Look, I’ll shoot it to you straight. I run a 2,000-acre corn and soybean operation right here in the Midwest, and out here, a truck isn’t a grocery getter—it’s a piece of farm equipment. My rig is a 2019 Chevrolet Silverado 3500HD dually with the 6.6L L5P Duramax V8. It hauls 20,000-pound gooseneck flatbeds loaded with skid steers, pulls massive grain wagons out of muddy fields, and does it all in 100-degree summer heat and sub-zero winter freezes.
If you’ve been around modern diesel trucks for more than five minutes, you already know the drill. The factory emissions equipment on these trucks—specifically the DPF (Diesel Particulate Filter) and the DEF (Diesel Exhaust Fluid) systems—are absolute chokeholds on the engine’s reliability. We all know the drill: the exhaust falls off in the barn, we plug in an EZ Lynk or HP Tuners, flash a custom tune, and suddenly the truck can breathe out the tailpipe.
But here is the dirty little secret that a lot of guys, especially new L5P owners, tend to forget: Fixing the exhaust is only half the battle. You’ve still got a massive problem under the hood, and it’s choking your engine from the intake side.
I’m talking about the EGR—the Exhaust Gas Recirculation system.
If you hang around Reddit’s r/Diesel or the Duramax forums, you’ll see the same debate raging every single week: “Do I just slap some cheap block-off plates on my L5P EGR, or do I need to spend the money and bust my knuckles doing a full EGR delete kit?”
Grab a cup of black coffee, pull up a bucket in the shop, and let’s talk diesel mechanics. I’m going to break down exactly what this system is doing to your General Motors L5P, the pros and cons of both methods, and what you actually need to do if you want your truck to survive past 300,000 miles.
The Enemy Within: What the Hell is the EGR Actually Doing?
Before we talk about tearing parts off your engine, you need to understand what the engineers at GM (under heavy pressure from the EPA) actually built.
EGR stands for Exhaust Gas Recirculation. When your 6.6L Duramax burns diesel fuel, it creates nitrogen oxides (NOx) due to the extreme heat of combustion. To lower these NOx emissions, the factory bolts a valve and a massive metal cooler to the passenger side of your engine block.
This system literally takes hot, soot-filled exhaust gas straight from your passenger-side exhaust up-pipe, routes it through a cooler (which uses your engine’s vital coolant to drop the temperature of the gas), and dumps that dirty, abrasive exhaust right back into your fresh air intake manifold.
Think about that for a second. Imagine you are running a marathon, pulling a heavy plow behind you. Now imagine someone taping a hose from your backside straight into your mouth. That is exactly what your engine is doing. It’s choking on its own exhaust.
The “Black Sludge” Death Sentence
It gets worse. Your L5P also has a PCV (Positive Crankcase Ventilation) system that routes oily vapors from the crankcase back into the intake. When you mix the dry, powdery black carbon soot from the EGR with the wet, sticky oil vapor from the PCV, you create something unholy.
We call it “diesel sludge.” It looks like thick, black, gritty asphalt.
Over tens of thousands of miles, this sludge cakes the inside of your intake manifold. It coats your intake valves, restricting airflow, increasing your EGTs (Exhaust Gas Temperatures), and eventually, it can cause the valves to stick or not seat properly. When you are pulling a 15,000-pound fifth-wheel up a 6% grade in the Rocky Mountains, you need cold, dense, clean oxygen. The EGR gives your turbocharger hot, dirty, oxygen-depleted trash.
So, you’ve decided it has to go. You’ve got the custom tunes ready to turn off the check engine lights. Now you are standing at the workbench looking at two very different hardware options.
Option A: The EGR Block-Off Plates (The Quick, Cheap Bandaid)
Let’s start with the route that most guys on a budget take first. The EGR Block-off Plate kit. If you search Google for “L5P EGR delete,” this is usually the first thing that pops up for under fifty bucks.
What is it?
This is exactly what it sounds like. Instead of removing the massive, heavy EGR cooler assembly from the engine valley, you simply trap the system. You unplug the electrical harness from the EGR valve so it stops opening. Then, you unbolt the flange where the exhaust up-pipe meets the cooler, and you slide a laser-cut, stainless steel “blind plate” (a solid piece of metal with no hole in it) between the flanges and bolt it back down tightly. You block the exhaust from entering the cooler, and usually, you put another plate on the intake side.
The Pros of Block-Off Plates:
- ✓ The Price Tag: You can buy a set of high-quality stainless steel plates for the cost of a steak dinner. We are talking $30 to $50. Compared to a full kit, it’s pennies.
- ✓ Ease of Installation: I can put a truck in my shop, pop the hood, and have block-off plates installed in about 45 minutes to an hour. You don’t need to drain the engine coolant. You don’t need to rip apart the whole passenger side of the engine bay. It’s a fast, dirty, easy DIY project for a Friday night.
- ✓ The “Stealth” Look: A lot of guys like this method because when you pop the hood, the engine still looks 100% factory stock. The massive EGR cooler is still sitting right there. Unless someone knows exactly where to look for those thin metal plates, it looks like an emissions-compliant truck.
The Critical Flaws (Why I Don’t Like It):
If I’m being honest, I view block-off plates as a temporary band-aid, not a permanent mechanical fix. Here is the massive glaring issue: The EGR cooler is still bolted to your engine block, and it is still full of engine coolant.
The factory EGR cooler is essentially a miniature radiator. It has tiny, fragile fins inside it. On the older LML Duramax engines, these coolers were notorious for rupturing. While GM did make the L5P cooler much more robust, it is still a weak point.
