The 4 Most Common Cummins 5.9L Engine Problems

The 5.9 Cummins 12v was known for its bulletproof reliability. However, Cummins had to modernize the engine and add newer technology to keep up with the diesel power race of the 90’s, bringing the new 24v common rail engine.

The switch from a 12v to 24v and to common rail fueling lead to a number of problems with the fueling system on the 5.9 Cummins 24v engine. Furthermore, it suffered from some serious issues with the block cracking and some less serious issues with the exhaust manifold and accelerator sensor.

I’m going to discuss overall reliability of the 24v engine and discuss which of the below problems are actually problems based on my experiencing owning and modifying these engines.

Cummins 5.9L 24v Engine Problems

• Fuel Lift Pump Failure
• Leaking Fuel Injectors
• Exhaust Manifold Leaks
• Accelerator Pedal Position Sensor Failure
• Bonus: Engine Block #53 Cracking

If you would rather consume this content via a video, check out our 5.9 24v Cummins Common Problems video below:

1. Lift Pump Failure

1998 to 2004 model year 24v’s experience common lift pump failure. These early years had their lift pumps attached to the engine block. Because of this, they were: (1) subject to a lot of excess heat, and (2) had to pull fuel a very long distance. This created a lot of undue stress on the pump, ultimately resulting in premature failure. 2005+ model years relocated their lift pumps to inside the gas tank which greatly improved reliability.

The bigger issue is that the lift pump tends to take the VP44 injection pump out with it. The later model CP3 injection pumps hold up better and do not fail as frequently, making the 2003+ models with the CP3 a bit more reliable, with 2005+ being the most reliable.

Failure Symptoms

• Engine misfires
• Lean AFR’s
• Rough idling and poor performance
• Hard starting or engine stalling while running
• Boost below target
• P1693 fault code

Lift Pump Replacement Options

Aftermarket lift pumps are very common among Cummins owners. While aftermarket systems can get rather expensive, most will relocate the pump closer to the gas tank. By doing so, the issues with the engine heat and pull distance are mitigated. With that being said, given the cost of replacing the OEM pump, you’re better off with the added reliability (and performance) created by aftermarket systems.

2. Leaking Fuel Injectors

Fuel injectors are known to fail around the 150,000 mile mark. The fueling system is the most problematic part of these engines, although they did tend to affect the VP44 versions more. The injectors are just naturally prone to leaking but dirty fuel is also a common cause for them failing as early as they do.

Faulty Fuel Injector Symptoms

• Hard start or no start
• Cylinder misfires
• Fuel in the engine oil
• Poor idling and performance, surging, etc.

Replacement Options

Unfortunately fuel injectors run $300+ per injector….which equates to nearly $2k per set without install. When fuel injectors fail, they fail one at a time but we usually recommend replacing all of them.

Your two options are: replacing the injectors with an OEM set, or running upgraded performance injectors. If you’re trying to make more power, get an upgraded set. Otherwise, OEM is fine and will be a bit cheaper than upgraded injectors.

3. Exhaust Manifold Problems

Unlike it’s competition, Cummins uses a inline engine, meaning all 6-cylinders are in one line. With 6-cylinders and 5.9-liters of capacity, the engine block is very long. And therefore, the exhaust manifold is also very long as there is one manifold connecting all cylinders.

The engine block is made of cast iron. Cast iron is very rigid and inflexible, which causes the block to actually expand as the engine heats up. While the block will only expand by a fraction of an inch, the constant expansion and contraction causes a lot of stress on the exhaust manifold. As a result, the manifold (also made of cast iron) can crack.

When the manifold cracks air leaks out, pressure is lost, and the engine loses vacuum.

Cracked Exhaust Manifold Symptoms

• Loss of performance, poor idling
• ARF running rich
• Boost below target
• Loud noises coming from the manifold, noise increases with RPM’s
• Engine misfires

One the manifold cracks, it’s only a matter of time before the crack worsens. The only option here is to replace the full manifold and to do it quickly to prevent further engine damage.

4. Accelerator Pedal Position Sensor (APPS) Failure

The accelerator pedal position, referred to as APPS, is responsible for telling the ECM how depressed the pedal is. This controls throttle and tells the ECM to open or close the throttle body, affecting engine RPM’s. When the sensor fails, the ECM doesn’t receive a signal from the pedal, therefore not knowing what to do with engine speeds.

This problem is mostly limited to 1998-2004 5.9 Cummins diesel engine. When the sensor fails it usually sends no signal to the ECM. However, it can occasionally flash back on and send a signal to the computer which can cause the engine to surge or lunge forwards. Driving with a failed APPS will be nearly impossible and certainly dangerous.

Failure Symptoms

• P0121, P0122, P0222, P0223 Engine Codes
• Pedal goes dead or non-responsive
• Engine surging

Replacing the accelerator pedal position sensor is a pretty simple DIY. The APPS sensor is around $100-250 depending on whether you get an OEM or aftermarket replacement.

Bonus: #53 Engine Block Cracking

This problem is only common for a small number of 5.9 Cummins so I’m adding it as a bonus as you should beware of the issue if you are looking at buying an older 24v. From 1998.5 to 2002, Cummins had two manufacturers producing their engine blocks: TUPY in Brazil, and Teskid in Mexico.

The blocks manufactured by TUPY in Brazil have a #53 etching on the side of them and are prone to cracking. While this isn’t a guaranteed failure, it is common. The thickness of the water jacket walls was too think which caused them to crack for a number of reasons. Coolant pressure, corrosion, frequent towing, and increase power are all common causes of the block cracking. Additionally, failing to let the engine properly warm up prior to running the engine hard can also cause this.

You can read more about Cummins #53 engine block problems here.

5.9 Cummins 24v Reliability

Outside of the fuel system, the 5.9 24v is a very reliable engine. The engine internals such as the pistons, rod, and crankshaft will last a lifetime and can hold up beyond 500,000 miles. Outside of the fuel system components, you should expect to replace common items like water pumps, hoses, belts, etc. over the course of ownership. Outside of these items, maintenance is generally very manageable and inexpensive due to the lack of emissions related systems.

The turbo, which is a common failure point on a lot of diesels, is very strong. However, it is not a very power-capable turbocharger which leads a lot of folks to upgrade this item when searching for significant power gains.

Approx. 50% of these engines will last beyond 350,000 miles without any catastrophic failure, making them very reliable. Just know that you will likely need to replace some expensive fueling parts long before you hit that kind of mileage.