If you've spent any time poking around under the hood of your truck lately, you've probably realized that an lmm duramax pcv reroute is one of those "must-do" modifications that owners talk about constantly. It isn't about adding a hundred horsepower or making the exhaust scream; it's about fixing a design flaw that, frankly, makes a total mess of a perfectly good engine. If you own a 2007.5 to 2010 Chevy or GMC with the LMM engine, you're dealing with a system that literally forces the engine to breathe in its own oily sweat. It's about as gross as it sounds, and over time, it can lead to some real headaches.
What is the PCV System Actually Doing?
To understand why everyone is obsessed with rerouting the PCV (Positive Crankcase Ventilation) system, you have to look at what it's doing from the factory. In a perfect world, the crankcase stays pressurized, and the PCV system vents those gases. On the LMM, GM decided the best place to put those gases—which are full of oil mist and moisture—was right back into the intake bridge, just before the turbocharger.
The idea was to burn those vapors off for emissions purposes. While that sounds fine on paper, the reality is that you're sending a constant stream of hot oil vapor through your turbo, into your intercooler, and eventually into your intake manifold. If you've ever taken your intercooler pipes off and seen a pool of black, nasty oil sitting at the bottom, that's exactly what we're talking about. It's not a sign that your turbo is blowing seals (usually); it's just the PCV system doing exactly what it was designed to do, even though it's hurting your truck in the long run.
Why You Should Care About the Oily Mess
You might be thinking, "So what? A little oil won't hurt anything." But it's not just a little oil. It's a constant coating that builds up over thousands of miles. This leads to a few specific problems that can actually cost you money and performance.
First, there's the intercooler. The intercooler's job is to drop the temperature of the air coming out of the turbo before it hits the engine. Air that's cooler is denser, which means more power and better efficiency. However, oil is a fantastic insulator. When the inside of your intercooler gets coated in a film of oil, it can't dissipate heat nearly as well. You're essentially losing the efficiency of your cooling system because of a thin layer of grease.
Then there's the issue of the boots. The rubber or silicone boots that hold your intercooler piping together hate oil. Over time, that oil mist soaks into the rubber, making it soft and slippery. If you've ever had a boot blow off while you're towing a trailer up a hill, you know how much of a nightmare that is. It sounds like a gunshot, and suddenly you have zero power. An lmm duramax pcv reroute fixes this by keeping those pipes dry, so the boots stay tight and the rubber doesn't degrade.
The Sludge Factor: PCV Meets EGR
If you haven't deleted your EGR system, the PCV oil is even more of a villain. The EGR (Exhaust Gas Recirculation) dumps dry, soot-filled exhaust back into the intake. When that dry soot meets the wet oil mist from the PCV system, they combine to create a thick, black sludge that looks like wet asphalt.
This gunk builds up on the intake valves and inside the intake manifold, eventually narrowing the passages where the air is supposed to flow. It's like trying to run a marathon while breathing through a straw filled with peanut butter. By rerouting the PCV, you're removing the "glue" that allows that soot to stick, which keeps your engine's internals significantly cleaner over the long haul.
How the Reroute Actually Works
Doing an lmm duramax pcv reroute is actually a pretty straightforward process. You're essentially bypassing the intake mouthpiece and venting those crankcase gases somewhere else—usually down toward the ground or into a catch can.
Most kits come with a few basic components. You'll get two fittings that replace the factory PCV boxes on top of the valve covers. You'll also get some heavy-duty hose and a plug for the hole where the PCV used to enter the turbo mouthpiece. You pull the old plastic lines out, pop the new fittings in, and run the hose down the side of the engine to the frame rail.
It's a "set it and forget it" kind of mod. Once it's done, the engine is breathing fresh, clean air from the air filter, and all that oily mist is being vented safely away from your expensive turbocharger and intercooler.
Is There a Downside?
I'd be lying if I said there were absolutely no trade-offs, though they're pretty minor in the grand scheme of things. The most obvious one is the "drip." Since you're venting oily vapor out of a hose under the truck, you might eventually see a small drop of oil on your driveway if you park in the same spot every day. It's usually not much—just a tiny bit of condensation—but it's something to be aware of.
The other thing is the smell. Every now and then, especially when you're stopped at a red light with the windows down, you might catch a whiff of those crankcase gases. It smells like hot oil. Most diesel guys don't mind it—it's just part of owning a truck—but if you're particularly sensitive to smells, you might notice it more than others.
There's also the legal side of things. Technically, modifying your PCV system is messing with emissions equipment. Depending on where you live and what kind of inspections your state requires, this could be a factor. Most people who do this mod consider it "preventative maintenance" for the longevity of the engine, but it's always worth checking your local regulations before you start tearing things apart.
Installation Tips and Tricks
If you're going to tackle the lmm duramax pcv reroute yourself, it's not a difficult job, but it can be a bit tight. The PCV boxes on the LMM are tucked away, and getting the factory clips off without breaking them can be a test of patience.
One thing I always tell people is to make sure you have a good set of picks and maybe some long-reach pliers. Also, when you're routing the hose down to the frame, make sure you stay away from the steering shaft and anything that gets excessively hot, like the exhaust manifold or the downpipe. You don't want your vent hose melting or getting tangled in moving parts.
While you're in there, a lot of guys also choose to do a "resonator delete." The LMM has a big plastic box on top of the engine that says "6.6L" on it. Its only real job is to quiet down the turbo whistle. Since you're already messing with the intake area for the PCV reroute, it's the perfect time to pop that resonator off and replace it with a solid plug. It cleans up the engine bay and lets you hear that turbo a bit more.
Final Thoughts on the Reroute
At the end of the day, an lmm duramax pcv reroute is one of those mods that just makes sense. You aren't doing it to win races or look cool at a truck show; you're doing it because you want your LMM to last as long as possible. Keeping oil out of your turbo and intercooler is just common sense if you plan on keeping the truck for the long haul.
It's a relatively cheap fix for a problem that can cause a lot of "death by a thousand cuts" style wear on your engine. Your intercooler will stay efficient, your boots will stay on, and your intake will stay free of that nasty sludge. If you can handle a little bit of an oil smell every now and then and a tiny drip on the driveway, the benefits to your engine's health are well worth the effort. It's a small price to pay for knowing your Duramax is breathing clean air.