2025 GMC Sierra 2500HD Coolant Capacity & Type

The 2025 GMC Sierra 2500HD holds approximately 18.7 quarts (17.7 liters) of coolant with the 6.6L gasoline engine and approximately 25.5 quarts (24.1 liters) with the 6.6L Duramax diesel engine.

Quick answer

6.6L L8T Gasoline Engine:

• Total Cooling System Capacity: 18.7 quarts (17.7 liters)
• OEM Coolant Spec: DEX-COOL (Orange/Reddish-Orange, OAT, GM Standard GM6277M)
• Mix Ratio: 50/50 Pre-mix or 50/50 concentrate with distilled water

6.6L L5P Duramax Diesel Engine:

• Total Cooling System Capacity: 25.5 quarts (24.1 liters)
• OEM Coolant Spec: DEX-COOL (Orange/Reddish-Orange, OAT, GM Standard GM6277M)
• Mix Ratio: 50/50 Pre-mix or 50/50 concentrate with distilled water

Capacity by engine option

Flush interval

For the 2025 GMC Sierra 2500HD, the recommended coolant flush interval under normal driving conditions is typically every 5 years or 150,000 miles, whichever comes first. However, if your vehicle operates under severe duty conditions (e.g., frequent towing, heavy hauling, extreme temperatures, dusty environments, or off-road use), it is advisable to inspect your coolant more frequently and consider a flush as early as 3 years or 45,000 miles.

Always refer to your owner's manual for the most accurate and up-to-date maintenance schedule for your specific vehicle configuration.

How to refill & bleed

Refilling and bleeding the cooling system on a 2025 GMC Sierra 2500HD requires careful attention to ensure all air is removed, which is critical for proper cooling system operation. While the exact procedure can vary slightly, here's a general guide:

1. Safety First: Ensure the engine is completely cool before attempting any work on the cooling system. Wear appropriate personal protective equipment, including gloves and eye protection.
2. Drain the System (if flushing): If you are performing a full flush, open the radiator drain petcock (usually located at the bottom of the radiator) and remove the radiator cap. Catch the old coolant in a suitable drain pan and dispose of it properly.
3. Flush (if needed): If flushing, close the drain and fill with distilled water, run the engine to operating temperature with the heater on high, then drain again. Repeat until the drained water is clear. You can use a cooling system flush product if desired, following its instructions carefully.
4. Close Drain and Refill: Close the radiator drain petcock. Begin refilling the system slowly with the recommended 50/50 DEX-COOL coolant mixture through the radiator fill neck. For the Duramax diesel engine, ensure the secondary cooling system (if applicable) is also filled.
5. Bleed the System: This is a crucial step to remove air pockets. Start the engine with the radiator cap off (or partially open coolant reservoir cap). Set the cabin heater to its highest temperature and fan speed. As the engine warms up, the thermostat will open, and coolant will begin to circulate. You may see air bubbles rise to the surface. It can be helpful to gently squeeze the upper and lower radiator hoses to dislodge air.
6. Check for Bleed Screws (if applicable): Some vehicles have coolant bleed screws on the thermostat housing, radiator, or heater hoses. If your Sierra 2500HD has them (consult your service manual), open them one at a time until a steady stream of coolant, free of air bubbles, emerges, then close them.
7. Maintain Fluid Level: Continuously monitor the coolant level in the radiator or expansion tank and top off as needed until it stabilizes. Once the engine reaches operating temperature and no more bubbles appear, install the radiator cap.
8. Final Check: Allow the engine to cool completely, then recheck the coolant level in the overflow reservoir and top off to the "FULL COLD" mark if necessary. Drive the vehicle and monitor the temperature gauge. Recheck the coolant level after the first few drives.

For the 6.6L Duramax, a vacuum fill tool is highly recommended for efficient and thorough air removal, especially given the complexity of the diesel engine's various cooling circuits.

Common cooling system issues

While the 2025 GMC Sierra 2500HD is a new model, based on previous generations and similar GM trucks with the L8T and L5P engines, common cooling system issues might include:

• Thermostat Failures: Thermostats can stick open (leading to slow warm-up or engine running cool) or closed (leading to overheating). This is a general wear item.
• Water Pump Leaks: The water pump is a mechanical component with seals that can eventually wear out, leading to coolant leaks. Bearings can also fail, causing noise or impeller issues.
• Radiator Leaks: Radiators can develop leaks over time due to corrosion, stone chips, or stress fractures in plastic end tanks (if applicable).
• Hose Leaks/Failures: Coolant hoses can degrade, crack, or soften with age and temperature cycling, leading to leaks or bursts.
• Coolant Reservoir Leaks: The plastic overflow reservoir can sometimes develop cracks, leading to slow coolant loss.
• Degraded Coolant: Neglecting coolant flushes can lead to the breakdown of additives, reducing corrosion protection and heat transfer efficiency, potentially causing blockages or premature component wear.
• Diesel-Specific Issues (Duramax): The Duramax engine, with its multiple cooling systems (engine, transmission cooler, intercooler, etc.), can be more prone to air pockets if not properly bled. Issues with auxiliary coolers or their lines could also arise.

Regular maintenance and using the specified DEX-COOL coolant are key to preventing these issues.

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