You know how RAM guys always swear up and down that the Cummins engine in their truck is the best thing on earth since sliced bread and that it’s totally indestructible and basically a gift from God himself? Well, let’s try to flip that on its head. Yes, Cummins engines are very good, I think there is no debating that, but let’s take a look at everything wrong with them, and more specifically, let’s look at everything wrong with the 6.7L Cummins.
Before we get deep into this, I want to mention that we won’t be looking at any transmission problems related to the 6.7L Cummins in this article, rather we’ll be looking at just the engine itself and its problems. If you’d like to see another article on why Cummins automatic transmissions have been incredibly problematic until recent years, be sure to drop a comment down below letting me know!
With that out of the way, let’s look at the most problematic parts of the 6.7L, starting with the components that everyone loves to hate, which are the emissions systems and more specifically the EGR system.
#1 EGR Cooler
Starting with the EGR Cooler itself, we see nearly the same problems that you’ll find on a Duramax or Powerstroke engine with their EGR Coolers, and that’s the whole thing getting clogged up from excessive soot.
Now as a quick refresher, the EGR cooler has one job, and that’s to cool exhaust gases down before they enter back into the intake. Otherwise, you’d have blazing hot exhaust gases entering the engine and that would cause a whole host of other issues on top of being really bad for performance.
For those who don’t know, the main function of the EGR cooler is to lower combustion temperatures by basically diluting the air/fuel mixture with a small amount of exhaust gas. With this in mind, it’s easy to understand why an EGR cooler is needed because cooler exhaust gases will help lower combustion temperatures more than hot exhaust gasses would.
The whole point of this is to reduce NOx emissions output, and you might think that introducing exhaust gas into the intake would drastically reduce performance, but it’s actually only a 1% or 2% reduction in power, so the difference is pretty much imperceptible, so no worries there.
Luckily, as compared to something like a Powerstroke, the 6.7’s EGR Cooler is actually an easy-to-fix problem. While some other engines have EGR coolers crack or overheat from other coolant or oiling issues, on the 6.7L it’s really just an issue of it getting clogged, which means it’s super easy to fix or prevent entirely with semi-regular cleaning of the EGR Cooler.
It also helps to drive the truck hard and limit idle time. With lots of idle time, exhaust gas temperatures will remain super low, which means soot can stick to everything in the EGR system without being burned off, which then ultimately leads to clogging. With hard-driving or even driving in a heavy load scenario such as towing, you’ll be able to get your EGTs high enough for enough time to burn off the excessive soot in the EGR Cooler to prevent the clogging from happening in the first place.
#2 EGR Valve
That leads up to the next EGR system part which is prone to having issues for literally the exact same reasons, and that’s the EGR valve. This little valve is responsible for controlling the flow of the entire EGR system. As you can imagine, if the EGR system was kept wide open in all situations, you’d run into issues such as too much exhaust gas entering the intake idle, to the point that it could cause your truck to stall or idle rough.
And that’s exactly what can happen when the EGR valve gets stuck or breaks. And again, this is almost entirely from excessive soot buildup, which generally occurs on trucks with a lot of idle time or trucks that are driven very lightly. Luckily this can be solved with scheduled cleaning, and even once it gets stuck, it’s often fixable with some cleaning. In worst-case scenarios, you’d replace the valve.
Better yet, drive your truck hard or tow more often to help burn off the excessive soot in your EGR system to prevent these issues from happening in the first place!
#3 DPF issues
That leads us further down the emissions systems pipeline, far away from the EGR system and the intake system, deep down into the exhaust system, where we have the diesel particulate filter. And again, pretty much all diesel engines after midway through 2007 received this system, so this isn’t a Cummins specific problem, it’s just what diesel engines have to deal with thanks to the EPA introducing regulations.
The job of the DPF is simple: capture excessive exhaust soot and then burn it off occasionally through a regeneration mode. It does the regeneration through two modes: an active regeneration mode and a passive regeneration mode.
Passive regeneration occurs when the exhaust temperatures are naturally high enough to oxidize the soot collected in the DPF faster than the soot is collected. This typically occurs when the temperature of the DPF is above 300°C or 572°F. This occurs during highway driving or when driving with heavy loads.
Active regeneration occurs when the exhaust temperatures are not naturally high enough to oxidize the soot collected in the DPF faster than it is collected. This works by injecting a small amount of diesel fuel into the exhaust stream, which is then oxidized by the diesel oxidation catalyst.
Unfortunately, though, it’s fairly common for the DPF to be permanently clogged up with time, which can then force your truck to active regen more often, which then leads to even more issues. And what’s even worse is that a new diesel particulate filter isn’t cheap. They’re generally anywhere from $1500 and up.
And unfortunately, the DPF related issues don’t stop there, because for some odd reason Cummins decided to use the late injection method for their active regeneration method, which means they’re injecting fuel in the cylinder during the exhaust stroke as a way to inject raw fuel in the exhaust system to heat up the DPF enough to burn off the soot.
As you can imagine, spraying diesel fuel into the cylinders during the exhaust stroke, but not igniting the fuel, can cause some issues, most notably, not all of that fuel is going to make it out of the cylinder and into the exhaust system, which means you have small amounts of diesel fuel washing down the cylinders and into the engine oil.
