5 Most Common 5.9 Cummins Bad Injector Symptoms & How To Prevent Them

If you’re into diesel engines, then you know there’s nothing quite like the sound of a diesel engine running smoothly. But what happens when that sound is replaced by rough idle and misfiring? That’s usually an indication that something is wrong with the injectors.

Now, Cummins engines are known for their durability and power – the 5.9 especially, but even the best engines can have problems from time to time. Injectors are one of the most common issues with diesels, and bad injectors can cause a whole host of problems.

If you’ve noticed that your 5.9 Cummins started blowing black smoke or is no longer running as smooth, it’s important to diagnose the problem as soon as possible. Ignoring the problem for too long will only make it worse and could eventually lead to costly repairs or even a complete engine failure – not fun.

In this article, we’ll take a look at the most common 5.9 Cummins bad injector symptoms. We’ll also discuss what you can do to fix the problem and how to prevent it from happening again in the future.

Most Common 5.9 Cummins Bad Injector Symptoms

1. Long Crank Time

One of the most common symptoms of bad injectors in a 5.9 Cummins is longer crank time.

When the injectors get worn out over time internally (the return check valve no longer seals), they will start to return too much fuel. When that happens, your 5.9 will struggle to build fuel pressure in the rail or even fail to build pressure if the injector is worn out badly.

Because of that, you’ll have longer crank times since the fuel needs to be pressurized to 4000 PSI for the engine to start and it can take a lot of cranking to build that pressure with a bad injector.

If it’s only one injector that’s returning too much fuel, then you’ll usually build enough pressure after cranking for a while, but if it’s a few failing injectors, then you likely won’t be able to start your 5.9 Cummins at all.

Whichever the case, it’s a good idea to get replace or rebuild all of the injectors. The thing is, once one of the injectors goes bad, the rest are about to fail as well. You’ll save yourself time and money by doing all of the injectors at once.

2. Excessive Black or White Smoke

Excessive smoke is another common symptom of 5.9 Cummins bad injectors. If you notice that your 5.9 is blowing more black smoke than usual or if the exhaust is particularly sooty, then that’s usually a sign that something is wrong with the injectors.

Black smoke is caused by incomplete combustion and is a result of too much fuel being injected into the cylinders or wrong timing. When the injectors are worn out, they can start to drip fuel or have a slight delay in injection timing. That extra fuel then gets into the combustion chamber and doesn’t get burned completely, leading to black smoke.

If you’re noticing excessive black smoke, especially under load, then it’s a good idea to check the injectors. A cylinder balance test would be the best way to quickly narrow down which injector is acting up.

3. Rough Idle or Slight Miss

Another symptom of 5.9 Cummins bad injectors is a rough idle. If the 5.9 is not idling smoothly or has a slight misfire, then it’s a sign that there’s an issue with the injector’s armature valve, plunger, or the nozzle pintle.

A rough idle can be caused by a few different things, but one of the most common causes is an injector that’s leaking internally. When an injector leaks, usually due to a worn nozzle pintle & seat or debris between them, it allows extra fuel to enter the cylinder, which then throws off the air to fuel ratio. That, in turn, will cause a misfire and a rough idle.

If you’re noticing a rough idle, it’s a good idea to check the injectors and see if any of them are leaking. You can do a cylinder balance test or an injector leak down test to narrow down which injector is causing the problem.

A leaky injector is a serious problem and it can cause the pistons to crack or melt from the excess heat.

4. No Start

Similar to the long starting time above, a no start usually means that either one or multiple injectors are either stuck open and leaking fuel into the combustion chamber or returning too much fuel.

When an injector is stuck open, it will constantly drip fuel into the cylinder, which can cause a hydrolock. That’s when a lot of fuel gets into the combustion chamber and the piston rod bends when it’s trying to compress it.

If you’re trying to start your 5.9 diesel and it won’t turn over, then it’s a good idea to avoid cranking your engine over for too long in case the injectors are stuck open.

5. Loud, Noisy, Clattering Engine

All diesel engines are inherently noisy when compared to their gasoline cousins, but when the injectors start to get worn on common-rail systems, you can usually tell by the increase in diesel-specific engine noise.

Usually, the engine will start to sound louder, with more clattering, more ticking noises, especially when you lug the engine.

This is often caused by a worn armature valve or control plunger, as they get a slight delay in their movement which affects the injection timing.

Will a Bad Injector Throw a Code on a 5.9 Cummins?

A bad injector can throw a code on a 5.9 Cummins, but only for the electrical side of the circuit. The electrical side is constantly monitored by the ECU for opens, shorts, or incorrect voltage, and if the electrical side of the injector goes bad, then you should see a DTC.

As for the mechanical side, which is the most common point of failure – 5.9 Cummins diesel injectors are extremely precise and even a small change in spray pattern, a slight leak, or a delay could upset the engine, but the ECU has no way of detecting mechanical injector issues.

Why Do 5.9 Cummins Injectors Fail?

In most cases, 5.9 Cummins injectors fail due to mechanical wear.

The 5.9 Cummins uses high-pressure and high-precision common-rail injectors, which are VERY sensitive to fuel quality and dirt. They are well made and should last for 300,000 miles, but their longevity largely depends on following proper maintenance practices and only using high-quality diesel fuel.

How to Prevent Injector Failure on the 5.9 Cummins

Because these injectors are made so precisely and with extremely tight tolerances, they need 100% clean fuel that’s free of dirt particles and high-quality diesel fuel. Because of that, you must change your fuel filter often (every oil change is recommended) and make sure that the fuel filter is high quality.

You do not want any particles passing through the filter and entering the rail, as that will easily lead to injector failure.

Next, using good diesel fuel from larger and more popular gas stations is a must. These injectors need proper lubrication in order to minimize mechanical wear, and the only way they get lubrication is with diesel fuel. If you’re filling up your truck with bad diesel fuel, then you will reduce the lifespan of your injectors, and a new set of injectors costs thousands of dollars.

5.9 Cummins Fuel Injector Replacement Cost

Replacing the injectors on a 5.9 is not an easy task and it should take 6 to 8 hours. If you’re doing it yourself, then you can expect to pay around $1,800 – $2,500 for a set of remanufactured injectors or $2,500 – $3,000 for brand new.

If you’re having a shop do it, then you can expect to pay around $2,500 – $4,000 for the job. Obviously, this is just an estimation and the actual price will differ depending on the state you’re in and the shop.

Final Thoughts

Common-rail diesel injectors in the 5.9 Cummins are a finicky bunch. They need clean and high-quality diesel fuel with proper lubrication to run well.

If you don’t follow these practices, then you will drastically reduce the lifespan of your injectors and end up spending thousands of dollars on a new set.

Bad injectors can cause a 5.9 Cummins to run rough, have low power, cause long crank times, and eventually lead to engine failure if left untreated.

If you’re having any of these issues, then it’s time to have your 5.9 Cummins injectors checked by a professional.

Do you have any experience with 5.9 Cummins injector failure? Let us know in the comments below!

Eddie Westheimer

Eddie is the co-founder of CarCareCamp.com, and the site’s primary contributor.

Under his belt, Eddie has a bachelor’s degree in Automotive Electronics Engineering and almost a decade of experience working as a  semi-truck technician (specializing in electrics).