Diesel Heart Transplant: Duramax CP3 Swap

Replacing the problematic Duramax LML CP4.2 injection pump with its more reliable predecessor, the CP3, can save thousands in repair costs down the road while remaining emissions-legal. 

The diesel equivalent of a heart attack waiting to happen. That’s becoming the common perception of the Bosch CP4.2 high-pressure fuel-injection pump found in 2011 to 2016 Chevrolet and GMC Duramax diesels, the same year Ford 6.7-liter Power Strokes and 2014 to 2016 Ram 1500 EcoDiesels.

The “CP4” injection pump, as it’s called by many, is the heart of these diesels, and in the past few years, it has garnered a reputation for catastrophic failure, resulting in repair costs north of $8,000. Failure of the injection pump, while the engine is covered under the manufacturer’s engine warranty, isn’t that big a deal to the owner, other than the inconvenience of having the truck laid up for a week at a dealership getting the fuel system replaced. But when the CP4 falls outside of warranty coverage, it’s a different story altogether, as the truck owner is faced with a sizable repair bill.

Chronic Heart Failure

Luke Langellier, co-owner of Michigan’s S&S Diesel Motorsport and a former Cummins R&D engineer, claims the “fatal flaw” of the Bosch CP4.2 pump is in its design. “The CP4.2 uses a two-lobe cam and two actuators to pressurize the fuel. Each actuator, or bucket, has a metal roller (tappet) that rides on the lobe,” he says.

“Contact is metal-to-metal on those rollers, with a thin film of diesel for lubrication. If there’s insufficient lubrication, they start to skid instead of roll. That high friction creates high heat, which causes a bunch of metal debris on those two surfaces that are touching and causes the CP4.2 to quickly self-destruct,” Langellier explains. “The fatal flaw of the CP4 is not only in the bottom-end design but how it routes that debris into the actual pumping elements and then to the fuel rails, the high-pressure lines and on into the injectors, wiping everything out.”

Preventive Care

The good news is that the problematic CP4.2 injection pump can be replaced before it fails. The fix is to do a CP3 transplant. The CP3 was a stalwart performer in 6.6-liter Duramax diesels from 2001 to 2010, and it’s been the heart of Cummins 5.9-liter and 6.7-liter engines since 2003.

Replacing the CP4 with Bosch’s CP3 is not actually a step backward, as the numerical designation might imply. Rather, it’s a proven technology that flows more fuel, and it has a stronger and more reliable design than the newer and more cheaply built CP4.2. Should one decide to “tune” the engine, a CP3 can support more horsepower than the CP4 because of the higher fuel volume it’s able to feed the injectors. The CP4.2 is said to support fuel for only about 525 horsepower, while a CP3 can support up to 800 horsepower.

A number of aftermarket diesel-performance parts suppliers have kits to do a CP4.2-to-CP3 conversion. The kits cost around $2,000 and require about two days of shop labor to make the swap.

The only 50-state, CARB-certified CP4.2-to-CP3 conversion kit that we know of at the time of this writing is from S&S Diesel Motorsport. The other conversion kits currently on the market require removing emissions-related components and uploading custom tuning into the ECM, which are against federal emissions laws.

Prescribed Treatment

Langellier, along with former Bosch engineers at S&S, developed modifications to the CP3 that allow it to be a bolt-in, smog-legal replacement in the LML Duramax. The S&S CP3 has been modified inside and out to work seamlessly with all of the LML’s emissions components, including properly feeding the “ninth injector” that is critical to the engine being able to do the “regens” to keep the diesel particulate filter clean.

The S&S CP3 has also been modified internally so it matches the critical fuel-flow curves of the factory Duramax LML injection pump to meet the requirements of the piezoelectric Bosh injectors so OEM fuel economy and power remain unaffected.

Such modifications are the key to why the S&S CP3 Conversion Kit ($2,500 MSRP) is a drop-in CP4.2 replacement, and why RVers towing with the 6.6-liter LML GM HD Duramax should find it an appealing replacement, especially those living in California and other states that have strict registration and licensing laws backed with emissions tests.

