Installing a pcv reroute lly kit is one of those small modifications that makes a massive difference in how clean your Duramax engine stays over the long haul. If you've ever popped off your intake resonator or looked inside your turbo mouthpiece and seen a thick, black film of oil, you know exactly what I'm talking about. It's a common sight on the 2004.5 and 2005 LLY engines, but just because it's common doesn't mean it's something you should ignore.
The factory Positive Crankcase Ventilation (PCV) system is designed to take the pressure and oily vapors from the crankcase and dump them right back into the intake. From an emissions standpoint, it makes sense because the engine just "re-burns" those vapors. However, from a performance and longevity standpoint, it's a bit of a headache. Over time, that oily mist coats your turbo blades, gunk's up your intercooler, and makes your rubber boots soft and prone to blowing off under boost.
Why the stock setup is kind of a mess
To understand why everyone talks about doing a pcv reroute lly, you have to look at what's happening inside the engine when it's under load. As the pistons move, a bit of pressure escapes past the rings into the crankcase. This pressure carries a fine mist of engine oil with it. In the stock configuration, GM routed this directly into the turbo inlet.
The problem is that the LLY already runs a bit hot, and adding hot oil vapor into the intake air doesn't help. When that oil hits the cool surfaces of your intercooler, it condenses. It starts to pool in the bottom of the intercooler, which reduces its efficiency. Even worse, that oil film gets onto the boots that connect your intercooler piping. Once those boots get soaked in oil, they lose their grip. You'll be driving down the road, hit a decent amount of boost, and—pop—a boot flies off, leaving you dead in the water until you can crawl under there and tighten it back down. It's a mess that nobody wants to deal with on the side of the highway.
What a pcv reroute lly actually does for your truck
The concept of the reroute is incredibly simple, which is probably why it's so popular. Instead of letting those oily vapors go into your turbo, you reroute them to the atmosphere. You basically plug the holes in the intake and run a hose from the valve covers down toward the ground, usually ending somewhere near the back of the engine or the frame rail.
By doing this, you're ensuring that only clean, fresh air is entering your turbocharger. This keeps the compressor wheel clean, which is great for maintaining balance and efficiency. It also means your intercooler stays dry and your intake manifold doesn't end up filled with a sludge-like mixture of oil and soot (especially if you still have an active EGR system). It's essentially "de-cluttering" the air your engine breathes.
Why LLY owners specifically should care
The LLY is a fantastic engine, but it's well-known for having a few "quirks" related to heat and airflow. Most LLY owners eventually upgrade their turbo mouthpiece because the stock one is incredibly restrictive. When you're in there swapping the mouthpiece, it is the absolute perfect time to do the pcv reroute lly.
Since the LLY is getting older now, those factory PCV hoses are likely becoming brittle. They can crack, leak, or just become a general eyesore. Replacing them with a reroute kit freshens up the engine bay and removes several potential leak points. Plus, since the LLY turbo is a variable vane design (VGT), keeping the internals as clean as possible is always a good idea. You don't want extra carbon or oil residue gumming up the works if you can help it.
Talking about the installation process
Installing a pcv reroute lly isn't a job that requires a master mechanic certification. It's mostly about reaching some tight spots near the back of the valve covers. You'll usually get two fittings that replace the factory PCV boxes (or work with them, depending on the kit), some heater hose, and a plug for the intake mouthpiece.
The hardest part is usually just getting the old hoses off. They've been heat-soaked for nearly twenty years, so they might be a bit stubborn. Once you get the new fittings in place, you just T them together and run a single hose down the side of the engine. You want to make sure the hose has a nice, downward slope so that any liquid oil can drain out and doesn't create a "trap" where oil can pool and block the airflow. If the hose gets blocked, pressure can build up in the crankcase, and that's when you start blowing out gaskets—which is exactly what we're trying to avoid.
To catch can or not to catch can?
There is a bit of a debate in the Duramax community about whether you should just vent the hose to the ground or use a catch can. If you vent to the ground, you might notice a little "haze" coming from under the truck when you're sitting at a red light, and you might see a small drop of oil on your driveway every now and then. It's not a lot, but for some people, it's annoying.
A catch can sits in the middle of the line. It catches the oil mist and lets the air escape. This keeps your driveway clean, but it adds another maintenance item because you have to remember to empty the can every few months. For most guys, a straight pcv reroute lly to the atmosphere is the go-to because it's "set it and forget it." If you're worried about the environment or your pristine concrete garage floor, the catch can is a solid middle ground.
Common mistakes and things to watch out for
One thing I see people do wrong is using the wrong kind of hose. You can't just use any old garden hose or cheap plastic tubing. It needs to be oil-resistant and able to handle the heat of the engine bay. Most kits come with high-quality reinforced hose, which is what you want.
Another tip is to make sure you zip-tie the hose away from any moving parts or hot exhaust components. You don't want your vent line melting onto your manifold or getting caught in the steering shaft. It sounds like common sense, but when you're cramped up in the engine bay, it's easy to overlook where that hose is dangling.
Also, be mindful of where the hose ends. You want it far enough back that the oily smell doesn't get sucked into your HVAC vents when you're stopped. Most guys run it down past the transmission bellhousing, which usually keeps the smell and the mist away from the cab.
Is it worth the effort?
At the end of the day, a pcv reroute lly is one of those "peace of mind" mods. It's relatively inexpensive, it's not that hard to do, and it solves a problem that otherwise just gets worse with time. If you plan on keeping your truck for a long time, keeping the "veins" of the engine clean is worth every penny.
It's not going to give you a massive boost in horsepower or make the truck sound like a race car, but it will keep your turbo efficient and your intercooler boots where they belong. In my book, anything that makes a truck more reliable and reduces the amount of greasy sludge I have to clean off during my next service is a win. If you're looking for a weekend project that actually does something productive for your LLY, this is definitely the one to pick.