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How to Convert LML Duramax CP4 Injection Pump to CP3

Pumped Up

Bruce W. Smith
Feb 14, 2019
Photographers: Bruce W. Smith
The black ’15 Chevrolet Silverado 2500HD work truck we encountered at Dunks Performance in Springfield, Oregon, is the typical calling card of a successful turf business. It is used every workday to pull trailers and haul loads in the bed, and when weekends roll around, the trailers attached to the receiver hitch carry a variety of toys, from a boat to ATVs.
The owner keeps it well maintained, but his concern about the longevity of the 6.6L Duramax LML engine’s CP4.2 high-pressure fuel-injection pump finally needs to be addressed. As the truck’s odometer reads just a little more than 130,000 miles, the odds of the pump’s failure grows greater by the day.
The catastrophic failures of the Bosch CP4.2 pump are well-documented and are the focus of ongoing class-action lawsuits in Texas and California against Bosch, GM, Ford, and other vehicle manufacturers on behalf of individual diesel owners whose vehicles use that pump.
The CP4.2 pump, which flows about 20 percent lower volume than the stock CP3 used in earlier Duramax engines, is said to fail because it needs to have a certain level of lubrication between the components that pressurize the fuel headed to the fuel rails. The low volume and ultra-low-sulfur diesel prevent the CP4.2 from doing its job.
Over time, the roller tappets on the actuators, which are forced up by the cam-like driveshaft inside the pump, start wearing from lack of proper lubrication. Whether it’s air in the fuel, a lack of fuel, or lack of lubrication in the fuel, there’s typically little or no warning when “wear” turns into costly catastrophic failure.
A grenaded CP4.2 creates an estimated $8,000 to $12,000 in repairs because the entire fuel system—from tank and lines to injectors—has to be replaced because minute metal debris from the failed pump usually makes its way through the system before the engine shuts down. When the truck’s age or mileage exceeds the warranty limits, the owner foots the repair bill.
Photo 2/23   |   The Bosch CP4.2 (right) has proven to be a cause of catastrophic failures that require more than $8,000 in repairs to replace the entire fuel system in late-model GM diesel trucks. The alternative for performance-upgraded 6.6 Duramax LML engines is converting to a venerable CP3 (left) high-pressure fuel pump such as Fleece Performance Engineering’s PowerFlo 750.
There’s another reason to replace the CP4.2 with a CP3: power. In addition to its durability shortfalls, performance-minded owners will quickly find the CP4.2 can’t provide enough fuel to maintain adequate rail pressure to support the more than 500 to 525 hp coming from aftermarket performance upgrades and modifications. That’s why the sure cure for reliability and performance is to be proactive and do the conversion.
The venerable CP3 is used in previous generations (’01-to-’10) of Duramax trucks, and all the 6.7L, 24-valve (’03-to-present) Cummins powerplants. It’s proven to be durable and can be modified to flow more than enough fuel to meet the demands of bigger injectors, higher-flowing turbochargers, and, with aggressive ECM programming, has been known to support up to 900 hp.
Long-term reliability and more power are exactly what this performance-oriented owner has in mind for the engine in his truck and the reason he and many other diesel performance enthusiasts find Fleece Performance Engineering’s Duramax LML PowerFlo 750 CP3 conversion kit so appealing.
Fleece says the PowerFlo 750 CP3 easily handles the rail pressures and fuel flow to support 750 hp across the entire rpm range. Fleece basically turns a new CP3 into a stroker by machining new actuators (buckets) and suction valves, along with porting the fuel-pressure regulator and machining a new shaft assembly to increase the stroke from the factory 8.2 mm to 10 mm.
With these types of modifications, a stroker CP3 flows upward of 260 lph on the bench, compared to a stock Duramax pump that maxes out around 200 lph. That 30-percent increase in flow should be more than enough to support the fuel this truck owner will need with his plans for future power upgrades that include compounds and fatter injectors.
Photo 3/23   |   The CP4.2’s weakness is ultra-low-sulfur diesel fuel’s inability to provide sufficient lubrication for the single-roller tappet (foreground) on the bottom of the two actuators that provide the 29,000 psi of rail pressure. Tappet wear is typically immediate and catastrophic with little or no warning signs.
The CP4.2 to CP3 conversion is relatively straightforward (as much as it can be when accessing an injection pump sitting at the very front of a 6.6L Duramax LML engine’s valley). Robert Shepler, Dunks’ technican who showed us how to do this conversion, says if all goes smoothly it’s a full two-day, 16-hour job with the right tools—not counting the time for the custom tuning.
This conversion also requires disconnecting the ninth injector because there’s no provision in the CP3 for supplying the 80 to 90 psi to feed it. It also requires a custom tune (Dunks uses EFILive) to reprogram the ECM so the CP3 plays well with the piezo injectors.
All this is information is for a DIYer’s benefit. Plan on spending a long weekend under the hood. We should also note this particular conversion is for “off-road/competition use only.”
All In Truck Performance in West Texas, specializes in Duramax LML and Allison ECM tuning and custom builds. Aaron Weibe is an expert at manipulating the LML’s complicated tuning protocols for EFILive uploads, which this owner is using.
Writing a custom tune is a difficult and time-consuming endeavor, as is incorporating the current changes in the CP3 that control rail pressures so there are no surge issues in its new home. To help with that, Fleece provides the necessary custom calibrations required for its PowerFlow CP3 750 to work properly with the LML.
Photo 4/23   |   Converting from a stock CP4.2 to CP3 typically requires about 16 hours, with most of that time spent removing parts to get to the pump that sits at the front engine valley.
