2018 GMC Sierra 2500HD Coolant Capacity, Type & Flush Guide

The 2018 GMC Sierra 2500HD holds between 16.2 and 31.4 quarts of coolant, depending on whether your heavy-duty truck is equipped with the 6.0L gas V8 or the massive 6.6L Duramax diesel engine. As an ASE-certified master technician, I regularly service these heavy-duty haulers, and keeping their cooling systems in top shape is absolutely critical—especially if you are towing heavy loads or operating in extreme temperatures.

Below is everything you need to know about the coolant requirements, flush intervals, and proper bleeding procedures for the 2018 GMC Sierra 2500HD.

Quick answer

Here is the fast breakdown of what your 2018 GMC Sierra 2500HD cooling system requires:

• Total Cooling System Capacity: 16.2 quarts (15.3 L) to 31.4 quarts (29.7 L) depending on the engine.
• OEM Coolant Spec: DEX-COOL® (Orange in color). This is an OAT (Organic Acid Technology) coolant meeting General Motors specification GMW3420.
• Mix Ratio: 50/50 mixture of DEX-COOL concentrate and clean, drinkable water (distilled or deionized is highly preferred to prevent mineral buildup). You can also buy it pre-mixed.

Capacity by engine option

Because the 2018 Sierra 2500HD was offered with two vastly different engines, the coolant capacities reflect the different cooling demands of a gas engine versus a high-torque turbo-diesel.

Note: The 6.6L Duramax diesel capacity includes the primary cooling system, heater core, and the secondary cooling loops utilized by the L5P for charge-air cooling. Always buy a little extra to account for topping off after bleeding.

Flush interval

General Motors uses extended-life DEX-COOL, but "extended life" does not mean "lifetime."

• Normal Duty: Flush and replace the coolant every 5 years or 150,000 miles, whichever comes first.
• Severe Duty: If you frequently tow heavy trailers, idle for long periods, or drive in dusty, extreme-temperature environments, I highly recommend dropping the service interval to every 3 years or 75,000 miles to protect the water pump and prevent the coolant from turning acidic.

How to refill & bleed

Modern heavy-duty cooling systems, particularly on the 6.6L Duramax L5P, are complex and prone to trapping air. Air pockets can lead to severe localized overheating and component failure.

1. Drain the system: Make sure the engine is completely cold. Open the radiator petcock and drain the old coolant.
2. Vacuum Fill (Highly Recommended): For the L5P diesel especially, using a pneumatic vacuum coolant filler (Airlift tool) is the professional standard. It pulls the system into a deep vacuum, tests for leaks, and draws the new coolant in without introducing any air pockets.
3. Manual Fill & Bleed (If no vacuum tool is available):
   • Slowly pour the 50/50 DEX-COOL mixture into the coolant surge tank until it reaches the "COLD FILL" line.
   • Leave the surge tank cap off. Start the engine and let it idle.
   • Turn the cabin heater to the maximum heat setting and the blower to low. This ensures coolant is flowing through the heater core.
   • As the engine warms up and the thermostat opens, the coolant level will likely drop. Slowly add more coolant to keep it at the proper line.
   • Gently rev the engine to 2,000 RPMs for a few seconds to help force air bubbles out of the system.
   • Once the air stops bubbling out and the heat blows consistently hot in the cabin, install the surge tank cap tightly. Check the level again after the truck's next cold soak.

Common cooling system issues

For the 2018 generation of the Sierra 2500HD, keep an eye out for a few known cooling system quirks:

• Radiator End Tank Leaks: The crimps where the aluminum radiator core meets the plastic end tanks can weaken over time and start seeping orange coolant. Check the lower corners of the radiator during your oil changes.
• Water Pump Failures: While the L5P Duramax has a more robust water pump than the older LML generation, they can still experience bearing wear and weep-hole leaks as they approach the 100,000-mile mark.
• Surge Tank Cracks: The plastic coolant surge tank is subjected to extreme under-hood heat and pressure cycles. Over time, the plastic can become brittle, leading to hairline cracks or cap thread failures that allow pressure to escape.

Maintaining your coolant is cheap insurance for your heavy-duty truck. If you don't feel comfortable tackling a heavy-duty flush or using a vacuum-fill tool yourself, don't risk a blown head gasket from trapped air. Find a trusted shop on Repairius.