Why the 5.9L Cummins is Overrated (Everything Wrong with It)

1989. That’s the year Cummins and Dodge teamed up. Dodge had previously experimented with selling diesel pickup trucks with Mitsubishi engines, and while the Mitsubishi 6DR5 that you could find in the D150 and D250 was a good engine in its own right, it was only a four-liter and had very limited power production, which ultimately led to it failing.

Because of the stiff competition between GM and Ford, Dodge needed something to kick their sales into high gear, which was the whole point behind offering a Cummins engine in their trucks. Today, more than 80% of RAM 2500 trucks sold use a Cummins power plant and that number is even higher for 3500 models.

The first engine in their partnership was the 12 valve 5.9L, which was then followed up by the 24 valve 5.9L. And while diesel guys will never fully agree on which engine is best, as most of them are blinded by brand loyalty, the general consensus is that the 5.9L is the best pickup diesel engine ever.

But, no engine is perfect, so it’s time to take a deep look at everything wrong with the 5.9L Cummins. Let’s get into it.

The 12V Cummins Genesis

When the 6BT Cummins debuted in 1989, it was a game-changer. Even though it was stuffed into the outdated AD Dodge chassis, it gave the Ram trucks a new foothold in the pick-up truck market. The 6BT packed a turbo, direct injection, and a whopping 400 lb-ft of torque, which exceeded anything Ford and GM were offering at the time with their V8 diesel engines.

With the new Cummins option for Ram trucks, Dodge began to see an increase in sales for the first time since the Ram’s launch in 1981.

The original 12-valve Cummins predates the P-pumped version, but it definitely set the tone in the diesel truck segment for years to come in terms of power, fuel efficiency, and long-term durability. In the first year, the Cummins engine option was equipped on over 18,000 trucks, which was nearly double what Chrysler initially expected.

One of the nice features of the 5.9L is the gear-driven camshaft system, which has since become the standard for all road-going diesel truck engines, which is also where you’ll find the biggest problem of this engine, which is the killer dowel pin, also known as KDP.

#1 KDP

One of the funniest part about the KDP issue is that not even a mechanical part of the engine. It’s quite literally a teeny tiny little metal dowel inserted into the front of the engine block that is used to align the front gear housing during initial assembly. As I mentioned a moment ago, it’s located near the geartrain on the front of the engine, more specifically it’s located above the camshaft gear and just to the left of the injection pump gear.

In another engine location, this little pin probably wouldn’t cause too much of an issue, but because of where it’s located, it can quite literally cause catastrophic engine failure. What happens is that over time, many thousands of miles and hundreds or thousands of running hours, that tiny little dowel pin can wiggle out of its hole from vibration.

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 worst-case scenario occurs, it can cause a chain reaction of the valves and the pistons violently meeting together and ultimately ending your engine’s life. In some situations, the result of this is repairable, but in most situations, it can end your engine and even the cylinder block itself.

Luckily, Cummins and Dodge figured out this problem, so it’s not really something you see on the later 24 valve engines, however, it can happen on some of the earlier 24 valve engines, just not the later engines.

The most common years where you see this problem a lot is on the 1994 to 1998 model year trucks, which is the engine with the Bosch P7100 pump. Supposedly the P7100 pump models have different harmonics compared to the earlier models with the VE44 rotary injection pump. That being said, it’s still present on the earlier engines, it’s just supposedly slightly less common.

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, which means any 5.9L from 1989 to 2002 is a ticking time bomb. That is unless you install a simple solution to keep that dowel pin in place.

This is known as 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. These types of kits are relatively inexpensive, but fairly labor-intensive, as it requires the removal of the front cover, which isn’t really that bad if you have the tools to do it but can certainly be a daunting task for someone who is new to working on their own truck.

#2 24V VP44 Issues

Moving away from the killer dowel pin, let’s take a look at two issues that you’ll find with the 24 valve engine, which is the VP44 injection pump and the lift pump. Starting with the first problem, we have the Bosch VP44 injection pump. Because of the fact that the 12v was mechanically injected and the 24v is electronically injected, the injection pumps are quite different.

Where the 12 valve injection pumps were known for basically being bulletproof and hard to kill even if you were trying, the VP44 is known to have a handful of weak points that can cause it to fail entirely. And I don’t mean to say that in a negative way, as the VP44 injection pump has its pros and is actually a very good injection pump once the weak points are addressed.

And because we all love throwing shade at the EPA, they’re partially to blame for the VP44 being added to the 5.9L, as stricter emissions standards are what ultimately forced Cummins to switch the 5.9L to electronic injection. Okay, enough complaining about the EPA. The majority of problems with the VP44 can actually be tacked back to the PSG, which is the computer you’ll find on top of the injection pump that controls the whole thing.

Similar to the dowel pin problem, with lots of miles, running hours, and heat cycles, issues with the PSG can pop up. Most notably, the soldering on the PSG’s circuit board can create issues with electrical signals, which can then result in a handful of issues, such as long cranking when trying to start an already hot 5.9L engine.

Past the PSG issues, there are also plenty of examples of the rotor seizing in the distributor part of the VP44 pump. This normally happens from surface damage due to poor de-burring during manufacturing. With the rotor seizing up, it will break the drive plate, which then leaves the VP44’s input shaft spinning, but no pressurized fuel making its way to the injectors, which means if this happens, your truck will immediately shut off and not turn back on.

That takes us to another part of the VP44, which is the diaphragm located at the distributor head inlet. This supplies fuel to the bores of the distributor shaft, and unfortunately, is a fairly common failure point on the earlier VP44 pumps due to its poor material that would crack with time and miles. Luckily, this particular issue in the VP44 doesn’t affect the later year models, as the diaphragm was reinforced to prevent this issue entirely.

