If you've noticed your temperature gauge acting a bit wonky or your truck is taking forever to warm up, it's likely time for a 2017 duramax thermostat replacement. Dealing with cooling issues is never fun, but on the L5P engine—which debuted in the 2017 model year—it's a job that most backyard mechanics can handle if they have a little patience and a few basic tools.
The 2017 Chevy and GMC 2500/3500 trucks were a big jump forward in power, but the cooling system still relies on those tried-and-true mechanical thermostats. When they fail, they usually stick open, which keeps your engine from reaching operating temperature. This might not sound like a huge deal, but it can tank your fuel economy and eventually trigger a P0128 trouble code. On the flip side, if they stick closed, you're looking at a potential overheat situation, which is a much more expensive headache.
Why the Duramax has two thermostats
One thing that catches a lot of folks off guard when they start looking into this is that the Duramax doesn't just have one thermostat; it has two. They sit right next to each other in the same housing, but they have different temperature ratings.
The front one usually opens at about 180°F, while the rear one opens at 185°F. The reason GM designed it this way is to allow for a more gradual flow of coolant. It helps the engine warm up more evenly and prevents that "thermal shock" you get when a massive amount of cold radiator fluid suddenly hits a hot engine block. When you go to buy your parts, make sure you get the kit that includes both. It's honestly not worth the effort of taking everything apart just to replace one and then having the other fail two months later.
Getting your tools together
Before you get your hands dirty, you'll want to make sure you have everything ready. There's nothing worse than having your truck torn apart and realizing you're missing a specific socket.
You're going to need a 10mm and 12mm socket, a few extensions, and probably a universal joint (swivel) for those hard-to-reach bolts. A flat-head screwdriver or a nut driver for the hose clamps is a must, too. Since you'll be opening up the cooling system, grab a clean drain pan and at least a few gallons of Pre-Mixed Dex-Cool. If you buy the concentrate, don't forget the distilled water—tap water is a no-go for these cooling systems because of the mineral buildup.
Tearing things down
First things first, make sure the engine is stone cold. Opening a hot cooling system is a recipe for a trip to the ER. Once things are cool, you'll want to drain some of the coolant. You don't necessarily have to drain the whole system, but you need the level to be below the thermostat housing so you don't end up with a lake of Dex-Cool on your driveway.
On the 2017 L5P, you're going to have to move the air intake out of the way to get a clear shot at the housing. Loosen the clamps on the intake tube and pop it off. You might also need to wiggle the wiring harness out of the way. It's a bit cramped in there, but compared to some other jobs on these trucks, it's actually not too bad.
Swapping the thermostats
Once you have access to the thermostat housing (located on the front of the engine, follow the upper radiator hose), you'll see the bolts holding the cover on. This is where those extensions and swivels come in handy. Be careful not to drop the bolts into the abyss of the engine bay—finding a dropped bolt on a 4WD Duramax can take longer than the actual repair.
Pop the housing cover off, and you might have to give it a little tap with a rubber mallet if the old gasket is stuck. You'll see the two thermostats sitting there. Take a quick mental note (or a photo with your phone) of how they are oriented. The "jiggle pins" or bleed valves on the thermostats should usually be positioned at the top to allow air to pass through.
Pull the old ones out and take a look at the mating surfaces. You want those to be perfectly clean. Use a plastic scraper or a scotch-brite pad to get any old gasket material off. If you leave even a tiny bit of junk on there, it's going to leak, and you'll be doing this whole job over again.
Drop the new thermostats in, making sure they're seated correctly. Put the new gasket in place—most quality kits come with a rubberized gasket that doesn't require extra RTV silicone. In fact, using too much silicone can actually gunk up the new thermostats, so it's usually better to just rely on the gasket itself.
Putting it all back together
Bolting the housing back on is straightforward, but don't over-tighten them. These are relatively small bolts going into aluminum, and it's surprisingly easy to strip the threads. Just get them snug.
Once the housing is on, reattach your hoses and the air intake. Double-check all your clamps. If you missed one, you'll know pretty quickly once the system gets up to pressure, and it's a mess you don't want to clean up.
The "burping" process
Now comes the part that requires the most patience: getting the air out of the system. Air pockets are the enemy of a 2017 duramax thermostat replacement. If there's an air bubble trapped near the thermostats, they won't open properly, and the truck will overheat even with brand-new parts.
Fill the surge tank up to the "Cold Fill" line. Leave the cap off for now and start the truck. Let it idle and keep an eye on that fluid level. As the engine warms up and the thermostats open, you'll see the level drop in the tank. Keep topping it off with Dex-Cool.
A pro tip here is to turn your heater on full blast. If you're getting hot air out of the vents, it's a good sign that coolant is flowing through the whole system. If the air stays cold even when the engine is warm, you've probably got an air lock. You might need to squeeze the upper radiator hose a few times to help "burp" the air out.
Once the truck is up to operating temperature and the level in the tank seems stable, put the cap back on and take it for a short drive. Keep a close eye on the temp gauge. When you get back, let it cool down completely and check the level one last time. It's very common to need a little more fluid after that first heat cycle.
Final thoughts
Replacing the thermostats on an L5P isn't exactly a fun Saturday morning, but it's a great way to save a few hundred bucks in labor costs at the dealership. It's one of those maintenance items that makes a huge difference in how the truck runs, especially when the weather gets cold.
Just remember to take your time, keep everything clean, and make sure you get all the air out of the lines. If you do that, your Duramax will stay at that perfect 185-190 degree mark for a long time to come. It's a satisfying feeling when that needle stays right where it should, and that "Check Engine" light finally disappears for good. Plus, having a heater that actually works in January is a pretty nice bonus.