In the world of diesel engines, there are a few names that are synonymous with incredibly reliable engines with equally strong cult followings. Think something like the 6.6L Duramax, International DT466, Detroit Diesel 60 Series, 3406 Cat, and so on. You get the point, there are a lot of good diesel engines out there.
One of the engines that we haven’t discussed on this channel is the Cummins 6CT, and by that name, you might think it’s closely related to the Cummins 6BT, also known as the 5.9L, but it’s more like the big brother to the B-Series engines, even though they don’t share much of anything in common.
And while it’s a great engine, just like most Cummins engines from this timeframe, it isn’t perfect, so today, we’re going to look at everything wrong with the Cummins 6CT engine.
And to preface this, it’s worth mentioning what exactly the Cummins C-Series is, the applications it was designed for, and some basic info, so you’re up to speed before we look at everything wrong with it.
Starting with the basics, we should clarify that, like the 6BT, the 6CT is really one engine in a family of engines, including the 6C, 6CT, and 6CTA. Across the board, though, these engines are incredibly similar, with the only notable differences coming in the injection systems, turbochargers, and running gear.
And to really understand this engine, we need to rewind the clock back to 1985, which is when Cummins first began production of the original C-series engines, but it wasn’t until 1998 that we saw the “ISC” version debut, which was built on the same basic design, but with loads of new features to bring it into the modern age.
In terms of specs, all C-series engines are very similar, with an inline-six cylinder design for improved reliability and simplicity, and a displacement of 506.5 cubic inches or 8.3L thanks to the bore and stroke of 5.31″ by 4.49″. The block is constructed from heavy and strong cast iron, as is the cylinder head.
Of course, with any large displacement engine with a fully cast iron construction, things start to get heavy. Like, really heavy, to the tune of roughly 1600lbs for this engine when dry. Add fluids, and that weight figure grows even larger. Keep in mind, the weight changes depending on the engine variant and application, but you get the idea. It’s a heavy engine.
The upside of using an inline-six configuration with cast iron materials is that it’s very strong, allowing this engine to output 240hp up to 400hp and 670lb-ft up to 1075lb-ft. On the surface, it’s just like many other medium-duty engines of the time, which is quite fitting considering the applications of buses, motorhomes, firetrucks, trash trucks, marine, heavy equipment, and so on.
Original variants of the C-series Cummins 8.3L are known as the 6C8.3. These earlier variants share the same basic design and construction as the later ISC 8.3L engines, but, the earlier engines were fully mechanical and quite a bit simpler.
If you’re deep into diesel engines, it’s at this point that you might notice that the 6C8.3 and 12-valve Cummins from nearly the exact same time frame are both fully mechanically and we’re both later phased out for the “ISC” variants which introduced electronics into the equation.
Of course, Cummins made other changes and updates for the ISC version of the 8.3L, including a new variable geometry turbocharger for improved power output and engine response, and the addition of common rail fuel injection in 2004 which further improved performance and reduced emissions output.
As always, die-hard diesel fans will about always opt for the fully mechanical engine, but it’s hard to deny the performance of the ISC engines. I think it’s also worth noting the similar nomenclature between the B-Series and C-Series engines with their respective ISB and ISC updates.
Another fantastic feature found on this engine is the mid-stop cylinder liners. And you might be wondering why they added these, and it’s simply to improve the ease of rebuilds, which in turn makes the engine cheaper to keep running for longer.
Other updates to the 8.3 ISC diesel include direct piston cooling, crankcase breather, and roller cams. There are more updates to list for the ISC variants, but I think you get the point. They’re on the same base, but the newer engine features lots of innovations for improved power and reduced emissions output.
With all that in mind, that takes us to the problems with this engine, which, to be frank, there aren’t all that many.
#1 Combine Hole in Block
The biggest and most catastrophic issue we see with these engines occurs almost exclusively in combines. As to why this problem appears only in this specific application set, there’s no clear answer for that.
But, the problem seems to occur anywhere from 6,000 to 8,000 running hours on the engine, when we see a large hole in the side of the cylinder block. Of course, the hole doesn’t just appear there randomly but rather is caused by a connecting rod exploding out of the side of the block.
A connecting rod failure like this can generally be traced back to a lack of lubrication at the connecting rod bearing, but again, it’s not clear why this issue happens in combines but not in other applications like motorhomes.
Part of this could be caused by combines simply having higher hours and more useful than other applications, but it could also be traced back to the combine engines having overheating issues, which then could lead to the oil temperatures getting high, and that could eventually lead to bearing failure later down the line.
#2 No KDP Issues!
