At the time of its introduction in 2001, the LB7 Duramax was considered the most advanced diesel engine to be used by any OEM in the light duty truck market. The revolutionary V8 diesel used aluminum cylinder heads and a common rail fuel injection system with a high pressure CP3 injection pump that made everyone a skeptic. While that LB7 platform was replaced with the LLY in the middle of 2004, the basic internal design of the Duramax has been left virtually unchanged up to the 2016 model year. Sure, it received some updates and changes as the years went on to better fit emissions requirements, but there is no doubt that the Duramax engine has been a homerun for GM. It’s an engine that is super reliable and fully capable of doing anything a truck owner needs, whether it’s used for towing construction equipment around all week or tearing up the drag strip on Friday nights.

Complete Head Gasket Repair Kit From MA. The exhaust manifold and up-pipe gaskets shown here are also an option to the base kit.
This repair job will be getting all new Merchant Automotive injectors, replacing the factory injector lines is always a good idea. Since the factory ones are prone to internal corrosion and debris build up, Merchant offers new hard stainless lines that require no core’s to be returned. OEM style fittings ensure leak-free seal.
This is an extremely labor-intensive job, and in most cases, should only be handled by professional repair shops with experience in motor work. However, for someone mechanically inclined, with a large assortment of tools and previous experience pulling cylinder heads, it can be done without removing the cab or engine from the chassis.

That being said, the LB7 isn’t without its faults. Since it was taken out of production more than 10 years ago and mileage just continues to increase, there are two main points an LB7 owner should prepare for: blown head gaskets and bad injectors. These are both relatively common problems you might expect to deal with on the 2001-2004 LB7 Duramax, but you can rest a bit easier knowing the aftermarket has had a decade to develop and perfect the parts you need to correct it. It’s now better than it was from GM.

As an owner, or someone in the market for the LB7, what signs do you look for to investigate further into potential head gasket or injector problems? On the head gasket side, it’s just like any other internal combustion engine: When that head gasket seal is compromised, the motor will lose coolant, which could pressurize the degas bottle enough to blow the cap; and the upper radiator hose could also be hard—from excessive pressure—long after the engine has been shut down and cooled off. On the injector side of things, gray/white smoke on takeoff is usually the first visual sign of a problem, as excessive fuel gets dumped into the cylinders and isn’t burned efficiently. The engine oil could start being diluted by excessive amounts of fuel seeping past the piston rings to diagnose this. A basic injector balance rate test can be performed with a scanner in the OBD port. In additional, a trained mechanic can disconnect the injector return lines to check fuel return rates.

Obviously, the goal here was to get the cylinder heads off the engine so the head gaskets could be replaced. While the idea of it seems pretty straight forward, the labor time adds up quickly. Extreme care must be taken on disassembly so that foreign objects don’t get dropped into the motor and parts you’ll be reusing aren’t damaged.
There it is, the tell-tale signs of a blown head gasket. On this truck, considered low mileage at just 86,000, it’s obvious the passenger No. 7 rear gasket had let go allowing coolant to enter the combustion chamber. An updated GM head gasket supplied in the Merchant Automotive kit, along with some stronger ARP head studs should prevent this from ever happening again.

Head Gaskets

There are several reasons and a variety of opinions as to why the LB7 head gaskets fail, such as excessive overheating problems that could occur when towing or working the engine hard; and excessive boost and extreme cylinder pressures in a high-performance application that could allow the stock head bolts to stretch and actually lift the heads from the block. But one of the main contributing factors could be the head gasket design itself. In the pre-2006 Duramax engines, GM used a multilayer steel gasket that utilized a containment ring around each cylinder bore. This containment ring, for lack of a better word, used a “crimped” edge, where the outside layer would basically wrap around the other layers and crimp it all together around the cylinder bore. It has been suggested that this crimped edge would weaken after hundreds and thousands of heat cycles, eventually failing and allowing the combustion pressure to get between the gasket layers to create a leak. These leaks can happen internally, meaning coolant will find its way into the cylinders, or externally, where the coolant will seep through the head and down the sides of the engine block where you’ll see it.

