Swapping out an lly thermostat isn't exactly how I'd choose to spend a Saturday afternoon, but if your Duramax is starting to run hot or—even worse—won't warm up at all, it's one of those jobs you just can't ignore. The 2004.5 to 2005 LLY engines are legendary for a lot of reasons, but their cooling systems are definitely high on the list of things owners love to complain about.
If you've spent any time on the truck forums, you know the LLY has a bit of a reputation for overheating, especially when you're pulling a heavy trailer up a grade. While there are a bunch of factors that play into that, the thermostats are often the first line of defense. When they stop doing their job, things go south pretty quickly.
Why the LLY Uses Two Thermostats
It might seem like overkill, but the LLY actually uses a dual thermostat setup. You've got one sitting in the front and one in the rear of the housing. They aren't identical either, which is something that trips a lot of people up when they're ordering parts for the first time. Usually, one is set to open at 180 degrees Fahrenheit and the other kicks in at 185.
The idea behind this is to have a staged cooling response. The first one opens up to get some flow going, and the second one provides that extra burst of cooling capacity when the engine really starts working hard. It's a smart design on paper, but it means you have two points of failure instead of one. If the 180-degree stat gets stuck, you might notice your gauge hovering a little higher than usual. If the 185-degree one fails to open, you're looking at a serious overheat situation the next time you hit the highway.
Knowing When Yours Are Shot
Sometimes it's obvious when an lly thermostat has bit the dust, but other times it's a slow crawl toward failure. The most annoying scenario is when they get stuck open. You'll be driving down the road in the middle of winter, and your heater feels like it's blowing lukewarm air at best. Your engine temperature gauge might never even reach the quarter mark. Not only does this make the cab miserable, but it also hurts your fuel economy because the engine never leaves "warm-up" mode.
On the flip side, getting stuck closed is the scary version. You're cruising along, and suddenly that needle starts climbing toward the red. If you see your temps spiking and then suddenly dropping, that's a classic sign of a thermostat that's "hunting" or sticking before finally snapping open.
I've also seen cases where the rubber seal on the thermostat itself starts to disintegrate. When that happens, little bits of rubber can actually prevent the valve from seating properly. It's a mess, and it's usually a sign that the coolant hasn't been flushed in way too long.
Choosing Your Replacements
When it comes to buying new ones, you've basically got two paths: OEM AC Delco or something like a Mishimoto high-flow set. Most of the time, I'd tell you to stick with OEM. The AC Delco stats are built for this engine and they generally last a long time. However, because the LLY is so prone to heat issues, a lot of guys swear by the lower-temperature thermostats.
You can find sets that open at 174 degrees instead of the factory 180/185. The logic is that by getting the coolant moving earlier, you have more of a "buffer" before the engine gets dangerously hot. If you live in a place like Arizona or spend your summers towing a 5th wheel through the mountains, the lower-temp lly thermostat might actually be a solid upgrade. But if you're up in the north where it hits sub-zero temps, you might find that your truck takes forever to get the cabin warm in the morning.
Tips for the Installation
Actually changing them out isn't the hardest job in the world, but it is a bit of a "skinny-arm" task. The thermostat housing sits right at the top front of the engine, but there's a lot of stuff in the way. You'll likely find yourself leaning over the grille for an hour, so a fender cover or an old blanket is a must unless you want your belt buckle to scratch the paint.
One thing you absolutely have to watch out for is the bolts. These things have been heat-cycled thousands of times, and they love to seize up. I usually hit them with some penetrating oil the night before I plan to do the work. If you snap one of those bolts off in the lower housing, you're going to have a very bad day involving drill bits and extractors.
When you pull the old ones out, take a second to look at the orientation. You want to make sure the new ones go in exactly the same way. Also, don't be tempted to reuse the old gaskets. Most new lly thermostat sets come with the rubber seals, but double-check that before you start. There's nothing worse than getting it all buttoned up only to see a steady drip-drip-drip on the driveway.
The Air Pocket Headache
The biggest "gotcha" with the LLY cooling system is air pockets. After you replace the thermostats and top off the coolant, you aren't done. These engines are notorious for trapping air in the heater core or the upper part of the block.
If you just fill it up and drive away, you might see the temp gauge spike almost immediately because there's a big bubble of air sitting right at the lly thermostat housing, preventing the stats from sensing the actual water temperature.
The trick is to use the bleeder screw on top of the thermostat housing. You'll want to crack that open while the engine is running (and after it's reached operating temp) to let the air hiss out. Just be careful—that coolant is hot, and it'll spray. I usually use a long screwdriver and stay back a bit. Once you get a steady stream of liquid with no bubbles, you're usually good to go. It's also a smart move to keep a gallon of 50/50 mix in the truck for the next couple of days, as the level in the overflow tank will probably drop as the last few bubbles work their way out.
While You're in There
Since you're already messing with the cooling system, it's a good time to take a look at the rest of the "stack." The LLY has a tightly packed group of coolers—the radiator, the intercooler, the AC condenser, and the oil cooler are all sandwiched together. Over time, dirt, leaves, and "road grime" get stuck between them.
If your new lly thermostat doesn't seem to be fixing your high temps, it's probably because your radiator is choked with junk. I like to take a garden hose (not a pressure washer, you'll bend the fins!) and spray out the stack from the back side. You'd be surprised at the amount of mud that comes out of a truck that looks clean from the outside.
Final Thoughts
It's easy to forget about the thermostats until they cause a problem, but on an LLY, they're pretty much a maintenance item. If you've got over 100k miles on your current set, or if you've just bought a used truck and don't know the history, it's cheap insurance to just swap them out.
It's one of those relatively inexpensive fixes that can save you from a multi-thousand-dollar head gasket job down the road. Just take your time, don't force the bolts, and make sure you bleed the air out properly. Your Duramax will definitely thank you for it the next time you're pulling a heavy load up a steep hill.