One of the most feared powertrain failures in the modern diesel world is the CP4 high pressure fuel pump. CP4 pump failure can contaminate the entire high pressure fuel system with metal particles, destroying the fuel system as a whole. With repairs that can cost in the 5-digit range, saying that CP4 failure stings is an understatement. Fortunately, CP4 failure is easily preventable. While you cannot turn back the clock on an improperly maintained fuel pump, you can prolong the demise of a previously abused unit.
CP4 pumps can fail for a handful of reasons including air and/or water in the fuel. However, the most common reason for failure is insufficient lubrication. Lack of lube causes metal parts to wear down, not only in the pump, but in all metal-to-metal surfaces within the fuel system. Metal wear causes metal particles to contaminate the entire fuel system, thus causing more wear and more contamination. The result is a cascade of failures throughout the high pressure fuel system including the CP4 pump itself, fuel rails, fuel injectors, cam, etc. This chain reaction of failures necessitates the replacement of the entire fuel system. Step 1: remove the cab from the frame…and so it begins.
The cause of the insufficient lubrication is the fuel we use, or are required to use, in the U.S. Common domestic diesel fuel is rated as Ultra Low Sulfur Diesel (ULSD). Just as the rating suggests with no ambiguity, our diesel fuel lacks sulfur which is essential for lubrication CP4 fuel pumps. Of course, the low sulfur content was mandated to reduce emissions, but not because the sulfur is the targeted pollutant, per se. High sulfur content fuels damage the DPF systems on modern diesel engines, thus reducing their effectiveness at reducing pollutants.
Looking at the array of additives available for diesel engines, it is hard to discern which you should use and which is just snake oil. In this case, diesel fuel lubricant is definitely something you should use. Don’t fear, these lubricant additives are not just sulfur in a bottle waiting for a chance to destroy your DPF. The engineers were one step ahead of us on this one. However, just because an additive says it is low/no sulfur doesn’t mean it is free of other ingredients that can harm your DPF. Always check the bottle of any fuel lubricant to ensure it is labeled DPF-compatible. Follow the instructions on the bottle for best results, as always.
CP4 failure can be the ultimate boogeyman for modern diesel owners, but we do not have to lose sleep over it unless we choose to. Proper fuel pump maintenance from a bottle found at your local parts store is all we need to escape the CP4 failure nightmare.