After recognizing the issue, GM re-designed the head gasket to what is referred to as a ‘riveted’ gasket, which is still a multi-layer steel gasket, but it no longer uses that rolled edge containment ring. This change has offered better results in the later Duramax platforms. Replacing head gaskets in the Duramax engine is a labor intensive job and requires a good stack of parts to get it back together correctly.

Upon close inspection of the cylinder heads and engine block, it appears the No. 7 cylinder had been the only one leaking. At this point, the heads will be dropped off at the local machine shop to be gone through, cleaned, tested for cracks, valves resealed, and the surfaces milled flat.
The factory gaskets are a multi-layer steel design, which is most common in the light-duty diesel market. In the early LB7 head gasket, it’s common for the containment ring around each combustion chamber to be compromised and allow leaks between those multi-layers. The latest Grade C OEM gaskets provided in the Merchant kit are completely redesigned from the originals and will offer better sealing and longevity.

Luckily for Duramax owners, Merchant Automotive (MA) of Zeeland, Michigan, has specialized in everything Duramax since 2004, offering a complete head gasket kit that has the necessary parts to get this job done right. The kit includes OEM Grade C updated head gaskets, along with every other OEM O-ring, seal, gasket, and piece part you’ll want to replace. While the basic kit includes brand new stock head bolts, as an upgrade, MA suggests the ARP head stud kit, which offers a higher tensile strength and torque sequence, equating to a better clamping force on the cylinder head. When paired with the redesigned head gaskets, you should never have to worry about blown head gaskets again.


Unfortunately, for an LB7 owner, the injectors are difficult to access and relatively expensive to replace, so it’s a job that will take an experienced mechanic more than 12 hours to do.

When GM first discovered just how common the injector failures were, they owned up to the mistake and extended a 200,000-mile warranty on the LB7 injectors to customers. However, that warranty had a deadline of seven years, so even the latest 2004 model is well over the coverage period and you’ll be on your own to cover the expense of replacing them.


After a trip to the machine shop for resurfacing and cleaning, the valves were all cleaned up and the valve springs tested for proper pressure. Upon reassembly on the engine, the new head gasket is only as good as the clamping force placed upon it. So rather than risk a future failure with the factory head bolts, a couple hundred dollars more for ARP head studs offered as an optional upgrade with the Merchant Automotive kit is worth the investment for peace of mind.
While this truck had already had the original injectors replaced under the massive GM warranty recall with just 25,000 miles on the truck, some of the balance rates on these units were already starting to get a little too far out of spec for comfort. With only 60,000 miles on them, it was a bit surprising, but being this deep in the engine doing head gaskets made the decision to replace them while it was apart an easy one.
These injectors had been installed at a GM dealership about six years ago under warranty, but more than one of them showed signs of combustion leaking past the copper washers that seal them in their bores. This leakage means one of two things, they weren’t properly installed the first time, or the injector hold downs had stretched some and allowed the injector to push out just enough to create the leakage.

Like the head gasket issues, there are a few different theories as to why these injectors fail and no one, not even GM, has really offered an actual reason. Some say it’s a fuel filtration problem as most fuel filters out there only filter down to 7-microns. So perhaps small debris and contaminants make it through the injectors and cause the ball and seat to wear out prematurely? Some think it’s a lack of lubrication from the Low Sulphur and Ultra-low Sulphur diesel that is produced now for cleaner emissions and it doesn’t offer the lubrication the injectors need. Maybe it’s just poor fuel supply, where the injector doesn’t get great fuel quality and entrapped air in the fuel creates an issue? The addition of an aftermarket lift pump could help with all of this, as you get better filtration, a water separator and air bubbles can be removed and returned to the tank. Another suggestion is that the LB7 injector design being placed inside the valve cover subjects it to more heat, and the internals and electronics can’t survive 100,000+ miles. An inferior metal and insufficient heat treatment on the original release injector bodies may also be partly to blame.

