How to Convert LML Duramax CP4 Injection Pump to CP3
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.
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.
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.
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.