On the bright side, you’d think because this engine is the “big brother” to the 5.9L that, it might suffer from the same dreaded killer dowel pin issue, but that’s actually not the case. In fact, the 8.3L is completely void of that dowel pin entirely, which means there are zero KDP issues!
#3 Insanely Heavy
Something which is a nonmechanical problem, but still considered something wrong with this engine, is the weight. And as we mentioned earlier, it’s already heavy when it’s dry. But once you add fluids, the running gear, and everything needed to get it to run, you’re looking at an engine that is nearly 2,000lbs.
As compared to engines like the CAT 3126 or International DT466, it has a couple hundred pounds on both of them.
In a medium-duty application, that’s not terrible, as the frame, suspension, and everything around the engine is built with a heavy engine in mind, but that does mean the 8.3L Cummins is a pretty bad candidate for swapping into a non-medium-duty application like a pick-up truck.
It can be done and has been done, but the sheer weight of the engine is something to keep in mind.
#4 Injection System Problems
In terms of injection system issues, this is where the earlier fully mechanical engine really shines with their P-Pump. But, we won’t get into details about how the P-pump injection system works in this article.
But for those wondering, the 8.3L and 5.9L P-pumps are very similar, but you can’t plop an 8.3L P-pump onto a 5.9L engine and vice-versa, at least not easily, because of where the port for lubrication is located.
In 1998 they switched to the Cummins Accumulator Pump System, also known as CAPS. Functionally, this is a lot like the VP44 pump found on the smaller 5.9L Cummins, which also means it suffers from nearly the exact same issues with lift pump failure and fuel filtration issues.
This injection system isn’t fully mechanical like the earlier injection pump, which means there has to be a computer to control the whole thing, which is where we find more problems.
For the ECM, the big problem is that it’s mounted to the cylinder block. That means that it is subject to a lot of vibration and a lot of heat, which ultimately can cause the ECM to fail and your injection system to ultimately to work.
Combine the issues with the CAPS pump itself and the ECM controlling the whole thing, and you end up with an injection system that is very hard to diagnose and very, very expensive to fix when it has problems.
Combine all that even further with the fact that the 8.3L Cummins was produced in tiny numbers as compared to the 5.9L Cummins, and you can see where this might be a problem. Not only is the injection system hard to diagnose correctly, but the parts are much harder to find, and much more expensive.
It is worth noting that by 2004, Cummins introduced common rail injection to the 8.3L, which is also the same year they gave the same treatment to the smaller 5.9L Cummins.
#5 Sleeve Cavitation
One of the big upsides of the 8.3L Cummins is that it uses a wet-sleeve design, which means that you can very easily remove the cylinder sleeve entirely for an engine rebuild, whereas something like the 5.9L Cummins is a sleeve-less engine, which means that cylinder damage requires boring over and using larger pistons to match.
The wet-sleeve design works by using an o-ring on the outside of the sleeve right about in the middle of the sleeve, which separates the coolant side from the oil side of the sleeve.
But, all of this means that you can and will see tiny amounts of vibration in the piston and sleeve combo. Under the right conditions, this might not be a problem, but for the 8.3L Cummins, it can lead to major issues in the form of cavitation, which will slowly eat through the cylinder liner.
Micro-bubbles are formed in the coolant due to the shock and vibration of ignition in the cylinder. Because those bubbles are caused by a vacuum, when the bubbles break, it will take a small amount of the liner with it.
This generally only occurs on engines with wet sleeves, although it can occur in other sleeve variants, regardless, it’s not too surprising to see it here on the 8.3L Cummins.
However, outside of the issues we just covered, there’s really not much else wrong with the 8.3L Cummins. Sure, it’s heavy as hell, but it’s pretty dang reliable, easy to rebuild, the p-pump models are easy to tune and get more power out of, and for applications like motorhomes and busses, you’re more likely to see the vehicle break down and fall apart well before the engine fails.
For medium-duty applications, it’s a pretty solid engine with decent power output compared to comparable engines from other companies like Cat and International.
But, because of its hefty weight, it’s a bad candidate for swapping into anything like a 1500 or 2500 truck, although it can and has been done before.
The design of the 8.3L Cummins does make rebuilding it very easy and cost-effective thanks to the wet sleeve design, but that brings concerns of its own with cavitation, although that can be band-aided with the correct coolant additives and maintenance schedule.
Unfortunately, in the world of agriculture, the 8.3L Cummins doesn’t have quite the same reputation, as the engines found in combines are well known for having issues with heat management and connecting rods exploding out the side of the block around the 6k to 8k hour mark.
All in though, it’s not a bad engine by any means. In fact, I’d say it’s a pretty good engine in the right applications. It’s definitely best suited to on-road use and offers a great balance of performance and durability.