Thermostat Housing

Replace the thermostat housing assembly on the 5.9L Cummins diesel engine, including thermostat, housing gasket, and coolant temperature sensor if needed.

Warnings

Tools required

Parts

• Thermostat housing gasket × 1 — Use OEM specification
• Thermostat (180°F or 195°F) × 1 — Use OEM specification
• Coolant temperature sensor (if reusing, inspect O-ring) × 1 — Use OEM specification
• Upper radiator hose clamp × 1 — Constant-tension type recommended

Fluids

• HOAT coolant 50/50 — 3 qt

Preparation

1. Ensure engine is completely cold to the touch (wait at least 2 hours after last operation)
2. Park vehicle on level surface and engage parking brake
3. Place drain pan under lower radiator hose connection at radiator
4. Remove radiator cap slowly to release any residual pressure
5. Open petcock at bottom of radiator or disconnect lower radiator hose to drain approximately 2-3 gallons of coolant into drain pan

Procedure

1. 1

   Access thermostat housing
   Locate the thermostat housing at the front top of the cylinder head, where the upper radiator hose connects. Remove the engine cover if equipped by lifting upward on the front tabs and pulling forward. The housing is mounted to the front of the cylinder head. Verify housing material and orientation against the service manual before proceeding.
2. 2

   Disconnect upper radiator hose
   Use hose clamp pliers or screwdriver to loosen the clamp securing the upper radiator hose to the thermostat housing outlet. Slide the clamp back along the hose at least 2 inches. Twist the hose gently to break the seal, then pull it off the housing outlet. Allow any remaining coolant to drain into your drain pan. Inspect hose end for cracks or deterioration.
3. 3

   Remove coolant temperature sensor
   Disconnect the electrical connector from the coolant temperature sensor by pressing the release tab and pulling straight out. Use the correct size socket or wrench (verify sensor hex size against the service manual) to carefully remove the sensor. The sensor has an O-ring seal and may have coolant behind it, so have a rag ready. Inspect the sensor and O-ring for damage or corrosion.
4. 4

   Remove thermostat housing bolts
   Remove the bolts securing the thermostat housing to the cylinder head. Verify the exact bolt count and socket size against the service manual before removal. Note the bolt locations and lengths as they may vary. Support the housing as you remove the final bolt to prevent it from falling. Gently work the housing away from the block, twisting slightly if needed to break the gasket seal. The thermostat may remain in the housing recess or in the engine-side pocket; check both locations and remove it.
5. 5

   Clean mating surfaces
   Remove the old gasket material from both the engine block mating surface and the thermostat housing using a plastic gasket scraper. Be careful not to gouge the aluminum housing or block surface. Clean both surfaces thoroughly with brake cleaner and a shop rag. Use a wire brush gently if needed to remove stubborn residue. Ensure no debris falls into the coolant passages in the engine block.
6. 6

   Inspect and prepare new thermostat
   Remove the old thermostat from the housing and inspect the housing for cracks or damage. Verify the new thermostat is the correct temperature rating (stamped on the valve). Test the new thermostat by submerging in hot water to confirm it opens. Position the new thermostat in the housing with the spring side facing toward the engine, and orient the air bleed/jiggle valve per the service manual (typically at the top, 12 o'clock position).
7. 7

   Install new gasket and housing
   Place the new gasket onto the engine block mating surface, aligning all bolt holes. Position the thermostat housing with the thermostat installed onto the gasket, ensuring the thermostat air bleed is at the top. Start all housing bolts by hand to ensure proper thread engagement and alignment. The housing should sit flush against the gasket without gaps.
   Torque spec

   Coolant Outlet/Housing Bolts12 Nm (9 lb-ft)
8. 8

   Torque housing bolts in sequence
   Using a torque wrench, tighten the housing bolts in a crisscross pattern to the manufacturer-specified torque (verify value and bolt count against the service manual). Make a second pass in the same sequence to verify all bolts are properly torqued. Do not overtighten as the aluminum housing can crack.
   Torque spec

   Coolant Outlet/Housing Bolts12 Nm (9 lb-ft)
9. 9

   Install coolant temperature sensor
   Inspect the coolant temperature sensor O-ring. If reusing the sensor, replace the O-ring if damaged or hardened. Apply a thin coat of clean coolant to the O-ring. Thread the sensor into the housing by hand until snug, ensuring it does not cross-thread. Use the correct size socket and torque wrench to tighten the sensor to the manufacturer-specified torque (verify hex size and torque value against the service manual). Reconnect the electrical connector until it clicks into place.
   Torque spec

   Coolant Temperature Sensor20 Nm (15 lb-ft)
10. 10

    Reconnect upper radiator hose
    Slide the upper radiator hose fully onto the thermostat housing outlet until it bottoms against the housing flange. If using a new constant-tension clamp, position it approximately 1/2 inch from the end of the hose over the bead on the outlet. If reusing the original clamp, slide it into position and tighten to 3.0 Nm (2.0 lb-ft) using a torque wrench or tighten firmly by hand with a screwdriver.
    Torque spec

    Hose Clamps3 Nm (2 lb-ft)
11. 11

    Refill cooling system
    Close the radiator petcock or reconnect the lower radiator hose if it was removed. Using a funnel, slowly fill the cooling system with 50/50 HOAT coolant through the radiator cap opening. Fill until coolant reaches the bottom of the filler neck. Squeeze the upper radiator hose several times to help purge air bubbles. Add more coolant as needed. The thermostat air bleed will help release trapped air automatically.
12. 12

    Burp cooling system
    Leave the radiator cap off. Start the engine and let it idle while monitoring coolant level. As the engine warms and the thermostat opens (approximately 5-10 minutes), the coolant level will drop as air is purged. Add coolant as needed to maintain proper level. Watch for coolant circulation in the radiator and feel the upper hose for warmth indicating thermostat opening. Run engine until cooling fan cycles on, then turn off engine.

Reassembly

1. Install radiator cap once engine has cooled and coolant level is verified
2. Check coolant reservoir level and fill to COLD mark with 50/50 HOAT coolant
3. Reinstall engine cover if equipped by positioning rear tabs first, then pressing down on front tabs until they click
4. Wipe down any coolant residue from engine components and check all connections for leaks

Verification

• With engine cold, verify coolant level is at bottom of radiator filler neck and reservoir is at COLD mark
• Start engine and check for leaks around thermostat housing, temperature sensor, and upper radiator hose connection
• Allow engine to reach operating temperature and verify cooling fan cycles on normally
• Check that upper radiator hose becomes hot when thermostat opens, indicating proper coolant flow
• Monitor coolant temperature gauge to ensure it stays in normal operating range
• Recheck coolant level after engine cools completely and top off if necessary
• Test drive vehicle and monitor temperature gauge under load to confirm proper thermostat operation

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