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Saving Your Fuel System: The CPX Solution for 6.7L Power Stroke

The Ultimate CP4.2 For Your 6.7L Power Stroke

For more than a decade, dealerships, independent shops, and private truck owners have been dealing with CP4.2 failures. The twin-piston, high-pressure fuel pump employed on the ’11-present 6.7L Power Stroke (as well as the ’11-’16 LML Duramax) has a long history of self-destructing and wrecking the entire fuel system when it lets go. In an instant, the CP4.2, high-pressure lines, fuel rails, and injectors can all become scrap, with a tank cleaning, potential lift pump replacement, and roughly 30 hours of labor all being part of what can easily amount to a $10,000 repair bill. Aftermarket disaster prevention kits and CP3 conversions have become common work-arounds, but only recently has anyone attempted to correct the inherent flaws in the CP4.2’s factory design.

CP4.2 failure isn’t limited to the Ford 6.7L Power Stroke, nor is it exclusive to the LML Duramax, the ’19-’20 6.7L Cummins, or the 5.0L Cummins in the Nissan Titan XD. The CP4 platform, which includes the single piston CP4.1, has been plagued with failures the world-over—and especially in Europe. It’s an extensive catastrophic sequence of events, which unfolds extremely quickly and that is almost always caused by a vacuum event. Air in the fuel system, be it due to an improper fuel filter change, an insufficient fuel system purge, a gel-up in winter, or another low-pressure fuel supply issue, is the biggest culprit.
So what happens when air infiltrates the CP4.2? The air gap between the cam lobe and the roller lifter causes the lifter bucket to rotate, usually 90 degrees, which changes the roller’s parallel relationship with the cam lobe to a perpendicular one. From the factory, there is no provision to keep the lifter buckets from rotating in their respective bores.

Meet the CP4.2 Bosch should’ve built: the CPX from RCD Performance. Its lifter buckets are pinned to keep them from rotating—turned lifter buckets being the primary failure that leads to metal-on-metal contact between the roller and cam. But on top of that, a revised feed port prevents any contaminated crankcase fuel from entering the plungers and barrels in the unlikely event the pump does fail. And it gets even better. RCD Performance offers three versions of its CPX for the 6.7L Power Stroke: a stock replacement pump, which features the same displacement the ’15-’19 CP4.2 did, a 10-percent over pump for aggressively tuned trucks, and a 10.3mm stroker pump that displaces 38-percent more fuel than stock. Join us for a look at the work the company performs inside its bulletproofed CP4.2 pumps.

