When it comes to keeping your Duramax running strong, proper maintenance isn’t optional—it’s essential. One of the most overlooked but critical tasks is changing the coolant. Over time, coolant breaks down, loses its protective properties, and can lead to overheating, corrosion, or even catastrophic engine damage.

As with most routine maintenance tasks we always recommend consulting the owner’s manual for recommended intervals and then ensure that they are followed. Our workhorse of a 2015 Chevrolet Silverado 2500HD has lived a tough life, hauling trailers from coast-to-coast. GM suggests performing a full coolant swap at 5 years or 150,000 miles. While we missed the service at 5 years, having just crossed the 130,000-mile mark we figured it was past time to perform this simple service.

Follow along as we walk through the step-by-step process of draining and replacing the coolant in your Duramax, ensuring your truck stays cool under pressure and ready for whatever you throw its way.

Our 2015 Chevrolet Silverado 2500HD is fit with the company’s 6.6-liter LML Duramax diesel V8 engine. It has lived a tough life, hauling loads across the country. Recently, the truck found itself living the comfy country life, trading time on the road for leisurely ranch duty.

The first step in any automotive project is gathering supplies. In the case of our Duramax coolant swap this meant assembling seven gallons of premixed coolant, along with a catch can to capture the old fluid for recycling (spoiler: our bucket was far too small), along with any tools needed.

Duramax engines are designed to run using GM’s Dex-Cool, which is a blend of ethylene glycol-based coolant and specific corrosion inhibitors. For our truck we opted to use Peak’s Original Equipment Technology coolant formulated for North American vehicles. The company now also brands this coolant as its line of “GM-Approved ‘dexcool’” coolants. Regardless of the label, Peak’s orange-colored antifreeze and coolant comes in either pre-diluted (as we opted for) or full-strength gallons. This type of coolant utilizes organic acid technology (OAT) and is rated for a freeze point of -34 degrees, a boiling point of 265 degrees, and offers engine protection for up to 15 years and 400,000 miles.

To gain access to the radiator we first needed to lift the right front corner of the Silverado and remove the wheel and tire. Because Duramax radiators don’t have a specific drain petcock most of the work will be performed inside the wheel well. A level work surface is crucial to get a complete drain. Also be sure to support the truck with a properly rated jack stand before crawling under.

While many blogs and videos on the internet may suggest otherwise, we found the job to be so much easier with the inner fender liner removed. Pulling the liner is a simple task. Depending on the year of the Silverado or Sierra pickup, the inner fender is held in place with a collection of Phillips-head screws and/or push-in body clips.

With the fender liner out of the way you can easily locate the lower radiator hose. Using a small pic, pop the factory E-clip connector loose without damaging the hose. A flat blade screwdriver would also suffice (or basically any other flat implement); however, the process gets less elegant without the proper tool.

The E-clip is rather large but also can be quite delicate. It’s imperative that this part not get damaged during the removal process, or worse, lost. Either scenario will send you scrambling to the local dealer and it’s hit-or-miss whether they stock the part.

Once we got our thinking caps on straight and realized that our standard oil drain pan wasn’t going to be up for the task of catching the used fluid, we pitched it in favor of a 27-gallon tote. A 5-gallon bucket is the typical go-to for most DIYers, however, we’ve also heard of people using kiddie pools and everything in between.

Before removing the lower radiator hose it’s important to remove the coolant expansion tank cap. Doing this will allow the system to breath as the coolant drains instead of creating air bubbles that would otherwise block the flow of coolant out of the engine.

With the catch bucket properly aligned and expansion tank cap off the lower radiator hose can be removed. While less than ideal, this is the preferred method of draining the cooling system in Duramax-equipped pickups. Because of the volume of fluid in the system this step can take several minutes.

Depending on how much of the Duramax engine’s coolant you’re looking to remove, which in our case was all of it, you may need to remove the upper radiator hose and/or the thermostat housing to allow the remaining trapped coolant to flow out of the engine. If you want to get really nerdy, most Duramax blocks have coolant drain plugs as well, though the location of them varies by generation.

The refill process is relatively straightforward. As with its lack of a drain petcock, the Duramax radiator also has no traditional fill cap. With the upper and lower radiator hoses installed, coolant can be slowly poured back into the expansion tank. As the radiator refills, coolant will gradually reach the “full” line in the expansion tank. At this point the truck’s engine can be started and as it slowly comes up to temperature the engine and radiator will purge the remaining air and pull in additional coolant from the tank. For our 2015 LML Duramax a full system flush will require adding 21.6 quarts, or about 5.5 gallons, back into the system.

It’s not impossible that over the engine will continue to purge air bubbles and take in coolant during the course of several drive cycles. This is completely normal. If the truck’s “Low Coolant” warning light should come on simply top off the expansion tank once the engine has cooled (never remove the expansion tank cap while the engine is hot, doing so can result in extreme burns).

OAT vs. IAT: Which Is The Proper Duramax Coolant?

The coolant you choose matters more than most diesel owners think. Using the wrong type—or mixing incompatible coolants—can do serious damage to your cooling system. Let’s break down the differences between Organic Acid Technology (OAT) and Inorganic Acid Technology (IAT) coolants, why they’re not compatible, and how to tell them apart.

Organic Acid Technology (OAT) coolants are designed for modern engines like the Duramax diesel found in Chevy Silverado pickups. They use organic acid-based corrosion inhibitors, without the silicates or phosphates found in older formulations. They offer a longer lasting service life, up to 5 years or 150,000 miles, and are often labeled as Extended Life Coolant (ELC) or branded as Dex-Cool for GM vehicles. These can be either orange, red, purple, or dark green in color. OAT coolants provide superior long-term protection and are gentler on water pump seals and modern engine components.

Inorganic Acid Technology (IAT) coolants, on the other hand, are the old-school green stuff found in pre-2000 vehicles. These coolants rely on silicates and phosphates for corrosion protection and are designed for engines with more cast iron and copper components. They offer a shorter service life, typically about 2 years or 30,000 miles, and are often labeled as “conventional” coolant. These are typically bright neon green in color.

Mixing OAT and IAT coolants is a big no-go. The most serious issue is that the inhibitors in IAT coolant can react with those in OAT and neutralize each other. This reaction can lead to sludge, scale, or gel-like deposits in the cooling system. If left for an extended period this can result in clogged cooling passages, water pump damage, overheating, or even radiator failure.