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5.9 Cummins repair vs. 6.7 Cummins repair – Which One is Easier?

5.9 Cummins repair vs. 6.7 Cummins repair – Which One is Easier?

The two staples of Cummins diesel engines are the 5.9l and the 6.7l models. However, neither of them is without faults. So, which of these two engines should you choose and why? We will look at common problems, maintenance, as well as some other general characteristics. Ultimately, the choice is yours, and whatever you choose, you will always want to have a reliable mechanics shop to help you out.

PCE is a San Diego company experienced with everything diesel related. We have been around for over 40 years.
Here are some facts about the two versions of the Cummings diesel engine.

Fuel consumption

In terms of fuel consumption, both of these engines have been hailed for their great fuel to energy ratio. After all, the newer 6.7l engine was based on the 5.9l, and it kept a lot of the original parts and designs. 

Since the 5.9l is an older engine, you would expect it to be somewhat less efficient. However, due to the more stringent EPA emission rules, the 6.7l has additional filters and redactors. This makes it cleaner, sure, but the tradeoff is a noticeably lower fuel economy.


On the other hand, new technologies have enabled the 6.7l to be better in other areas. In numbers, the 5.9l has around 325 HP in the stock variety but can be tricked out relatively easily. However, since it is smaller, its turbo is laggier. 

On the other hand, the 6.7l has anywhere between 350 and 385 HP, depending on the specific model. With improved turbo technology and increased torque output, it has the 5.9l beaten on lower RPMs. 


The 5.9l uses a simple fixed geometry turbo. This makes it fairly easy to fix, but turbo lag at lower RPMs cannot be avoided. On the other hand, 6.7l uses a variable geometry turbo. The main benefit comes from this is a very responsive engine with little turbo lag, even at low RPMs.

Common problems

With 5.9l being relatively simple, the most common problems on it are the lift pump and the fuel injectors. The common rail system on these engines is somewhat lacking in general, so special care needs to be paid to this. 

The 6.7l is a bit more complex machine, which means there are more parts which can go wrong. Case in point, the ERG valve, which helps with gas emissions, can get choked by dust and other particulates. Keeping it clean is important for proper functioning of the engine.

The greater torque output comes with a price, too. It comes in the form of the increased pressure on the cylinders. This causes a common issue: a blown head gasket.


The beauty of the Cummings engines is that they are very easy to upgrade. Case in point, all of the issues with the 6.7l version of this engine are easily avoided with a few choice aftermarket pieces that strengthen the faults of the mint engine. Seeing how it has become a standard, replacing the 5.9l version, chances are you will run into this version of the engine.

No matter which version of this engine you choose, and whether you decide to put aftermarket parts in it or not, you are going to need a solid mechanic. PCE has been around for over four decades, and the expertise of our employees guarantees the best possible service. Contact PCE to learn more.

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