“What we have done with our stock version of the LML CP3 is doing the development work in the back end to make it as easy to install as possible,” Langellier says. “It provides LML owners a very reliable solution to the CP4.2 that is a drop-in, emissions-intact replacement, without having to do any other modifications or tuning.”

The Duramax is like an onion because it has many layers of parts that need to be peeled away to get to the CP4.2 fuel-injection pump. Replacing the pump takes about 16 hours.

Among the layers is the EGR system. During this project was a good time to inspect the EGR valve assembly, the EGR tube (shown) and the EGR cooler for excessive carbon buildup because these components have to be removed to get to the problematic injection pump.

The Bosch CP4.2 pump is located at the front of the Duramax’s engine valley and behind the water pump. Removal of the fuel-injection pump requires removal of the coolant Y-bridge that holds the thermostats. This is a good time to replace the two thermostats, which cost about $130 for the pair.

It usually requires the use of two small pry bars to force the CP4.2 rearward out of the aluminum drive housing because the O-ring around the drive-gear side of the pump seals is very tight.

The CP3 high-pressure fuel-injection pump (left) is slightly larger bodied than the factory CP4.2 found on the 2011 to 2016 Duramax LML. The CP3 is considered more reliable and robust than the CP4.2, which is why S&S Diesel Motorsport offers it as a 50-state-legal, CARB-compliant bolt-in replacement.

S&S Diesel’s engineers made multiple internal and external modifications to the factory CP3 pump so it would match the fuel-flow requirements of the LML Duramax’s piezoelectric injectors and work seamlessly with the factory EGR system, including the “ninth injector.”

S&S Diesel’s CP3 Conversion Kit includes a new Bosch pump, along with a new high-pressure feed line (foreground) and low-pressure main fuel feed line, because the CP3 pump is longer from front to back than the CP4.2. The CP3 also has just one high-pressure feed to the fuel rail, where the CP4.2 has two.

Making the conversion required removing the drive gear and nut from the CP4.2 pump and reinstalling them onto the CP3.

Torque on the drive-gear nut is 52 ft-lb. S&S Diesel includes detailed instructions with the kit.

Before installing the CP3 injection pump, remove the fuel regulator from the old CP4.2 and inspect the metal-screened filter on the end. If there is any sign of metal debris on the screen, the fuel tank has to be removed, flushed and the entire fuel system replaced, including high- and low-pressure fuel lines, fuel rails and injectors.

The S&S-modified Bosch CP3 pump is truly a drop-in replacement for the problematic CP4.2. Making the swap is good insurance against having to pay north of $8,000 in fuel-system repairs when a CP4.2 fails.

The CP3 bolts into the same location as the factory injection pump, using the OEM bolts. The pump-to-block mounting bolts are torqued to 18 ft-lb.

While other CP4.2-to-CP3 conversion kits require the removal of all the emissions components, the S&S CP3 pump has been modified to keep the LML Duramax emissions system intact, including a pressure feed line from the CP3 to the hydrocarbon Doser, aka the “ninth injector.” It’s torqued to 89 in-lb.

The OEM low-pressure fuel-feed supply line in the valley is replaced with a shorter one that comes with the S&S Diesel kit. This main fuel-supply line is prone to leak if not installed properly. Lubricate the threads and torque the compression fitting to 26 ft-lb.

Another critical area for correct installation of the new CP3 is the high-pressure fuel line to the fuel rail on the passenger’s side of the block. S&S provides a new li e, which is installed on the forward feed port , while the S&S-supplied block-off nut goes on the adjacent feed port . It’s critical that both nuts are torqued to spec (22 ft-lb), as are the other injector lines. Under- or overtightening will result in fuel leaks.

The S&S CP3 reuses the low-pressure fuel-return U-hose. It’s important that the rubber hose doesn’t touch any of the steel lines, which would eventually wear a hole in the rubber.

