Few topics start a louder argument at a diesel meet than CP4 versus CP3. One owner calls the CP4 a ticking time bomb. The next one has a high-mileage CP4 and a receipt for every fuel filter. They can both be telling the truth.
This guide skips the shouting. It explains how the two pumps differ, what your real options are, and how to decide whether a conversion is worth your money.
Built Differently on Purpose
The CP3 uses three radial pistons around an eccentric shaft, pushed by a polygon ring. Picture three people taking turns pushing on a merry-go-round hub. It’s simple, proven and fuel-lubricated.
The CP4 uses a camshaft to drive one plunger (CP4.1) or two (CP4.2) through roller tappets. Bosch built it to make high rail pressure efficiently in a compact package. The trade-off is that the roller-on-cam contact depends on a healthy film of fuel, so poor lubricity, water or air hit it harder.
| Feature | CP3 | CP4 |
|---|---|---|
| Pumping elements | Three radial pistons | One (CP4.1) or two (CP4.2) plungers |
| Drive | Eccentric shaft and polygon ring | Camshaft and roller tappets |
| Lubrication | Diesel fuel | Diesel fuel |
| Reputation | Forgiving, long track record | Efficient, sensitive to fuel quality |
| When it fails | Often wears more gradually, but can still shed debris | Can fail suddenly and spread metal through the system |
| Found on | Duramax LB7 through LMM, 2003-07 5.9 Cummins, 6.7 Cummins 2007.5-18 and 2021-24 | Ford 6.7 Power Stroke applications beginning in 2011, Duramax LML, Ram 6.7 2019-20, 3.0 EcoDiesel and others |
Did You Know? Ram used the CP4.2 on the 6.7 Cummins for 2019-20, then went back to the CP3 for 2021. GM moved from the CP4.2 on the LML to a Denso HP4 on the 2017 L5P.
Owner’s Tip: Not sure which pump you have? Read the tag. Bosch CP3 part numbers start with 0 445 020, and CP4 numbers start with 0 445 010.
Your Three Real Options
| Path | What it involves | Upside | Watch out for |
|---|---|---|---|
| Keep the CP4 and protect it | Fuel discipline, filters, water draining, possibly a protection kit | Retains factory configuration | Fuel habits have to stick |
| CP3 conversion kit | CP3 pump, drive and mounting hardware, lines, sometimes tuning | Proven pump with a long track record | Emissions legality and application-specific configuration require verification |
| Stanadyne DCR kit | Stanadyne’s own common-rail pump built as a replacement | Newer design, sold as a kit for specific engines | Application-specific fitment and verification needed |
Protection kits add a filter to the pump’s return path, aiming to catch debris before it spreads. They’re worth a look, but know their limits.
CP3 conversions are sold for the LML Duramax and the 6.7 Power Stroke. Depending on the kit, you may need a calibration change.
Stanadyne DCR kits replace the CP4 on the Ford 6.7 and the Denso HP4 on the L5P Duramax. The DCR is a two- or four-plunger pump rated to 2,400 bar.
Shop Talk: Rail pressure is the fuel pressure stored in the common rail, waiting for the injectors. It’s measured in bar or psi, and one bar is about 14.5 psi, so 2,000 bar is roughly 29,000 psi. That’s why nobody touches a common-rail line with the engine running.
When a Conversion Makes Sense: The Checklist
A conversion usually makes more sense when:
- Your CP4 already failed and you’re replacing the system anyway, and the complete fuel system already needs attention.
- The truck is out of factory warranty.
- You tow heavy, drive remote roads or buy fuel from places you’d rather not.
- You plan to keep the truck for years.
It usually makes less sense when:
- The fuel system is still under factory warranty.
- You live where emissions inspections are strict and the kit isn’t approved for street use.
- Your CP4 is healthy, your fuel habits are good and you’ll sell the truck soon.
- The money would do more as filtration and a fuel-supply upgrade.
One more truth: no pump fixes a contaminated system. If a CP4 has already shed metal, the injectors, rails and lines still need attention, whatever pump goes back on. See CP4 failure: what has to be replaced.
The Emissions Question
On a road vehicle, changes to emissions-related parts or calibrations can run afoul of federal rules, and California requires an Executive Order for many aftermarket parts used on the street. Before you buy, check whether the exact kit carries approval for your vehicle or is labeled for off-road use only. If a kit needs tuning that changes the emissions calibration, treat that as a red flag for a street truck.
Bench Notes: CP4s that arrive after a failure usually show the same story: cam and tappet damage and metal in the housing. CP3s aren’t immune either. Water and bad fuel score their internals too. The pump is never the whole fuel-system plan.
When to Send It to Us
Send your pump in if you want a straight answer on its condition before deciding. We test CP3s and CP4s and report what we find. See the CP3 page, the CP4 page, the Denso HP4 page and the Stanadyne brand page, plus the Duramax, 6.7 Power Stroke and 6.7 Cummins engine guides.
Questions about CP4 vs. CP3
Is the CP3 bulletproof?
No pump is. The CP3 has a reputation for tolerating less-than-perfect fuel, but water, air and dirt still hurt it. Treat it with the same care.
Does a CP3 conversion need tuning?
Some kits do and some are built to work with the factory calibration. Ask the kit maker, and check whether any required tune affects emissions approval.
Can I keep my CP4 and just protect it?
Yes, and plenty of owners do. Clean fuel, frequent filter changes, a drained water separator and never running the tank dry are the foundation, with or without a protection kit.
CP4, CP3 or DCR, the best pump is the one matched to how you use your truck. Get the facts on yours first. Start a Mail-In Repair or request a repair evaluation.