The Bosch CP4 high-pressure fuel pump used in many Ford, Chevrolet, Ram, and Nissan diesel engine applications is notorious for failures. It’s not a matter of if, but when. Some replace them with the earlier CP3 pumps, and there are aftermarket pumps that correct the problem. But there is also a bypass kit that adds to the CP4’s longevity. But more importantly, protects your engine from the inevitable shrapnel that will destroy other parts of the engine, making for a very costly fix. 

Why do diesel CP4 fuel pumps fail?

Bosch CP4 pumps are not able to function over time because they were designed for European diesel fuel. Diesel fuel in the U.S. is dryer. That means it has fewer lubrication properties which are vital for the pump to live. The list of diesel engines it is in is long, taking in Ford Power Stroke, Ram HD, Chevrolet Duramax, Nissan Titan Cummins applications, and even several BMW vehicles. 

Some of those manufacturers have issued recalls to replace the CP4. Others are part of several class action lawsuits. So it is an almost industry-wide problem that some manufacturers, like Ram, are taking care of themselves, while others, like GM, are letting play out in court. 

Why is CP4 fuel pump failure so expensive?

Once a CP4 fuel pump starts its inevitable disintegration, the bigger problem is the debris that gets into the entire fuel system including the cylinders themselves. Built-in filters only work so well, and become overburdened as the pump continues to consume itself. But there is something you can do to save your engine as the fuel pump dies. 

A bypass system directs the low-side fuel at the MPROP or FCA into a bypass filter canister immediately after the pump itself. Then the clean fuel is directed back through the MPROP and through the fuel rails, injectors, and into the cylinders. The MPROP controls the fuel pressure going into the injectors. 

What are CP4 bypass kits?

If you wait until your truck stops running, you’re past the time you need to install a bypass system into the truck. Towing in and replacing the CP4 pump and adding the bypass system won’t get your truck back running. Instead, you’re now facing an injector, fuel cooler, fuel rail replacement, and possibly more. 

These bypass system kits are available for all of the affected diesel engines sold by Ford, Chevrolet/GMC, Nissan, and Ram. They include all hardware, hoses, and (usually) detailed instructions. Several instruction videos are available on the interweb, for the DIY-ers out there. 

How hard is it to install?

It is a fairly straightforward repair involving removing components like the air box and upper/lower intake manifolds. If any modifications are necessary to components like the upper or lower intake manifolds, it is extremely minor. These mods are for slight clearance problems and don’t involve much more than using a Dremel tool or die grinder. 

Again, these kits won’t keep your CP4 fuel filter from failing. It will fail. But it can be the difference between spending $2,000 to replace it, or $10,000 to replace your entire fuel delivery system.