The CP4.2 pump has a fuel-temperature sensor screwed into the bottom rear of the housing. That sensor is removed and plugged back into the OEM harness. The CP3 doesn’t use this sensor. The plugged- in sensor is secured with a cable-tie next to the pump on the driver’s side, where it’s held out of the way.

The LML Duramax with its new heart — an S&S Diesel CARB-compliant Bosch CP3 — is ready to start pumping. The investment in time and money protects against a possible failure of the OEM CP4.2 fuel-injection pump.

Open-Heart Surgery

S&S Diesel’s CP3 kit was installed in a 2011 GMC Sierra HD 2500 at Tony’s Garage in Florence, Oregon. The 2011 Crew Cab had suffered an engine failure (due to a non-fuel-related issue), requiring replacement. Anticipating a CP4.2 issue in the future, the S&S CP3 kit was installed on the new GM engine as parts were being reinstalled.

Replacing the CP4.2 can be done at any time, but the best time, short of failure, is when the factory EGR cooler, turbo, injectors, or other components require replacement. The Duramax is like an onion, with many layers of parts, plumbing and wiring harnesses. The fuel injection pump is at a layer deep inside, at the very front of the engine valley, beneath the EGR cooler, and behind the thermostat-housing Y-bridge. Getting to the pump requires nearly a half-day of shop time.

If you are already paying for shop labor to replace a failed or plugged EGR cooler or turbo, swapping out the problematic injection pump at the same time is almost labor-free. The photos show only the highlights of the CP4.2 replacement steps during the engine swap.
These steps should be helpful for any diesel tech contemplating the S&S CP3 pump conversion or knowledgeable DIYers who aren’t afraid to tackle such diesel repair work.

The cost of doing just the LML CP4.2-to-CP3 conversion is about $3,500 for parts and labor. That might take the breath away from many Duramax owners, but compared to the $8,000-plus it costs should the CP4.2 fail, it’s a bargain. The investment in such a conversion should also bring lasting peace of mind and assurance that the LML Duramax will not suffer a catastrophic fuel-injection pump failure like others have experienced.

Power Stroke Bypass

The 2011 to 2016 Ford 6.7-liter Power Stroke utilizes the same CP4.2 found in that era of the Duramax. But because of the way it’s implemented in the Power Stroke, it can’t be swapped out easily. So S&S Diesel Motorsport has designed a bypass kit to prevent the debris from a grenaded CP4.2 from wiping out the Power Stroke’s injectors and flowing through the entire high-pressure side of the fuel system, as it does in a Duramax.

“Basically our ‘CP4 fail-safe kit’ is an aluminum block that goes on the pump that redirects the factory fuel flow,” explains Luke Langellier, S&S co-owner and development engineer. “When the CP4 fails, our Power Stroke filter block prevents that debris from going through the high-pressure system, saving the cost of replacing injectors, lines, and fuel rails. But it doesn’t prevent the CP4 from failing.”

S&S Diesel’s Ford CP4.2 Bypass Kit retails for $360.

How to Keep a Healthy Heart

One tip we learned during the research for this article is never run a diesel engine with a CP4.2 injection pump low or out of fuel. That’s the death of a CP4.2. Any air introduced into the fuel system is a sure way to deprive the two actuator roller tappets inside the pump of proper lubrication. Air in the fuel system is such a concern that Bosch has stringent technical procedures to ensure there’s no air in the fuel supply, even when putting these pumps on a test bench. For that same reason, it’s prudent to install an aftermarket lift pump, such as those offered by FASS and AirDog (pictured), whether using a CP4.2 or CP3. Lift pumps keep the fuel air-free and take some of the workload off the high-pressure injection pump.

Special thanks to Tony’s Garage in Florence, Oregon.

See Related Story: Diesel Tech Q&A: Lift Pumps

A respected automotive and RV journalist and longtime Trailer Life contributor, Bruce W. Smith has held numerous editorial titles at automotive and boating magazines, and authored more than 1,000 articles, from tech to trailering. He considers his home state of Oregon a paradise for RVing and outdoor adventure.