This causes oil dilution, which ultimately means a decrease in lubricity for your oil, especially with modern diesel fuel which doesn’t have nearly as much lubricity as the old high-sulfur diesel fuel. With less engine lubricity can come accelerated engine wear and the need for more frequent oil changes.
GM and Isuzu managed to get around this issue with their Duramax trucks by using the 9th injector method rather than the late injection method, which means they quite literally have a fuel injector on the exhaust system as a way of getting fuel into the exhaust system, which completely eliminates the cylinder washing and oil dilution issues.
Of course, the solution to all of the EGR and DPF issues is to delete the systems of your truck but be warned, it’s not legal. Sure, some states, cities, and counties don’t require emissions testing to register your vehicle, so you might think it’s legal to delete your truck’s emissions systems, but just because your local government doesn’t care, doesn’t mean the federal government.
In fact, it’s a federal crime to modify or delete vehicle emissions systems, which means you can face a pretty big fine and/or jail time. So, just keep that in mind. The EPA is regularly coming after companies who sell emissions delete tunes and devices, so tread lightly with this stuff.
#4 Turbo Failure / Sticking VGT
That takes us to the last soot-related issue, which again, affects pretty much all diesel trucks with similar parts, and that’s the turbocharger. The 6.7L Cummins uses a variable geometry turbocharger which uses internal vanes to effectively change the size of the turbocharger, which means it simply has better response and better power as compared to a standard fixed geometry turbocharger.
Unfrtouantly though, soot can clog up everything in the turbocharger and cause the turbo vanes to stick. When this happens, you’re either stuck with effectively a small and good responding turbo, or an effectively big turbo with bad response. Like the other soot-related issues though, this can generally be fixed or prevented with hard-driving, limited idle time, and occasional cleaning.
#5 Head Gaskets
Next up, we have head gaskets, which isn’t something that was ever an issue on the older 5.9L Cummins, so it’s a bit of a surprise to see it on the 6.7L.
If you poke around online, you’ll see people blaming all sorts of things, from the way the heads were milled, to the regen cycle cranking up cylinder pressure, to the turbo sticking and creating excessive cylinder pressure, increased power as compared to older Cummins engines, emissions systems increasing the load on the coolant system, and much more.
Realistically, there could be a ton of different things that cause this issue, but there doesn’t appear to be a singular cause. The solution, of course, is to upgrade to head studs rather than head bolts. You can also do little things like adding fire rings to the cylinder head for improved sealing.
While you’re doing that work you can also address some valvetrain weaknesses such as the factory pushrods which can very easily wear out prematurely or even bend. While the head is off you can also ditch the factory pressed in freeze plugs for some thread-in plugs to eliminate that possible issue down the road.
All of that combined with a nice multilayer headgasket can pretty much entirely solve the head gasket or head lifting issue. Again though, there doesn’t really appear to be a singular cause for this issue, rather it’s a multi-source problem that can just be prevented with the aforementioned upgrades to the head bolts and head gasket itself.
#6 CP4 Injection Pump Failure
The very last issue of the 6.7L Cummins only affects the 2019 and 2020 model years, where the engine was strangely switched from the Bosch CP3 injection to the Bosch CP4.2 injection pump. If you remember from our Duramax Guide article, we saw the LML Duramax also use this CP4.2 injection pump and it was also very problematic in that application as well. You’ll also find this injection pump in the 6.7L Powerstroke.
It’s arguably the most popular injection pump available for manufacturers to use, but it’s also known for being quite problematic. If the pump self-destructs, it will send metal fragments through the lines and rails, into the injectors, and then out the return system. Detecting this failure before it happens is basically impossible, and by the time it’s through, you’ll need to replace the pump, lines, rails, injectors, and more.
The problems typically occur when contaminants get into the fuel. The whole reason that manufacturers chose the use the CP4 pump over the CP3 pump is production cost and weight. The CP4 pump is much lighter thanks to its aluminum construction and also cheaper. Typically these pumps see failure at or before the 100k mile mark, depending on the quality of the fuel and how well the truck is maintained.
While there isn’t really a singular source of the problems with the CP4 pump, a lot of it revolves around debris and metal shavings from cam erosion. Those metal shavings are eventually passed through the whole fuel system and as time goes on, it’s really only a matter of time until something fails entirely.
Luckily though, Cummins realized that this injection pump wasn’t as reliable as the previous CP3 injection pump, so for the 2021 models they switched back to the CP3 pump, and the CP4.2 pump was recalled by RAM, and ironically the fix is to revert those recalled models back to the CP3 pump.
Summary
So, that’s every major issue found on the 6.7L Cummins. I’m going to make a bold statement here: The 6.7L is arguably as reliable as the 5.9L once deleted. The emissions systems are the source of the majority of problems. And once deleted, you could really make a case that the 6.7L is better than the 5.9L, as the larger displacement and better factory turbos allow it to make an insane amount of power with small bolt-on parts and tuning.
Regardless if you think the 5.9L or the 6.7L is better, there is no denying that the 6.7L Cummins has been one of the most reliable engines on the diesel market since its release. There really aren’t many problems with these engines and most of them go their entire life without experiencing any major failures.