Aaron has a vast library of custom tunes compiled from a multitude of All In’s own builds, which include mild Duramax LMLs with a single CP3 and stock turbocharger to ground-shaking sled pullers and drag trucks with multiple turbos and CP3 fuelers. They also provide further customization of those tunes to meet each individual owner’s needs.
Unfortunately, the final results for all of that will have to wait. As luck (or bad luck, however it’s viewed) would have it, we ran into an unexpected hurdle. Concerned about how the heads would hold up under the higher cylinder pressures generated from a set of Wehrli Custom Fabrication compound turbos and the increased fueling of 30-percent-over Dynomite Diesel Products injectors that are next on the upgrade list, the truck’s owner had Robert pull the heads in preparation for installing ARP studs.
Surprise! We discovered seven pistons with small cracks at the edge of the fuel bowl. They appear to have been damaged by high EGT.
Now the owner has some costlier decisions to make before his beloved rig flexes its newfound muscle on the dyno’s rollers. Fortunately, everyone involved seems to be going with the flow.
Stay tuned, because our plan is to follow this project all the way to the dyno tuning stage.
Photo 5/23   |   Unfortunately, a CP3 doesn’t have a way to provide the Duramax LML’s ninth injector the 80 to 90 psi of fuel pressure it requires to operate. The EGR cooler is removed to get to the pump and can be reinstalled if needed. Unbolting the up-pipes requires a lot of patience.
Photo 6/23   |   Dunks Performance technician Robert Shepler removes the thermostat Y-bridge to access the CP4.2 mounting bolts at the water-pump housing. This is a great time to replace the two thermostats if the truck has more than 150,000 miles on it.
Photo 7/23   |   A CP4.2 high-pressure injection pump is tucked at the very front of the Duramax’s valley behind the water pump (arrow).
Photo 8/23   |   All the fuel lines are disconnected, including the low-pressure feed line. Robert uses 22mm and 24mm wrenches to break the flare nut loose on the feed line. Fleece provides a new feed line in its CP3 kit.
Photo 9/23   |   The CP4.2 is held in place by three 13mm bolts through the water pump housing. There’s also an O-ring on the injection pump’s face, so removal requires gentle, even prying to free the pump from the geardrive housing.
Photo 10/23   |   Important note: The stock fuel-temperature sensor (on the bottom of the CP4) is removed from the pump, plugged into the factory connector, and secured in a safe location. It is not used on the CP3.
Photo 11/23   |   The Fleece CP4.2 to CP3 conversion kit includes everything needed to make the swap, including the O-ring that seals the pump to the new adapter plate.
Photo 12/23   |   Since a CP3’s bolt pattern is not the same as the LML’s stock injection pump, Fleece includes a CNC-machined aluminum adapter plate, and the new O-ring that seals the pump to the water-pump housing.
Photo 13/23   |   Robert pulls the drive gear from the stock CP4.2 and reinstalls it and the retaining nut on the Fleece PowerFlow 750 CP3.
Photo 14/23   |   We were concerned about the way the CP4.2’s drive-gear retaining nut looked on the CP3. But Fleece’s instructions say it does not interfere with the front cover and provides the same amount of thread engagement.
Photo 15/23   |   Robert ensures the CP3 gear and retainer nut are torqued to 70 ft-lb. He uses a pair of angle-nosed pliers to hold the gear while he applies the torque.
Photo 16/23   |   The Fleece PowerFlow 750 CP3 supplies enough fuel volume and pressure to handle more than 750 hp and, in this configuration, bolts right back into the 6.6L Duramax LML as if it is a stock part.
Photo 17/23   |   The Fleece-supplied low-pressure fuel feed (right), which is a modified stock line, is shorter than the original to give the longer CP3 room to fit in the valley of the block.
Photo 18/23   |   Robert used this short Fleece fuel-feed line to replace the entire stock fuel-filter assembly. This truck already had a FASS Fuel Systems 150-gph lift pump. Next on the upgrade list are Wehrli Custom Fabrication’s compound turbochargers, so removing the filter and plumbing is an easy way to really clean up the engine compartment.
Photo 19/23   |   The kit includes an O-ringed plug to block off the fuel-return line, located along the passenger side of the engine valley. The Fleece CP3 doesn’t have provisions for this line.
Photo 20/23   |   With the new CP3 in place, Robert installs the fuel-rail block-off cap and special plug that Fleece includes with the conversion pieces. The cap closes off the area where the second CP4.2 high-pressure feed is normally connected. A CP3 only has a single high-pressure feed line.
Photo 21/23   |   The new Fleece PowerFlow 750 CP3 sits comfortably in the engine valley, ready to meet whatever demands are placed on it.
Photo 22/23   |   Midway through Day 2, Robert was about to begin replacing all the parts he removed for the CP4.2 to CP3 conversion when the Silverado’s owner decided to have ARP head studs installed. When the Duramax LML’s heads were pulled, we discovered seven cracked pistons. This saga will continue. Stay tuned!

Give Your CP a Lift

One way to help take the load off a 6.6L Duramax LML engine’s high-pressure fuel pump—be it the stock CP4.2 or a CP3—is to install an aftermarket lift pump sourced from companies such as AirDog. A lift pump pushes clean, air-free fuel to the injection pump, so the injection pump doesn’t have to suck fuel all the way from the tank, which typically leads to cavitation. A lift pump pulls fuel from a vacuum, which also lessens the mechanical strain put on the positive-displacement gears in the injection pump.
Photo 23/23   |   The gears inside a Bosch CP4.2 high-pressure injection pump work hard to pull fuel from the tank. Installing an aftermarket lift pump helps lessen the extra load and eliminates cavitation concerns.

Sources

Fleece Performance Engineering
Brownsburg, IN 46112
317-286-3573
http://www.fleeceperformance.com
Dunks Performance
541-726-1006
http://dunksperformance.com/
All-In Truck Performance
432-209-7095
allintrucks.com

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