#3 24V Lift Pump Failure

And that takes us to the second issue, which is the lift pump. Because of how the VP44 operates, relying on a steady flow of fuel pressure to lubricate the injection pump and keep it cool, if your lift pump fails, it can ultimately cause your injection pump to fail. Simply put, a little lift pump failure can domino into possible engine failure.

And unfortunately, lift pump failure is actually fairly common on 24v Cummins, at least until they introduced the common rail variant. The standard lift pump you’ll find on the 24v is a self-priming electric Carter lift pump, but oddly enough, it’s mounted directly to the engine block, and once again, miles, hours, heat, and vibration, can cause it to fail prematurely.

This was made even worse by the factory fix, which was to replace that aforementioned lift pump with an in-tank lift pump which ultimately didn’t provide the fuel pressure required to keep the VP44 injection pump happy.

Luckily, the fix is actually pretty simple, just install an aftermarket lift pump. Aftermarket electric pumps from Fuelab, FASS, and AirDog have proven more than capable of delivering adequate pressure compared to the original Carter electric pump or the in-tank pump.

Plus, if you’re talking a non-stock 5.9L Cummins, you’ll want a high-flow lift pump in the first place, otherwise, you’ll run into even more issues with the VP44 injection pump being starved out. If you’re on a stock 5.9L with a stock lift pump, a simple modification to monitor the health of the lift pump is to install a fuel pressure gauge, which will clearly let you know if your lift pump is supplying adequate pressure or not.

#4 53 Block

That takes us to the next fatal flaw of the 5.9L Cummins, which specifically only affects the 24 valve engine, which is the number 53 block. Cummins had multiple companies casting their engine blocks, one of which was a Brazilian company known as TUPY, which cast approximately 100,000 engine blocks.

The only issue is that the TUPY cast blocks have thinner water jacket walls as compared to earlier and later 5.9L engine blocks, which ultimately makes these blocks very susceptible to cracking, which then leads to coolant leaking and could quickly result in your engine overheating, especially in a high-load situation such as towing.

While the issue can theoretically happen anywhere on the water jacket walls, it’s most common to occur on the passenger side below the freeze plugs. If your engine ends up cracking, there is a bandaid fix known as stitching, where a machine shop will drill a hole at each end of the crack in order to stop it, however, this is really more a sketchy bandaid than a real fix, with the only real fix literally being an entirely new engine block, which, as you can imagine is not only expensive to buy but extremely expensive on labor.

Because of this, most people looking to buy a Cummins-powered truck avoid these specific trucks like the plague, and luckily they are pretty easy to identify with a big “53” cast into the driver side front of the block near the injection pump.

#5 47RE Transmission Issues

And that brings me to my last point, which takes us outside the scope of the engine and to the transmission, which as you may know, Dodge transmissions are known for being pretty bad. And more specifically, I want to look a the 47RE, which can be found in 1994 to 2002 12 valve and 24 valve powered trucks.

Problems with the transmission range from getting stuck in gear, torque converter failing to lock, hard shifts, and so much more. And frankly, a lot of the issues surrounding this transmission are simply due to the fact that its components, such as the shafts, clutches, and converters, are all much smaller than they are on competing transmissions from GM and Ford trucks.

While GM and Ford used freshly designed transmissions for their trucks, the 47RE re-uses an old design, including the use of bands to shift gears instead of using clutch packs like other modern diesel transmissions were. Really, the 47RE was out of date the second it came out, and that combined with using an older design, spells out disaster.

Starting with the main culprit for the 47RE’s poor performance, we have the low-pressure line, which is what actuates the bands in the transmission, and simply doesn’t have enough pressure in stock form to hold any decent amount of power. A stock unit will only hold around 100psi of fluid pressure, but, an upgraded valve body can hold upwards of 250psi.

And then there are the problems with the valve body pressure solenoid which often results in the truck getting stuck in first gear. The solenoid works by energizing and de-energizing to governor pressure, and the transducer sends info back to the ECU on fluid pressure and temp. The issue is when the transducer fails and sends the wrong info to the ECU, which then results in low pressure and the transmission being unable to shift up.

If the solenoid can also fail and produce incorrect amounts of pressure it can then leave the truck stuck in third gear.

Then the transmission bands also create even more issues. For example, the band on the front planetary gear set can become loose, and then that causes issues shifting from 1st to 2nd and then 2nd to 3rd as well. To put it really simply, the bands inside the 47RE are weak and very often prematurely wear out and can mess up shifting.

All of this, on top of poor electrics, all the components being too small, and more, it’s easy to see why the 47re automatic transmission behind the 5.9L Cummins is well known for being problematic and essentially the weak point of the entire truck. The manual transmissions, on the other hand, are pretty much just fine.


Without a doubt, the 5.9L Cummins best light-duty diesel engine ever. I don’t care what you say, it is absolutely more reliable than any GM Duramax or Ford Powerstroke, but to be fair, all older pre-emissions trucks are pretty reliable. But, the 5.9L Cummins is undoubtedly the most reliable, with the 7.3L Powerstroke and LB7 Duramax coming up in second and third.

That being said, it’s pretty basic and old school, so if you’re looking for something newer, cleaner, and quieter, this definitely isn’t the way to go, but that’s fine because I doubt anyone out there is getting into a 5.9L truck with the intentions of having a quiet, clean, and perfect truck.

And although it has some minor issues, once the killer dowel pin is dealt with, it’s an incredibly reliable engine, it just happens to be backed up with a lackluster automatic transmission. Besides the issues we mentioned in this article, there aren’t really any other major issues. There’s other small stuff, such as it constantly leaking oil, but nothing major.

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