Table of Contents

What the LML EGR System Is

The LML was GM's first Duramax with a full modern emissions package: EGR, a diesel particulate filter, and selective catalytic reduction running on diesel exhaust fluid. For other model years, see our guide to Duramax problem years explained.

The EGR side takes exhaust gas from the engine, runs it through a cooler that uses engine coolant to pull heat out of it, and sends it back into the intake. Putting inert exhaust gas back into the cylinder displaces some of the oxygen and lowers peak combustion temperature, which is how it controls nitrogen oxides.

That design puts two very different things next to each other. Inside the cooler, hot exhaust runs past coolant passages. The cooler sees constant thermal cycling, and it is the part GM built special tooling to test.

GM's service tooling tells you something about the hardware. The EGR cooler pressure tester adapter set, EN-48974, shipped to dealers in December 2007. In December 2010 GM shipped a supplement, EN-48974-20, for the multiple-cooler EGR system, because those coolers have more ports than the earlier design. The same bulletin that covers the LML trucks confirms the engine uses that EGR system.

Test the Cooler Before You Buy Anything

This is the step most people skip, and GM's service bulletin is blunt about why it matters.

Bulletin 08-06-04-003C warns that a leaking EGR cooler can pass coolant through the exhaust system and into the combustion chambers on the right bank. It adds that the cylinders affected depend on which exhaust valves happened to be open when the engine was shut down, which is why the same failure can show up in different places on two different trucks.

Then it gives the instruction that should decide your order of operations: you must test the EGR cooler for leakage before attempting any engine repair, because skipping that test leads to a misdiagnosis for coolant in the combustion chamber. In plain terms, a shop that finds coolant in a right bank cylinder and jumps to a head gasket conclusion may be condemning the wrong part.

The test is mechanical, not a guess. A technician partially drains the cooling system, fits the EN-48974 hose adapter and cap plugs on the cooler hose barbs, connects the J-46091-1 regulator, and pressurizes the cooler to 42 psi. Close the shutoff valve and watch the gauge for five minutes. If the gauge shows pressure decay, the cooler is leaking and gets replaced. If it holds, the leak is somewhere else and diagnosis continues.

Get that test done before you spend money on anything. If the cooler holds pressure, a delete kit will not fix a problem you do not have. Our LML EGR delete kit buyer's guide also explains why to test the cooler before buying a kit.

Symptoms Worth Acting On

Coolant loss with no puddle

A leaking EGR cooler sends coolant into the exhaust or the intake side instead of onto your driveway. The signs:

  • White smoke or steam from the tailpipe.
  • A coolant level that keeps dropping with nothing under the truck.
  • A low coolant message or light that keeps coming back.

Do not keep driving on it. GM's cooling system bulletin, 16-NA-204, states that running low on coolant can damage the engine, and adds that head gasket failure is in most cases the result of operating the engine with a low coolant condition. A slow EGR cooler leak is exactly the kind of thing that turns into a much larger repair if it is ignored.

Intake carbon

Recirculated exhaust carries soot, and soot deposits in the intake manifold and on the EGR valve. This builds up gradually and does not announce itself. The truck just gets lazier over time. Trucks that idle a lot, run short trips, or work hard under load tend to build it faster.

Codes

EGR faults on the LML can set flow and position codes in the P0401 through P0406 range. Treat a code as a starting point for diagnosis, not as proof the hardware has to come off. A failed sensor and a clogged cooler are different problems.

What a Complete LML EGR Delete Kit Includes

Important: the parts below are for off-road, race, or non-public-road use where permitted, as set out in the notice at the top of this page.

A kit has to do four jobs, and a kit that skips one of them will leave you sourcing parts mid-job:

  • Cooler and valve block-off hardware. A delete pipe or a block-off plate seals the EGR cooler position and the EGR valve port so exhaust gas cannot re-enter the intake.
  • Coolant reroute parts. The factory cooler is plumbed into the engine's coolant circuit. Remove it and you have opened that loop, so the flow path has to be handled rather than left open.
  • Gaskets and clamps for the new joints. Fresh intake gaskets matter because you are opening a sealed system and do not want a boost or vacuum leak after reassembly.
  • A matching tune. Covered in the next section, because the LML has a specific answer here.

EngineGo lists the LML EGR delete hardware in the 6.6L Duramax LML EGR delete kit collection. Check the product page for what is in the box and confirm fitment for your year and body style.

What the Tune Actually Needs on an LML

The LML ECM watches EGR valve position and infers EGR flow from what the mass airflow sensor reports. When the ECM commands the EGR valve open, it expects a drop in fresh air volume, because recirculated exhaust displaces incoming air. Pull the hardware without changing the calibration and those expectations no longer match reality. You get codes, and the truck can drop into reduced power.

Here is where LML owners get bad advice. You will see people told they need an ECM unlock for this job. They do not. The LML uses an earlier generation ECM that flashes through the OBD-II port. Our own ECM unlock platform guide lists all Duramax generations from 2001 to 2016, LB7 through LML, as not needing an unlock. Budget for calibration, not for an unlock service you do not need.

EngineGo lists LML tuning through the LML Duramax tuner collection. The packages run on the EZ LYNK AutoAgent 3 platform, which delivers tune files over Wi-Fi to a phone app rather than requiring a laptop. Confirm your model year and body style are covered, and ask about shift-on-the-fly if you want switchable power levels, since that needs a separate switch.

The Install, Step by Step

Tools and setup. You are working on a hot exhaust and coolant system under a heavy truck. Use a lift or rated jack stands, never a jack alone. Let the truck cool completely before you open anything. Wear gloves and eye protection.

