If you own a 2017 or newer L5P Duramax, there is a part on your engine that can take the entire fuel system with it when it lets go. It is the CP4.2 high pressure pump, and it is the same basic pump that gave Ford owners nightmares. The L5P has been more reliable than the early Ford application, but more reliable is not the same as safe, and when a CP4 does fail it does not fail quietly.
Why the CP4.2 Is a Problem
The CP4.2 runs with very tight tolerances and very little lubrication margin. Our diesel fuel is drier than what these pumps were designed around overseas. When the internals lose lubrication, metal starts rubbing metal, and the pump grinds itself into shavings. Those shavings do not stay in the pump. They get pushed downstream into the injectors, the rails, and the lines, and once that metal is in the system the only real fix is to replace nearly all of it. That is a bill that runs into the thousands.
The Fix: Convert to a CP3
The CP3 is the pump the older Duramax and Cummins trucks ran, and it is one of the most proven high pressure pumps ever put on a diesel. It is tougher, it tolerates our fuel, and when it does eventually wear out it does it gracefully instead of sending shrapnel through your injectors. Converting an L5P from the CP4.2 to a CP3 removes the single biggest catastrophic risk in the fuel system, and it does not cost you anything in drivability.
| Factory CP4.2 | CP3 Conversion | |
|---|---|---|
| Failure style | Catastrophic, sends metal downstream | Gradual, contained |
| Tolerance for dry fuel | Low | High |
| Worst case repair | Full fuel system replacement | Replace the pump |
| Power headroom | Limited | Plenty |
What to Run
For the L5P trucks the kit we point people to is the S&S Diesel 2017-2025 Duramax L5P CP3 Conversion Kit. S&S has been doing these conversions longer than almost anybody, and the kit comes with what you need to do the swap right the first time. If you are on an older 2011 to 2016 LML truck, that platform has its own version, the S&S LML CP3 Conversion Kit, built for the same reason.
Who Should Do This
Every L5P owner should at least know the risk. If you tow with the truck, if you depend on it, or if you are adding any power at all, the conversion is the move. You are trading a part that can fail and cost you the whole fuel system for a part that has been proven on these engines for two decades. Do it before the pump makes the decision for you.
Shop the L5P CP3 Conversion Kit →