Again, thanks to Merchant Automotive, the LB7 Duramax platform can continue to survive and improve its lifespan with their newly released replacement injectors, which are completely new from the ground up. MA has used their years of knowledge and testing to develop their own injector that no longer requires a core return, as they don’t deal with re-manufacturing injectors, which may not have the lifespan you’d expect when doing such a labor intensive job. The new MA injectors use the best materials possible, including an injector body that has been developed to eliminate cracking. Inside, the injectors receive a chrome plated ball seat for better wear and corrosion resistance. At the nozzle, the needle tip gets a Diamond Like Carbon (DLC) Coating that offers unique properties for low friction and makes them highly corrosion resistant.

When installing new injectors, especially on higher mileage applications, it’s always a good idea to replace the injector cups as well. Again, this is a small investment to ensure you won’t run into future problems with coolant leaking into the cylinders or an injector that doesn’t seal quite right.
MA’s new and improved injectors. Best of all, being new, there is no core charge.
Going back to the signs of an external leak at the injector tip, the injector hold downs are going to be upgraded to the Billet kit from SoCal Diesel. Due to the extreme cylinder pressures at higher power levels, the billet hold down and stronger stud will allow a better clamping force on the injector and cup and keep them from lifting or moving.
Along with the better billet hold down, the SoCal Diesel kit comes with a higher tensile strength
ARP stud kit, this allows better clamping and holding force on the injector, without putting extra strain on the threads in the aluminum heads.

Along with the replacement injectors, the MA injector package will come with everything you need to do a full and thorough injector install. New injector feed lines will be required, as the originals are prone to debris and contaminants reaching the injector when they are removed. It’s also suggested that the MA Heavy Duty return line bolts be used to replace the factory bolts, which are prone to rounding off when removed. The MA bolts will be easier to torque to spec and ensure no under valve cover fuel leaks once the job is completed.

This particular truck is a super clean, virtually, show-room condition, low mileage 2004 model. At just 86,000 miles, the loss of coolant while towing was a sure sign of an internal engine problem and some rising injector balance rates would lead to the inevitable (breakdown?). Used mainly for towing a 40-foot, fifth-wheel travel trailer and large boat all summer long, the truck had the basic upgrades, like intake, exhaust, EFI Live Tuning, lift pump, and a built transmission, already done to it. The truck made a respectable 400hp in the tow tune and peaked at 480hp in the performance tune, so it wasn’t pushed to its absolute limits, and boost levels with the stock turbocharger were still limited to a ‘safe’ 25-psi. So, blown head gaskets were a bit of a surprise to the owner, but it just goes to show that their failure can happen in any LB7, regardless of mileage, use, and/or abuse.

While it’s almost a shame to hide these beautiful new parts inside the engine, reassembly must continue and the use of the all new Merchant injectors and billet SoCal Diesel hold downs should be the last time an injector ever has to come back out of this engine.
Another upgrade that will be going into this rebuild are the Heavy Duty Fuel Return bolts that will replace the shallow headed stock bolt. Since it’s common for the stock bolt to round off during removal, the redesigned MA bolt gives you twice the tool engagement and will ensure the proper torque can be reached keeping leaks from ever happening inside the valve cover.
With the cylinder heads off the truck, you have pretty easy access to the CP3 injection pump mounted in the valley of the engine, so it seemed a great time to also replace the original pump with something a little better. The Sportsman line of CP3 pump from Exergy Performance is a stock replacement unit with some minor internal modifications to increase fuel flow above 3000-rpms by 30 percent.
The new injection pump from Exergy offers some additional piece of mind to go along with those new injectors. With Max Effort EFI Live tuning from ATP Trucks, the new injectors and Sportsman pump should have no problem putting down more than 550hp

In next month’s follow-up article to the head gasket and injector replacements, the engine will get some further upgrades with additional airflow from some improved exhaust manifolds and up-pipes from, a better breathing ProMax SX-E turbocharger from High Tech Turbo, and a high flow Y-bridge, piping and intake kit from Wehrli Fabrication. DW





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