Thermostat

This procedure covers removal and replacement of the thermostat on the 5.9L Cummins diesel engine, including coolant draining, housing removal, and refilling the cooling system.

Warnings

Tools required

Parts

• Thermostat (180°F or 195°F as specified) × 1 — Use OEM specification
• Thermostat housing gasket × 1 — Use OEM specification
• Coolant temperature sensor O-ring (if equipped) × 1 — Use OEM specification

Fluids

• HOAT coolant 50/50 — 2 qt

Preparation

1. Ensure engine is completely cold before starting work
2. Position drain pan under lower radiator hose connection at radiator
3. Remove radiator cap slowly to release any residual pressure
4. Locate thermostat housing on front driver side of engine block

Procedure

1. 1

   Drain coolant from system
   Loosen the lower radiator hose clamp at the radiator and carefully slide the hose off to drain coolant into the drain pan. Alternatively, use the petcock drain on the radiator if equipped. Drain approximately 2-3 gallons to bring level below thermostat housing.
2. 2

   Remove coolant temperature sensor (if necessary)
   If the coolant temperature sensor is installed in the thermostat housing, disconnect the electrical connector and carefully unscrew the sensor using a deep socket. Inspect the sensor O-ring and replace if damaged.
   Torque spec

   Coolant Temperature Sensor20 Nm (15 lb-ft)
3. 3

   Remove thermostat housing bolts
   Remove the two or three bolts securing the thermostat housing to the engine block. Note the bolt locations and lengths for reassembly. Carefully pull the housing away from the block.
   Torque spec

   Thermostat Housing Bolts15 Nm (11 lb-ft)
4. 4

   Remove old thermostat and gasket
   Remove the thermostat from the thermostat housing recess in the cylinder head, noting the orientation of the thermostat (sensing/spring element faces down into the engine). Remove and discard the old gasket. Note any alignment pins or guides.
5. 5

   Clean mating surfaces
   Use a scraper or gasket removal tool to thoroughly clean both the engine block and thermostat housing mating surfaces. Remove all traces of old gasket material and debris. Wipe surfaces clean with a shop towel. Ensure surfaces are completely dry and free of oil or coolant.
6. 6

   Install new thermostat
   Position the new thermostat into the recess in the cylinder head with the sensing/spring element facing down into the engine. The thermostat should sit flat in the recess. Verify the temperature rating matches the OEM specification (typically 180°F or 195°F for the 5.9L Cummins).
7. 7

   Install new gasket and housing
   Place the new gasket over the thermostat, aligning with any locating pins or guides. Position the thermostat housing over the gasket and align the bolt holes. Start all bolts by hand to ensure proper alignment before tightening.
8. 8

   Torque housing bolts
   Torque the thermostat housing bolts in a crisscross pattern to 24.0 Nm (18.0 lb-ft). Do not overtighten as this may crack the housing or distort the gasket seal.
   Torque spec

   Thermostat Housing Bolts15 Nm (11 lb-ft)
9. 9

   Reinstall coolant temperature sensor (if removed)
   Apply a small amount of thread sealant to the sensor threads. Install the coolant temperature sensor into the thermostat housing and torque to 20.0 Nm (15.0 lb-ft). Reconnect the electrical connector ensuring it clicks into place securely.
   Torque spec

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

    Reconnect radiator hose
    Slide the lower radiator hose back onto the radiator connection and position the clamp properly. Tighten the hose clamp securely. Ensure the hose is fully seated on the connection.
11. 11

    Refill cooling system
    Using a funnel, slowly add HOAT 50/50 coolant mixture through the radiator fill neck. Fill the radiator completely, then add coolant to the overflow reservoir to the FULL COLD mark. Add coolant to replace what was drained (approximately 2-3 gallons) or until the system is full. Start engine and allow to warm up while monitoring coolant level.
12. 12

    Bleed air from system
    With the radiator cap off, run the engine at idle until the thermostat opens (coolant will begin circulating and level may drop). Add more coolant as needed to maintain full level. Allow engine to reach operating temperature while monitoring for leaks around the thermostat housing. Turn heater to maximum heat to help purge air from the system.

Reassembly

1. After engine reaches operating temperature and thermostat opens, shut off engine and allow to cool
2. Recheck coolant level when cold and top off to FULL COLD mark on overflow reservoir
3. Install radiator cap securely

Verification

• Start engine and verify no coolant leaks from thermostat housing or sensor
• Confirm engine reaches normal operating temperature (approximately 180-195°F)
• Verify heater produces hot air when set to maximum temperature
• Check that upper radiator hose becomes hot after thermostat opens, confirming proper coolant circulation
• Recheck coolant level after first drive cycle and top off if necessary
• Monitor for any coolant loss or overheating during the first few days of operation

Related trouble codes on this vehicle

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

Chasing one of these codes and not sure which part is guilty? Ask a master mechanic about YOUR car →

More procedures for this vehicle