This is what the camshaft within a CP4.2 looks like after a lifter bucket has rotated. When the lifter bucket turns in its bore, lubrication between the roller and cam lobe is lost and both pieces effectively begin eating into each other. In very short order, hardened metal filings become suspended within the pump’s crankcase and are allowed to flow into the high-pressure chambers (where the plungers are) as well as the return side of the fuel system.
When a lifter bucket rotates inside the CP4.2, the roller begins shedding material almost instantly. The roller remains pressed into its polished follower during the melee, but that doesn’t keep it from breaking down due to the intense friction it sees when coming into contact with the cam lobe. To be sure, this roller damage pictured here is relatively minor compared to some of the gouging we’ve seen this piece endure during a pump failure.
The path to contamination (and ruination) of the high-pressure lines, rails, and injectors in the 6.7L Power Stroke’s common-rail system is highlighted here. The blue zip tie represents the port that routes fuel up to the plungers and relief valve. After being pressurized by the plungers in the high-pressure chambers, fuel routes through the high-pressure outlet bound for the lines, rails, and injectors.
Before the folks at RCD Performance set out to address the lifter bucket issue, they took a look at the CP4.2’s design and developed a simple, machined solution to help limit the damage caused when one of these pumps fails. By machining a new feed passage in the pump housing for the volume control valve (VCV), and by blocking the old path, the company has effectively been including a built-in disaster relief kit with every CP4.2 it’s sold for nearly two years now. Here, dozens of CP4.2 housings are awaiting their turn in the Doosan VCF 850LSR II 5-axis CNC station that performs the required machine work.
After machining the new port in the pump housing, a steel ball is pressed in place to eliminate the factory fuel passageway. Another way of explaining the new path for fuel is by calling it an integrated bypass for lubrication fuel. So instead of reintroducing the lubrication fuel into the VCV and plungers, now the fuel goes to the return circuit and ultimately back the tank.
Here, you get a closer look inside the new feed port in RCD Performance’s CPX pump. It’s the same diameter of the original port it’s next to, which is now plugged. The strainer you see is the inlet into the pump from the low-pressure feed circuit.
To keep the lifter buckets from rotating, RCD Performance cuts a keyway in each unit and then chamfers the edges. The open slot accommodates a high-strength, tool steel pin (shown), which requires that two machined holes be present in each CP4.2 housing.
Each side of the CP4.2 pump’s aluminum housing is machined in a precise location to accept its lifter bucket pin. Here, a light is being used (from the outside looking in) to showcase where one of two holes were located in this pump. The pins are pressed into the housing during the course of each pump’s assembly process.
Prior to pressing the tool steel pin into place within the CP4.2’s housing, its respective lifter bucket is correctly orientated and installed within the high-pressure chamber bore. As you can imagine the pin is of a proprietary length, as it can only be allowed to protrude so far into the bore before clearance issues surface.
Before the pin is sent home, it’s hit with a liberal coating of assembly lube to ease its installation. Then with the lifter correctly aligned in its bore, the pin is pressed into place. After this one went in, the housing was rotated and the second lifter bucket and pin were installed on the other side.
This is what the pinned lifter bucket looks like inside the bore. Note the protrusion (or lack thereof) of the pin itself and that there is some clearance between the pin and the keyway. Now, no matter how bad an aeration scenario gets, the lifter bucket can never rotate—even during a gel-up.
At this point, the CP4.2 is pinned and as such officially becomes one of RCD Performance’s CPX pumps. Now the rest of the pump assembly could commence. For our visit, this particular pump would become one of the company’s horsepower-friendly 10.3mm stroker units.
Most of these internals are the exact parts you’ll find in every Bosch CP4.2, namely the high-tension springs, plungers, plunger heads, volume control valve (VCV), and high-pressure overflow valve you see here. What’s different in the CPX version is the pinned lifter buckets (top left), the integrated disaster relief port (not shown), and in this specific case RCD Performance’s 10.3mm camshaft.
RCD Performance’s 10.3mm camshaft is distinctively made to match or exceed the OEM camshaft’s overall quality and long-term durability. However, the obvious reason it’s available has to do with performance. The increased stroke this cam provides means it’s capable of displacing 38-percent more fuel than a factory ’15-’19 CP4.2. With the right injector and turbo combination, the 10.3mm pump can support well north of 800-rwhp.
For customers that don’t need the flow gains of a 10.3mm pump, RCD Performance offers a direct drop-in replacement CPX that features the stock (’15-’19 model year) 7.5mm camshaft—along with the aforementioned pinned lifter buckets and integrated disaster relief port. But in between its stock and 10.3mm CPX, the company also offers a 10-percent over stock displacement version, which boasts an 8.25mm camshaft. Pictured from left to right is RCD Performance’s 10.3mm camshaft, its 8.25mm camshaft, and the OEM Bosch 7.5mm camshaft.
Continuing on with the 10.3mm CPX assembly, the front bearing support was installed following the stroker camshaft. The front bearing support is fastened in place by way of four Torx head bolts, which were torqued to Bosch’s specs after first being driven in hand-tight.
From there, the high-pressure overflow valve was installed in the back of the pump housing. It performs several jobs, the first of which is to regulate the pressure within the system. Additionally, the high-pressure overflow valve sees fuel leakage from the high-pressure chambers as well as lubrication fuel from the crankcase, which it returns to the tank.
With a new O-ring in place on each side, the plunger assemblies were installed and torqued to spec. Each of the two assemblies consists of the plunger surrounded by a high-tension spring, the spring being responsible for returning the lifter bucket back to the resting position after each compression stroke. Although many assume the internals inside a CP4.2 are all inferior given the pump’s tendency to fail, the plungers are particularly durable and rarely fail. According to RCD Performance, the Tungsten Carbide plungers will last 400,000 to 500,000 miles, if not longer.
The final step in the CPX pump’s assembly calls for a new volume control valve (VCV). Removing this metering valve and checking its screen for debris is the first step in diagnosing a CP4.2 failure. Finding the 85-micron screen full of metal is a sure sign the pump came apart—and that you may be out $10,000 or more. Luckily for the recipient of this pinned pump, that’s a catastrophe the customer will never face.
To ensure its pumps are fully tested and validated before shipping out, every unit spends time on this Ditex test bench. While here, it must pass a comprehensive list of efficiency, leakage, and performance tests. Once completed, the full test report joins the CPX in the box.
Notice the purge and deaeration parameters that are part of the CPX testing process. It’s very important in stock CP4.2 pump testing to ensure that air is never allowed to be present—which can potentially rotate the lifter buckets. Thankfully that’s not a worry with RCD’s pinned CPX pumps, but it’s still conducted as part of the company’s automatic testing procedure nevertheless. Other points of interest in the test are the Backflow Pressure numbers and the Oscilloscope test.
Along with the reliability gains you get with an RCD Performance CPX pump, their affordability arguably makes them even more appealing. A direct, stock-for-stock replacement CPX retails for $999. The 10-percent over displacement option—developed to ensure peak rail pressure is always on the table in aggressively tuned trucks—retails for $1,299. And the 10.3mm CPX, a pump that can easily support more than 800-rwhp and do so in ultra-durable fashion, retails for $1,899.

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