The 4 Most Common Cummins 5.9L Engine Problems

The Cummins 5.9L “B-Series” diesel engine was produced from 1984 up until 2007. First generation versions were named the 6BT but also commonly referred to as the 12v, or 12-valve. Each cylinder had 2 valves, hence the name 12-valve. Midway through 1998, the 6BT was replaced with the ISB, which stands for “Interact System B”.

The ISB 5.9 featured an inline-6 cylinder engine with multi-valve pushrods, and 4 valves per cylinder. Therefore, the Cummins ISB 5.9 is also commonly known as the 24v, or 24-valve. The engine continued in production until 2007 when it was retired for the 6.7L Cummins, primarily due to increased emissions regulations.

The Cummins diesel engine is unique in the diesel truck world, as it is uses an inline design and only features 6 cylinders. Ford’s Powerstroke, and Chevy’s Duramax engine both use tradition V-shaped engines and have 8-cylinders instead of 6. Despite the smaller powerplant, these engines are still capable of producing significant power, and tend to be more reliable than its competitors.

4 Most Common Cummins 24v 5.9L Engine Problems

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

1. Cummins 5.9 Lift Pump Failure

The Cummins 24v fuel injection system has three primary components: a lift pump, an injection pump, and injectors. The injection pump, also known as a high-pressure fuel pump, is responsible for pressurizing the fuel which gets sent to the injectors and into the cylinders. Injection pumps will pressurize fuel anywhere from 15,000 to 30,000 psi, which is extremely high.

Lift pumps are responsible for pumping fuel from the gas tank to the injection pump. Without a lift pump, the injection pump has to pull the fuel all the way from the gas tank to the fuel injectors. This creates an immense amount of stress on the system and ultimately crushes injection pump reliability. In order to have a reliable fuel injection system, the Cummins 5.9 comes from the factory with an OEM lift pump.

However, 1998 to 2004 model year 24v’s experience common lift pump failure. These early year 24v’s 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.

Lift Pump Failure Symptoms

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

24v 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.

Aftermarket Lift Pumps:

2. Leaking Fuel Injectors – Cummins 24v

Despite the reliability of the Cummins 5.9, its fuel system is its Achilles heel. As discussed above, the fuel injectors are responsible for spraying highly pressurized fuel into the engines cylinders. With the 5.9’s fuel pumps flowing north of 25,000psi, the injectors face a significant amount of stress.

Diesel fuel isn’t always the cleanest or most refined fuel which means it’s not uncommon for it to contain some dirt particles and other unwanted sediments. If you don’t frequently change your fuel filters the dirt in the fuel can easily get clogged in the injectors. This can cause the injectors to either leak and drip fuel into the engine, or clog and not provide enough fuel to each cylinder.

Fuel injectors on the 5.9 24v are known to fail around the 150,000 mile mark.

Cummins 5.9 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. Because of this, it’s not uncommon for people to replace only the bad fuel injectors. While this might make sense if you have one fail really early on a new set, we don’t usually recommend it for older engines.

If you have 125,000 miles on your current set of injectors, they are likely all operating below spec from normal wear and tear. Dropping a new injector in with a set of old worn ones can also create some performance issues if the old injectors are also worn. At this point, its only a matter of time before the others fail.

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 Leaks

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 5.9’s 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 – Cummins 5.9L

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 Cummins 5.9’s. 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.

APPS Failure Symptoms

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 Cummin’s 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 this problem here:

Cummins 5.9 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.