Coolant Flush How-to: 2026 GMC Sierra 3500 HD AT4 6.6L V8 Turbo Diesel

Learn how to flush the coolant (antifreeze) in your 2026 GMC Sierra 3500 HD. Coolant / antifreeze circulates through Sierra 3500 HD radiator and engine to maintain the proper temperature. Coolant / antifreeze also passes through the heater core under your cowl to provide heat to the cabin of your Sierra 3500 HD. In addition to preventing your car from overheating, coolant / antifreeze also helps lubricate the seals in the water pump and hoses. Start your coolant flush by draining the coolant / antifreeze that is in your reservoir, then drain the coolant that is in your radiator by opening the petcock or removing the lower radiator hose. This video shows where the radiator drain plug (petcock) is located on a 2026 GMC Sierra 3500 HD. Fill up the radiator with distilled water and run the engine. Turn off the engine and drain the water, fill up the radiator with antifreeze until you have a 50/50 coolant / distilled water mixture and burp the system.

Like the oil in your Sierra 3500 HD, coolant gets dirty over time and loses its ability to cool down your 6.6 liter engine. When you flush your coolant, which is antifreeze, on your 2026, you are draining out all of the old dirty coolant and preventing costly radiator and cooling system repair down the road. The video above shows you where the petcock on your radiator is located, how to flush the coolant / antifreeze and how to burp the system to get the air out of your Sierra 3500 HD.

Coolant is toxic and animals are attracted to the sweet taste of coolant (antifreeze). If you spill coolant when you flush your Sierra 3500 HD, be sure to put down cat litter to soak it up to discard it. Flushing the coolant on a regular basis removes gunk and mineral deposits from your engine, while prolonging the life of your heater core, thermostat, hoses and water pump.