 miles), recheck coolant level when cold and top off if necessary
• Check overflow reservoir level remains at cold line when engine is cold
• Use coolant refractometer to verify 50/50 mixture provides proper freeze protection for your climate

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