The CP3 and CP4 are Bosch high-pressure fuel pumps for common-rail diesels. The CP3 has a long reputation for durability and tolerance of imperfect fuel, while the CP4 (and CP4.2) was designed to be more efficient but is widely reported to be more sensitive to fuel quality, lubricity and water. When a CP4 fails, it can send metal debris through the injectors and fuel system, which is why many owners of CP4-equipped trucks consider a CP3 conversion.
CP3 vs CP4 at a glance
| Factor | CP3 | CP4 / CP4.2 |
|---|---|---|
| Design | Radial three-piston pump | Two-piston design with roller tappets (CP4.2) |
| Reputation | Durable, tolerant of fuel quality | Efficient, but sensitive to fuel lubricity and water |
| Typical failure | Gradual wear, low pressure | Can fail suddenly and contaminate the fuel system with metal |
| Common trucks | 5.9 and many 6.7 Cummins, earlier Duramax | Later Duramax and Power Stroke engines, and the newest Cummins |
Pump use changed by model year and engine version, so confirm which pump your truck has before buying parts.
Why CP4 failures are so expensive
When a CP4 wears internally, metal particles can travel downstream into the rails and injectors. A proper repair then means replacing the pump, the injectors, the rails and lines, and cleaning the tank and supply system, rather than the pump alone.
How to protect either pump
- Keep supply pressure healthy with a good lift pump; see diesel lift pump.
- Change fuel filters on schedule and drain the water separator; see fuel filter.
- Buy fuel from busy stations and watch for water-in-fuel warnings.
- Investigate rail pressure codes promptly: P0087, P0088, P0089.
Signs of a failing injection pump
- Hard starting or long cranking, especially when warm
- Loss of power and reduced-power mode under load
- Low rail pressure codes and pressure that falls away when accelerating
- Metal glitter in the fuel filter, a serious sign on any common-rail pump
Stop and diagnose at the first metal-in-filter sign, because continuing to run can spread debris to the injectors.
CP3 conversions
Conversion kits replace a CP4 with a CP3 using brackets, drive parts and plumbing designed for the engine. They are popular on CP4-equipped trucks, but cost and installation complexity vary, and the engine calibration may need attention; check that any kit suits your exact engine and year.
CP3 problems
The CP3 is not immune: low supply pressure, dirty fuel and age wear it out, causing hard starts, low power and low rail pressure. See CP3 pump, 6.7 Cummins problems and 5.9 Cummins problems. If a pump failure has damaged the engine, see our Cummins engines.
More guides: bad diesel injector symptoms.
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