Step 1: Disconnect the batteries

Disconnect the negative terminals before you touch electrical connectors or remove metal parts near the harness. There are live circuits in the area you are working in, and isolating the system first costs you two minutes.

Step 2: Drain the coolant

Drain into a clean pan so you can see what comes out. If the cooler has been leaking, or the coolant is due, replace it rather than reusing it. Catch it properly and dispose of it per local rules.

Step 3: Remove the factory EGR hardware

Disconnect the electrical connector at the EGR valve. Remove the intake-side components blocking access to the valve and cooler, then unbolt the EGR valve and the cooler assembly and disconnect the coolant lines feeding the cooler.

Access is tight and the cooler sits low and toward the rear of the engine, so clearing intake and turbo-side components comes first. Expect carbon when the assembly comes apart. Keep soot and debris from falling into the intake ports, and if the intake manifold is heavily fouled, this is the moment to deal with it.

Step 4: Fit the delete hardware

Install the valve block-off plate and the cooler delete pipe or plate using the supplied gaskets. Route and connect the coolant reroute fittings exactly as your kit's instructions describe, so coolant flow through the circuit stays correct.

Do not guess at torque values. This assembly mixes aluminum, stainless, and cast iron, and those materials expand at different rates. Use the service information for your truck, and follow the sequence it gives for multi-bolt flanges so you do not distort a flange. Over-torquing a block-off plate is an easy way to crack it.

Check every joint on both the exhaust feed side and the intake side. Sealing one and leaving the other is the mistake that turns into an exhaust or boost leak and a second teardown.

Step 5: Reassemble the intake side

Refit the intake components and tubing you removed, using the fresh gaskets from the kit. Reconnect sensors and connectors. Check every clamp and fastener.

Step 6: Refill and bleed the cooling system

This is the step that decides the job. See the next section.

Step 7: Flash the tune

With the hardware sealed and the cooling system full, flash the calibration. Start the truck, bring it up to temperature, and watch for leaks and for the coolant level to settle.

Refill and Bleed: GM's Own Procedure

An air pocket in this cooling system is not a minor annoyance. GM's bulletin 16-NA-204 states that air trapped in the cooling system can cause cavitation that may damage system components. It also warns never to remove the cap with the system hot or under pressure, because the coolant can be well above its atmospheric boiling point and can boil out with force.

GM's bulletin exists because this engine is fussy about filling. The 2011 to 2016 pickups use a dual-sided surge tank, and coolant has to be on both sides for the system to hold pressure correctly. Normal system pressure is 5 to 20 psi.

The fill procedure for the pickups, as GM writes it:

  1. Remove the surge tank cap.
  2. Using a 50/50 mix of GM Dex-Cool and water, slowly fill the pressurized side of the tank until the system is full.
  3. With the cap still off, start the engine and race it to 2,000 to 2,500 RPM until it reaches normal operating temperature. It should reach 194 degrees Fahrenheit and the upper radiator hose should be hot.
  4. Keep running at 2,000 to 2,500 RPM for another five minutes.
  5. Let it idle for five minutes while you slowly add more coolant.
  6. Run at 2,000 to 2,500 RPM for another five minutes.
  7. At idle, add coolant until the pressurized side is full.

Then fill the overflow side to at least one third but no more than half full, and install the cap.

The purge cycle follows: run the engine at 2,500 RPM for three minutes after it reaches operating temperature, shut it off, and let it cool completely. Squeeze the upper radiator hose to confirm the system is free of pressure. Then check that the pressurized side is full to the top and the overflow side is one third to one half full. If the level dropped, wait until the engine is completely cool to ambient temperature and add coolant.

GM notes that the coolant level often will not reach the top of the bottle until all the air is out, so do not read a low level during the fill as a problem in itself. Work through the cycles.

EGR Only, or a Full Delete?

An EGR delete does not touch the DPF or the SCR system. Both paths need a tune.

  EGR delete only Full setup
Covers EGR valve and cooler EGR plus the DPF section
Tune needed Yes Yes
Parts to source EGR kit plus tune EGR and DPF together with tuning

The reason to do them at the same time is access, not marketing. Both jobs involve the same area of the truck, and doing them in one teardown saves you repeating the work. The reason to do EGR only is that it is the part actually giving you trouble.

FAQs

Do I need a tune for an LML EGR delete?

Yes. The ECM monitors EGR position and flow. Without a matching calibration it sets codes and can cut power. On this platform the tune flashes through the OBD-II port, and no ECM unlock is required.

Is an LML EGR delete legal?

Not for on-road use. Removing or disabling emissions equipment on a vehicle used on public roads violates the federal Clean Air Act and many state laws. An off-road use disclaimer does not change how an EPA-certified vehicle is treated under federal law. Check federal, state, and local rules for your situation before buying or installing anything.

How do I know the EGR cooler is actually the problem?

Have it pressure tested. GM's procedure pressurizes the cooler to 42 psi and monitors the gauge for five minutes. A gauge that shows pressure decay means the cooler is leaking. A gauge that holds means the coolant is going somewhere else.

Will an EGR delete hurt my engine?

The mechanical goal is to stop exhaust soot from going back into the intake and to remove a coolant-to-exhaust joint that can leak. The real consideration is legal compliance, not mechanical harm. Deleting EGR does raise peak combustion temperature, which is the tradeoff the system existed to manage.

How long does the install take?

The hardware side is a long day's work for a first-timer, and the cooling system fill takes longer than most people expect because it runs on thermal cycles rather than a single top-off. Do not rush the bleed to save an hour.