HOW TO INSTALL 01-23 6.6 Duramax MODIFIED OEM Thermostats

01-23 6.6 Duramax MODIFIED OEM Thermostats

Tools You’ll Need

  • 8mm / 10mm / 13mm sockets (depending on year)

  • Ratchet and extension

  • Screwdrivers (flathead + Phillips)

  • Pliers

  • Drain pan for coolant

  • Gasket scraper / soft brush

  • New thermostat gasket or O-ring (usually included with thermostat)

  • Torque wrench (recommended)

Optional: coolant funnel, gloves, safety glasses

Important Notes Before Starting

  • Engine must be cool to avoid burns

  • Catch all coolant — do not dump on ground

  • These modified OEM thermostats are a direct replacement; installation is nearly identical to stock units

Step-by-Step Installation

1. Drain Coolant

  • Place a drain pan under the radiator

  • Open the drain valve at the bottom of the radiator

  • Drain enough coolant so the thermostat housing can be removed without spilling

2. Locate the Thermostat Housing

  • Driver’s side of the engine

  • Follow the upper radiator hose to the thermostat housing at the intake manifold

  • On some years, there may be two housings (coolant bypass + main thermostat) — install in the main housing

3. Remove the Thermostat Housing

  • Loosen and remove the bolts holding the housing

  • Carefully separate the housing from the engine

  • Some coolant may still spill — have a rag or pan ready

4. Remove the Old Thermostat

  • Pull the old thermostat straight out

  • Note its orientation — spring side goes into the engine

  • Remove old gasket or O-ring from housing and mating surface

5. Clean Surfaces

  • Scrape off old gasket material carefully

  • Clean mating surfaces with a soft brush or rag

  • Ensure no debris falls into the coolant passages

6. Install the Modified OEM Thermostat

  • Place thermostat in housing same orientation as old unit (spring into engine)

  • Install the new gasket or O-ring included with the thermostat

7. Reinstall the Housing

  • Place housing back over the thermostat

  • Insert bolts and hand-tighten first

  • Torque to manufacturer spec (varies by year, usually ~89 in-lbs / 10 Nm)

8. Refill Coolant

  • Close the radiator drain valve

  • Refill with coolant mixture (50/50 distilled water + Dex-Cool or OEM coolant)

  • Fill slowly to avoid air pockets

9. Bleed Air From System

  • Start engine and let it reach normal operating temperature

  • Open any bleed screws if your truck has them

  • Watch for air bubbles and top off coolant as needed

10. Check for Leaks

  • Inspect thermostat housing bolts and hoses

  • Run the engine until thermostat opens (upper radiator hose will get hot)

  • Make sure no leaks are present

11. Test Drive

  • Drive the truck for 10–15 minutes

  • Ensure engine reaches normal operating temperature

  • Check for leaks one more time

Tips / Common Notes

  • Modified OEM thermostats improve flow / reliability without aftermarket adapters

  • Use new gasket every time

  • Do not reuse old thermostat

  • Keep coolant caps tight after bleeding