Pardon our dust! We are remodeling our site to ensure a better experience for you!

LML , duramax , diesel , CP4 fuel , fuel pump

Duramax LML CP4 Pump Problems

Duramax LML CP4 Pump Problems

Modern diesel engines, like the Duramax in 2001-to-2016 GM trucks, rely on a complicated, high-pressure, common-rail fuel delivery system to make optimum power with the least amount of fuel. 2001 to 2010 GM trucks running the LMM Duramax enjoyed exemplary reliability from the Bosch CP3 injection pump, but this pump was discontinued when the LML Duramax bowed in 2011 because General Motors opted for a more fuel-efficient injection pump to go with its all-new engine. This pump, the Bosch CP4, hasn’t enjoyed the same reputation for reliability as the stalwart CP3 and it’s led many owners down the rabbit hole of trying to fix annoying and expensive issues such as complete pump failure.

Luckily for Duramax LML owners, Merchant Automotive has a solution for this vexing problem. We took all of our Duramax experience and completely re-worked, reinvigorated the more reliable CP3 pump, and bundled it together with everything you need to completely solve CP4 pump problems.

Today, we’re going to cover everything you need to know about CP4 fuel injection pump issues in the Duramax LML and how to make sure that your truck is not the next victim of this flawed component:

  • How does the fuel system work on diesel trucks?
  • What’s the difference between the CP3 and CP4 pumps?
  • What’s a fuel system replacement kit?

If you’re ready to get a handle on your LML CP4 fuel injection pump issues, then so are we.

Let’s do this!

How Does The Fuel System Work On Diesel Trucks?


Diesel engines vary from their standard gasoline counterparts by how they deliver fuel to the engine and the combustion process itself. In a gasoline engine, air and fuel are mixed in the combustion chamber, compressed by the cylinder, and ignited by a spark plug. In diesel engines, like the Duramax, this process occurs without the use of a spark plug! In fact, diesel engines rely on the heat generated from air compression in the cylinders to ignite the fuel that is sprayed directly into the chamber itself. Yes, you read that right. The only thing being taken into the cylinder of a diesel-powered truck is air!

Air in a diesel engine is compressed 15:1 and 22:1, which results in anything from 300-500 psi in the cylinder compared to 120-200 psi in a gasoline engine. This compression causes a brief but intense rise in temperature. Fuel is introduced via the injector, is ignited by the heat and a small explosion happens that then moves the cylinder back down the stroke. Boom. Forward motion. The exhaust valve opens and the cycle continues over and over again.

CP4 Pump Issues

In order to introduce diesel fuel into this high-pressure environment, a high-pressure fuel pump and advanced fuel injectors must be utilized.

Recent Duramax engines utilize a fuel injection pump (CP3 or CP4) known as a “radial piston pump” to deliver fuel to the injection system. These unique pieces of machinery deliver perfectly measured doses of fuel to the common rail of the Duramax at a high pressure.

The injectors then move the fuel from the common rail, and into the cylinders at precisely timed intervals. This process repeats thousands and thousands of times a minute.

The fuel injection pump is one of the crucial elements of a Duramax truck and it needs to be seriously durable to keep up with one of the hardest working engines on the market. Unfortunately, the CP4 in LML Duramax trucks is not exactly known for its durability.

So what’s the deal with CP4 failure?

What’s the difference between the CP3 and CP4 pumps?

In short, the CP4 pump was designed to be a cost-cutting measure, since the LML runs a more efficient injector and requires less overall fuel flow. In theory, the CP4 makes perfect sense! However, in real-life scenarios, it is a MAJOR sore spot for LML owners.

The issue with the CP4 pump comes down to material durability. Debris created by cam erosion within the pump eventually passes into the injectors and then is recirculated throughout the fuel system! As the engine runs, more and more damage occurs until the lines, regulator, and injectors are completely and totally ruined. Due to the high pressure within the system, this can cause the pump to lock up, or completely explode. Whoa!

Check out the damage!

This issue seems to be tied to the lack of lubrication qualities present within modern low-sulfur diesel fuel, tied to poor maintenance of the fuel system. Evidence shows that Canadian Duramax owners don’t seem to have the same issues and it’s most likely tied to the formulation of their diesel fuel. All in all, it’s simply a recipe for disaster, and at over $10,000 to replace the fuel system, one hell of an expensive fix to boot.

What’s A Fuel System Replacement Kit?

So, what can you do about this pervasive problem? Lucky for LML Duramax owners, the diesel experts at Merchant - Automotive have a solution that will instantly solve all issues with the CP4 pump and fuel system.

Merchant - Automotive Fuel System Replacement Kit:

We’ve worked with the fuel injection expert at S&S Diesel to create a CP3 pump that replaces the problematic CP4, still meets EPA approval, and is already re-calibrated to match the LML stock CP4 fuel curve. Yes, this pump is CARB compliant and legal in all 50 states! Rather than rely on a poor design with stunning failure rates, the fuel system replacement kit from Merchant Automotive takes a bulletproof design and makes it even better.

In addition to the modified CP3 pump, this kit includes everything required to replace critical components in your fuel system with upgraded, competition-ready parts:

  • Fuel rails
  • Fuel injectors
  • Fuel lines
  • Master Filter Head Kit (Filters, filter head rebuild kit/spacer, WIF plug, bleeder screw, MA Additive)
  • All Gaskets
  • All Seals

Vehicle Fitment Guide:

Models: GM 2500HD and 3500 Duramax Silverado

Engines: LML

Years: 2011, 2012, 2013, 2014, 2015, 2016

Models: GM 2500HD and 3500 Duramax Express Vans

Engines: LGH Van

Years: Late 2010, 2011, 2012, 2013, 2014, 2015

Trust Your Duramax To The Experts

We know the possibility of CP3 failure is not something you should have to deal with, especially from a brand as well known for durability as Duramax. The reality is that this troublesome part can fail and cause damage that can easily exceed $10,000! Replacing your entire fuel system preemptively will help you avoid this costly repair and give you a fuel injection system that will last the life of your truck, even under heavy usage. The last thing you want to deal with when on a road trip, far away from home is an LML that is completely stopped in its tracks by a CP4 failure.

Trust the experts at Merchant Automotive, we’ve got your back for the long haul. 

join the discussion!