Save $10K On Your LML Duramax Diesel Engine: Install A Lift Pump

• November 23, 2021
• Story By
  Mike McGlothlin

The Bosch CP4.2 represents the evolution of high-pressure diesel fuel pump technology. It costs less for the manufacturer to produce yet produces much higher injection pressures than its predecessors. This means diesel engine manufacturers can meet extremely stringent particulate matter emission standards while keeping the overall production costs of their engines down. Unfortunately, the CP4.2 has a fatal flaw that can spell disaster for not only the pump itself, but the rest of the [expensive] injection system and possibly even the engine. One engine that is exceptionally susceptible to CP4.2 pump failure is the LML Duramax, the 6.6L diesel V-8 GM offered in its ’11-’16 model year HD trucks.

Although Ford’s 6.7L Power Stroke utilizes a very similar version of the CP4.2, it’s proven much more reliable in ’11-present Super Duty applications. The primary reason for this is the fact that the Power Stroke comes factory with an electric lift pump feeding it consistent low-pressure fuel supply. The LML Duramax doesn’t have a factory lift pump. Luckily, the addition of an aftermarket electric lift pump can help ensure that positive pressure (and no air) is supplied to the low-pressure side of the CP4.2—and can be installed relatively affordably. Below, we’ll explain why the CP4.2 fails, show you what it looks like when it self-destructs and then lay out how you can keep this failure from happening to you.

Bosch CP4.2: The Past, Present and Future High-Pressure Common-Rail Pump

Bosch’s CP4.2 is the twin piston high-pressure variant in its CP4 pump line (the CP4.1 is the single piston version, employed overseas in various diesel engines). Each cylinder of the CP4.2 features a piston assembly, plunger, spring and head. A single camshaft with two actuation lobes drives the pistons up and down within their respective bores. In the LML Duramax, the CP4.2 pressurizes diesel fuel as high as 29,000 to 30,000 psi. This extreme high pressure facilitates fine atomization within the engine’s cylinders. With future, tighter particulate matter emission standards in mind, Bosch designed the CP4 line to produce (and handle) as much as 39,000 psi.

The Problem

The camshaft drives the pistons by way of a roller tappet located in the bottom of each piston. Both roller tappets are press-fit into a polished follower. Not only does the roller tappet require a thin layer of diesel fuel to keep the point of contact with the cam’s lobe lubricated, but it must also remain parallel with the cam’s lobe. Unfortunately, when a lack of fuel lubrication is present or (worse) air infiltrates the high-pressure pump, the pistons float and are prone to rotating in their bores. When this happens, the roller tappet rides perpendicularly on top of the cam lobe and the inconsistent forces from up above due to highly pressurized yet aerated fuel lead to contact being made between the two components.

The Catastrophe

After the roller tappet in the piston and the cam make enough contact, both pieces begin to break down. Due to the design of the CP4.2 pump, metal debris is permitted to enter the high-pressure chamber and exit the high-pressure outlet. This means that shrapnel-laced fuel is allowed to flow into the fuel rails—fuel which is eventually sent to the injectors. At the same time, the return circuit and all fuel being routed back to the tank is contaminated. By the time you notice the problem, it’s already too late. Most CP4.2 failures consist of the engine shutting off unexpectedly, never to restart.

The Aftermath

As you may have already guessed, the cost of a CP4.2 failure can be expensive. Repairs can range from $6,000 to $10,000 depending on what is replaced (vs. cleaned and reused) and who handles the work. For optimal peace of mind, the high-pressure fuel system should be completely replaced. This means installing a new CP4.2, injectors, fuel rails, high-pressure fuel lines, return fuel lines and a thorough tank cleaning. Most independent diesel shops can perform the job in roughly 30 labor hours, and the labor obviously represents a solid chunk of the overall price of repairs.

First Things First: Check The VCV

One of the first things to check in the event of a suspected CP4.2 failure is the VCV, or volume control valve. This valve regulates the flow rate of the high-pressure pump and is equipped with an 80-micron screen in the LML Duramax engine. Finding the screen in this kind of shape, coated in metal debris, is usually a telltale sign that the CP4.2 has lunched itself. Exergy Performance developed a two layer, 25-micron screen to capture more debris and also shut the engine down sooner in the event of a pump failure. Unfortunately, while this can help limit the damage caused by a failing CP4.2 (making the repairs less expensive), it can’t keep the CP4.2 from failing.

Prevent CP4.2 Failure: Install A Lift Pump

An aftermarket lift pump system is the most affordable way to keep the CP4.2 alive. FASS, AirDog and Fuelab are the big names in town here. Each of them offer a complete system that not only provides sufficient fuel supply pressure for the CP4.2, but all of them feature improved filtering and air removal capabilities over the OEM equipment. Regular fuel filter and water separator changes and correct filter installation are also a big part of keeping air out of the LML’s injection system.

Chassis-Mounted Peace Of Mind

On this FASS Titanium Signature Series lift pump system, the pump, fuel filter and water separator are all part of the same assembly, which can be mounted to a cross member or along the truck’s frame rail. Its 2-micron fuel filter outperforms the factory 10-micron AC Delco unit and the lift pump allows the CP4.2 to enjoy a steady 8 to 10 psi of positive fuel pressure supply. As a bonus, even an entry-level lift pump system will support an additional 200 to 300 hp over stock.

200,000-Mile Example

GM’s 6.6L Duramax V-8 has long been known for its durability. With proper care and regular maintenance, the rotating assembly alone can go an easy half a million miles. On LML models (’11-‘16), the CP4.2’s lifespan can be extended tremendously with the simple addition of a lift pump. Seeking a bit of high-mile insurance, the owner of this ‘13 Silverado 2500 HD added a FASS system. The ¾-ton Chevy now has more than 200,000 trouble-free miles on the odometer and counting.

CP3 Swap—The Ultimate Solution

While the addition of a lift pump can keep the CP4.2 alive, long-term, the ultimate fix is to replace it with an alternative pump. By reverting back to the Bosch CP3, the high-pressure fuel pump employed on ’01-’10 Duramax engines and that proved extremely reliable, LML owners can keep a potentially cataclysmic event from occurring. Various CP3 swap systems are available in the diesel aftermarket and, while fairly pricey, offer the only 100-percent cure for CP4.2 failure.