Everything Wrong the with Cummins 3.9L 4BT

1983. This was the first year we saw the 4BT Cummins go on sale. It was initially marketed as an industrial and commercial engine, with primary applications aimed at off-highway construction, agriculture, marine, and so much more. It truly was and is a very versatile little engine that can do nearly anything you ask of it.

Because of its versatility, reliability, performance, and, most importantly, its size, the 4BT Cummins has become pretty popular in the automotive world as an engine swap for Jeeps, trucks, buggies, rock crawlers, and even the occasional little sporty car.

That being said, no engine is perfect, and while the 4BT Cummins is a very reliable engine, it’s been put on a pedestal by Cummins fans, much like the 12V and 24V Cummins. So, it’s time we ignore all the good things about this little engine and instead look at everything wrong with it. So, let’s get into it.

4BT Versions and CPLs

Okay, getting straight into it, I want to mention that there are multiple versions of the 4BT Cummins, as it was used in a wide variety of applications, many of which didn’t drive a vehicle but rather power generators, massive pumps, and other weird commercial applications like that.

With that in mind, we should quickly cover some of the differences between all the variants. This mostly boils down to the CPL number, which stands for “control parts list,” and that number is related to the specific components used on the engine during initial assembly, including emissions controls, injectors, injection pump, turbocharger, and more.

Despite the fact that there are 34 CPL number variants for the 4B Cummins, there are really only four versions used for automotive applications, which are 1839, 2001, 2195, and 2304. And while there’s a handful of differences between the off-road and on-road engine variants, a lot of it simply comes down to the injection pump’s governor, which limits RPM, as well as a different oil pan since the automotive applications have to clear either an axle or subframe. 

And much like the 12-valve 6BT Cummins, as we’ve covered in other videos, the 4BT has two different injection pumps to pick from based on the year and application of the engine, and they’re very similar to the 6BT injection pumps. These are the Bosch P7100 pump, also simply known as the P-pump, and the Bosch VE rotary injection pump.

It’s worth noting that there are some CPLs that have a completely different pump, CPL 1963, which uses a Bosch P3000 pump which a power output of 130hp.

With some of that basic info out of the way, let’s take a look at everything wrong with the 4BT, starting with the biggest and most problematic of them all, which is the killer dowel pin, which is quite literally the exact issue you see on the 12 valve Cummins, as the front of the engine is identical between the two.

#1 KDP

And while we’ve covered the KDP issue pretty in depth in our Everything Wrong with the 5.9L Cummins video, it’s worth reiterating here for those who don’t know about this issue. And the funniest part about the KDP issue is that it isn’t a mechanical part of the engine, or electrical, or anything like that. Rather, it’s basically an initial assembly defect.

Generally speaking, when any particular engine is being assembled when new, the manufacturer uses dowel pins to help align components properly to ensure proper and consistent assembly. One of those is a small metal down on the front of the 4BT, which helps align the front gear housing. This particular dowel is located above the camshaft gear and near the injection pump.

It’s because of this location that the dowel pin can cause catastrophic engine failure and how it earned the KDP nickname. The 4BT, much like the 6BT engine, vibrates a lot. In fact, the 4BT vibrates significantly more than the 6BT because it’s a four-cylinder. What happens when things vibrate a ton? Other parts become loose.

After hundreds or thousands of engine hours, there are a lot of heat cycles and engine vibrations throughout that time which can and will ultimately wiggle loose anything that’s not tightly secured to the engine. While this pin is pressed into place, all the vibration over significant amounts of time will wiggle it loose.

In the ideal scenario, the pin will wiggle out, possibly bounce off a few things and then land at the bottom of the front cover, which then will hopefully settle at the bottom of the oil pan. Unfortunately, that ideal situation doesn’t always happen, and the flip side of the coin is the pin falling out and getting lodged between the cam gear and injection pump gear.

If this happens, it can and will shred apart your engine, starting with the pistons and the valves meeting together in a rather violent fashion, as well as destroying many of the gears in the geartrain system.