When you block off the exhaust flow but leave the cooler full of hot, pressurized engine coolant, the cooler undergoes thermal cycling. Over time, the internal welds can weaken and crack. If that cooler ruptures internally, you now have a massive internal engine coolant leak. Best case scenario, your truck starts drinking coolant and blowing white smoke. Worst case scenario, it dumps coolant straight into the cylinders, hydro-locking your $15,000 Duramax engine and destroying the rods.
Leaving a useless, dormant cooler full of fluid bolted to your engine block is like leaving a ticking time bomb in your basement. Eventually, it’s going to cause a headache.
Option B: The Full EGR Delete Kit (The Surgeon’s Approach)
This is the path I take on all my farm trucks. It’s the path any reputable diesel mechanic will tell you to take if you plan on handing the keys to this truck down to your kids one day. The Full EGR Delete.
What is it?
A full delete kit (often from reputable aftermarket diesel fabricators) doesn’t just block the flow; it completely removes the cancer from the patient. You are ripping out the EGR valve, the massive soot-filled cooler, the crossover tubes, and all the factory brackets. In its place, the kit provides:
- A Pass-Through Plate/Up-pipe fix: To permanently seal the exhaust manifold.
- A High-Flow Intake Horn: This is the crown jewel. You replace the restrictive, sludge-filled factory cast intake pipe with a smooth, mandrel-bent aluminum tube or a CNC-machined billet aluminum block.
- Coolant Reroute Hose: Because you removed the cooler, you have an open coolant circuit. The kit provides a high-grade 5/8″ silicone bypass hose to route the engine coolant seamlessly back into the heater core or water pump, keeping the fluid flowing perfectly without the risk of a cooler rupture.
The Pros of a Full Delete:
- ✓ Total Peace of Mind: The risk of a blown EGR cooler hydro-locking your engine is now zero point zero percent. The cooler is sitting in a scrap metal pile behind the barn.
- ✓ Massive Space Under the Hood: The L5P engine bay is packed tighter than a tin of sardines. The factory EGR system takes up almost the entire upper passenger side of the engine. When you rip it out, it’s like a breath of fresh air. Suddenly, you can actually reach the passenger-side glow plugs! If you ever need to service the fuel injectors or want to upgrade your injection pump (HP4), the labor time is cut in half because the space is wide open.
- ✓ Better Breathing and Lower EGTs: This is what matters when you are towing. The factory intake routing is cramped to make room for the emissions gear. An aftermarket high-flow intake horn gives the BorgWarner turbocharger a smooth, massive pathway to force dense, pressurized air into the cylinders. Because the engine is breathing better, your Exhaust Gas Temperatures drop. I’ve seen my cruising EGTs drop by 50 to 100 degrees Fahrenheit on long highway pulls just from installing a full delete and a high-flow intake horn.
- ✓ Snappier Throttle Response: Getting rid of the turbulent air caused by the EGR valve makes the turbo spool up just a hair quicker. When I press the go-pedal to merge onto the highway with a trailer, the power is right there.
The Brutal Reality of Installation (The Cons):
I’m not going to sugarcoat this. Doing a full EGR delete on an L5P in your driveway will test your religion.
- • The Cost: A high-quality kit with a billet intake horn and good silicone hoses will run you anywhere from $300 to $800, depending on the brand and finish.
- • The Labor: If you pay a diesel shop to do this, expect a massive labor bill. If you do it yourself, clear your entire weekend.
- • The Knuckle-Busters: Taking the EGR cooler off an L5P requires reaching bolts that are buried deep against the firewall. You will be leaning over the fender for hours. You’ll need a massive assortment of 10mm, 13mm, and 15mm sockets, swivel joints, and long extensions. Usually, you have to take the passenger side plastic wheel well liner completely out just to reach up from underneath the truck to blindly undo the exhaust up-pipe bolts. It is a frustrating, filthy, exhausting job.
The Economics of Reliability: What’s the Right Call?
Let’s break this down from a business perspective, because out here, a truck is a business asset.
If you just bought a used L5P, you are tight on cash, you need the truck running tomorrow to get to the job site, and you just flashed your delete tunes—go ahead and throw the $50 block-off plates on there for now. It will stop the soot from entering the intake, and it will get you down the road. It’s better than leaving the EGR active.
But…
If you are like me, and you view your 3500HD as a long-term investment that needs to pull heavy loads for the next ten years, the block-off plates are a shortcut you shouldn’t take.
When you are hauling a massive payload, the engine coolant temperatures spike. The thermal stress on that dormant factory cooler is immense. The last thing you want is to be three states away from home, pulling a grade, only to watch your temperature gauge peg the redline and white, sweet-smelling smoke start billowing out of your exhaust because the cooler finally let go. A blown head gasket or a hydro-locked motor will cost you $10,000 to $15,000 to rebuild.
Is risking a $15,000 engine worth saving $400 and a Saturday afternoon of wrenching? Hell no.
My Final Advice to the L5P Community
Do it once, and do it right. Order the Full EGR Delete Kit. Get a buddy to come over, buy a case of cold beer, put the truck up on jack stands, pull the passenger-side fender liner, and get to work.
Strip that massive, useless hunk of metal out of the engine bay. Bolt on a beautiful, free-flowing intake horn. Reroute your coolant lines properly. When you finally turn that key, hear the turbo whistle clearly, and see how clean and open your engine bay looks, you’ll know you bulletproofed your rig the right way.
Your Duramax was built to be a legendary, million-mile workhorse. Stop forcing it to breathe its own dirty exhaust, rip the band-aid off, and let that big V8 breathe the clean country air. Keep your oil clean, keep your fuel filters fresh, and I’ll see you out on the road.
