Common Problems: The CP4 Time Bomb

December 2017 Tech Corner, Feature Trevor Mason

It’s always frustrating to finally get your hands on a brand-new truck (or at least, new to you) and find out there’s something wrong with it. It’s even more frustrating to learn that not only are you not alone in your suffering, but that it’s a common problem to your vehicle. I bet that every single one of you reading this had at least one or two things about your own truck pop into your head that you wish the manufacturer would fix.

Every generation of every major diesel truck has at least a few things that immediately get replaced upon purchase; at least, you should take a long, hard look at these things before you get too far along in your ownership. We wanted to start looking at these common problems and give you advice on what to do to take care of it. Check back every month for a brand-new installment. And if there’s something you think we absolutely have to cover, let us know!

CP4 Pumps

To kick things off, we’re going to look at something that’s very near and dear to our hearts: the CP4 injection pump on 2011-present LML Duramax engines. Boy, where to begin? People have taken a somewhat hyperbolic approach and refer to the CP4 as a time bomb, among other colorful terms. The thing is, they’re not too far from the truth. Even if you have a 100 percent stock pickup, there’s a [really] good chance that you’re going to be on the receiving end of a $10,000 bill when it finally goes out on you and destroys your entire fuel system.

Cost Savings?

So what’s going on here? Well, as with most things, the blame lies in the fact that the CP4 replaced the old reliable CP3 as a cost-saving measure. See, the newer model trucks have more efficient piezoelectric injectors, which means they require less fuel to run properly. Sounds good, right? The problem is that with less fuel volume comes less lubrication of the components in the pump. Adding to the trouble is the fact that the ultra low-sulfur diesel we have in the US has less lubricity than elsewhere in the world.


Another contributing factor is that stock Duramax engines don’t have lift pumps. Lift pumps help supply fuel directly to the injection pump, which then sends it on to the engine. This decreases the amount of work the injection pump has to do, which obviously leads to longer pump life. All that is a long way of saying that the injection pumps on Chevys have to do all the work with worse lubrication. Even a CP3 pump will have a hard time providing the necessary fueling without a little extra help. Without a lift pump, the fuel is under constant suction, which causes undue wear and tear on the pump and the injectors, as well as cavitation. Quick side note: besides being a word that my word processor doesn’t recognize, cavitation is defined as the formation of empty space within a liquid because of a propeller. In layman’s terms, it means air bubbles that show up because of the speed of the fan. It shows up more commonly with boats, where the propeller is the means of propulsion, but the same concept applies to any kind of fan-driven pump.


Add all these things up and it’s literally just a matter of time until your CP4 pump fails. As I said before, it can be catastrophic when this happens, because not only will it destroy the injectors, but it will leave metal shavings in your fuel lines (which is impossible to fix without replacing the entire system) and can even in rare cases crack the gear and throw it through the engine’s front timing cover.

At A Minimum

What’s a LML owner to do? If you don’t quite have the scratch for any serious upgrades, the only thing you can really do is use fuel additives that increase its lubricity. That’s nothing more than a stop-gap, though. To actually solve the problem, you have to look to the aftermarket.

Replacement Options

The most popular way to address this issue is to simply replace the CP4 with a CP3. Many companies, such as Fleece, BD Diesel, and Dynomite Diesel offer replacement kits that come with everything you need to change out the offending CP4 with a stronger, more reliable CP3. Some companies even offer the solution of a dual fueler kit, which is simply a CP3 that sits alongside the CP4 to help out with pumping duties. This is more popular with Power Strokes, though, so it may not work as well with a Duramax.

Give It A Lift

As always, the addition of a lift pump from companies such as AirDog, aFe, FASS, and Fuelab will help your situation considerably. Two heads are better than one, after all. It’ll provide constant positive fuel pressure as well as 100 percent positive fuel to the engine. Additionally, lift pumps have built-in water separators and fuel filters. Because the lift pump is pulling the fuel first, the injection pump isn’t cavitating and working overtime to pull fuel from a vacuum, which will increase its longevity.

Costs Involved

Price is obviously an issue for all this, and the cost is going to fluctuate based on how serious you get. As stated, simply adding fuel additives is the cheapest option, with prices ranging from around four dollars for an eight-ounce bottle up to 50 bucks for a gallon. Once you start adding components to the engine, prices go up considerably. CP3 conversion kits will run you anywhere from $600-$3000, depending on whether the kit actually includes the pump. Dual pump kits range between $2000-$3000, depending on whether you want a stock pump or a modified one. Lift kits are comparatively cheap, coming in around $600-$800.


Once you’ve decided what you’re actually going to do, the last issue is the installation. Fluids are a cinch, obviously, but anything else will probably require the aid of a mechanic. We were recently on hand when Matt Rosenberg replaced the CP4 of our EPA-friendly LML, and boy was that a pain in the ass. Lift pumps are a relatively straightforward install, but will still be a fair amount of work. “Straightforward” doesn’t necessarily mean “easy.”

Now that you’re equipped with this knowledge, it’s time to make a decision. Do you wait it out and hope you get lucky or do you defuse the time bomb?


aFe Power




BD Power


Dynomite Diesel Products


FASS Diesel Fuel Systems


Fleece Performance Engineering




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