And because of how this issue occurs at random with miles, hours, heat cycles, and vibration eventually causing it to wiggle loose, there’s no predicting this problem. Luckily, there are steps you can take to ensure this issue doesn’t occur, namely a KDP Disaster prevention kit, which is basically just a tiny metal plate used to keep the pin in place, and that plate is held in place by the bolt right next to the dowel pin.

But, because the killer dowel pin can damage your cylinders if the worst-case scenario occurs, that takes us to the next problem with the 4BT, which is the fact that it doesn’t have cylinder liners.

#2 No Cylinder Liners

The issue with running zero cylinder liners is that any damage to the cylinder walls results in the need for boring over the cylinders to a larger displacement. That’s not a terrible thing for performance or anything like that, but if there are only so many times, you can bore out a cylinder before you run into some major issues.

If you look online, there appear to be plenty of cylinder sleeve repair kits that effectively add a sleeve if there is ever a situation in which you cannot bore out the engine to properly repair the cylinder walls. It’s not that big of an issue at the engine of the day, but on an engine with so few problems, it’s worth noting.

#3 Cracked Head and Valve Seats

And for the next problem, it’s either very under-reported or over-reported, depending on who you’re asking, and that’s cracking with the cylinder heads. I want to say right here, off the bat, that cylinder head cracking issues on the 4BT are supposedly very uncommon and only account for a few perfect of all 4BT engines.

The generally occurs between the injector bore and the valve seat. The rule of thumb is that if the crack does not extend into the valve seats, then the cylinder head is reusable. If the crack does extend into the valve seats, the head has to be repaired and the valve seat replaced.

As to why this cracking problem occurs, it’s not exactly clear, but there’s speculation that the problem is caused by the use of 9mm injectors, and supposedly switching to 7mm injectors can eliminate the problem entirely or at least minimize the problem.

#4 Price and Performance

Aside from the mechanical and structural issue we’ve mentioned thus far in the video, I think it’s worth highlighting that the 4BT Cummins is one of the many engines you’ll find popularized on the internet that has become victim to its own popularity, and what I mean by this is that the 4BT is plain and simple massively overrated.

We’re talking about a 3.9L engine that will rattle your teeth out, barely makes 130hp stock and barely makes 180hp with simple bolt-ons, weighs over 750lbs, and costs upwards of $16,000 USD for a rebuilt example. I’m sorry, but that kind of money for a heavy, old, leaky engine that barely makes enough torque to keep itself alive is embarrassingly bad.

And I know there are a lot of you out there who are about to run to the comments saying that it’s worth that high price tag because of reliable and awesome it is but consider the fact that you can literally buy a new R2.8L Cummins directly from Cummins as a crate engine for the same price as a rebuilt 4BT.

Or the fact that you can buy a rebuilt 6BT Cummins for under $7,000. Now, it’s worth highlighting that not ALL 4BT Cummins is that expensive, as you can find long blocks around $4,000 USD and then transfer over any parts you need to. But the point still stands, for the weight and size of the engine on top of the embarrassing power figures, it’s an incredibly expensive engine, and frankly, it’s massively overrated.

But the same can be said about nearly any engine that has been popularized and placed on a pedestal on the internet. Hell, even something like an L92 engine has gotten ridiculously expensive, so this problem isn’t necessarily exclusive to the 4BT Cummins.


Other than what we have listed here, there really isn’t much else wrong with the 4BT Cummins. Sure there are random little issues, but really nothing majorly wrong with the engine outside of the KDP issue. Mechanically, it’s an incredibly strong little engine. At the end of the day, it’s a 6BT 5.9L Cummins than had two cylinders chopped off.

You can quite literally take a 5.9L exhaust manifold and modify it to fit a 4BT by chopping two cylinders off the manifold. Same with the intake plate; you can take a 5.9L plate and shrink it down accordingly to fit. This is a great way to add a 5.9L grid heater to improve cold starts, as many 4BT CPLs didn’t come equipped with a grid heater.

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