5.9 Cummins Running Rough / Misfire

There are several reasons why your engine might run rough, and symptoms vary depending on the cause. The most noticeable effects include engine hesitation, power loss, excessive exhaust smoke, decreased fuel efficiency and an illuminated check engine light. 

Here’s an in-depth look into common issues that could be causing your engine to run rough.

Fuel System Issues

If your truck is exhibiting hard starts, rough idling, hesitation or sluggish acceleration, these symptoms may be due to one of these fuel delivery issues:

• Clogged fuel filter: A dirty fuel filter may restrict fuel flow and lead to poor combustion, causing your engine to run rough. Replacing the fuel filter will fix the issue. 
• Weak or failed lift pump: A rough idle can be caused by insufficient fuel pressure reaching the injection pump. Check the fuel pressure with a Rail Pressure test and Balance Rate test, and replace the lift pump if necessary.
• Contaminated or poor-quality fuel: Your injectors can become clogged by dirty or low-quality fuel. To address this problem, use a premium fuel additive like Hot Shot’s Secret Diesel Extreme to clean the tank, lines and injectors. Diesel Extreme six-in-one product that also demulsifies water, control asphaltenes and boost cetane
• Buildup restricting fuel delivery: Carbon buildup on injector tips or waxy and polymeric deposits limit the amount of fuel reaching the engine. Use Diesel Extreme to dissolve deposits and restore injector function.

Faulty Injectors

Faulty injectors can also cause your engine to idle rough, decreasing engine performance and increasing exhaust smoke. Leaking injectors allow excess fuel to enter the combustion chamber, disrupting engine performance. Test your fuel injectors and replace any faulty ones.  

Air Intake or EGR System Problems

If your truck has poor acceleration, rough idling and excessive smoke from the exhaust, you might have an air intake or EGR system issue. Common causes include:

• Dirty air filters or intake manifold: A clogged air filter restricts airflow to the engine, leading to inefficient combustion and reduced performance. To address this issue, replace the air filter and clean the intake manifold for optimal airflow and engine efficiency. 
• Faulty exhaust gas recirculation (EGR) valve: An EGR valve that is stuck open can let too much exhaust into the intake, causing the engine to run rough. To fix this, inspect the valve, then clean or replace it as needed. 

Faulty or Dirty Sensors

If your 5.9L Cummins engine is running rough or you notice poor fuel economy and inconsistent engine behavior, there might be a sensor issue. Faulty and dirty sensors are caused by: 

• Mass airflow (MAF) sensor issues: A dirty or failing MAF sensor affects the engine’s air-fuel ratio and must be cleaned or replaced. 
• Crankshaft or camshaft position sensor failure: These sensors provide critical information about your engine’s timing. You’ll need to test them and replace faulty ones. 

Low Compression or Mechanical Engine Issues

Misfiring and loss of power could be caused by:

• Worn pistons, rings or cylinder walls: Low compression in one or more cylinders can cause rough idling. Perform a compression test and repair or replace the damaged components.
• Timing issues: Incorrect fuel injection timing can also lead to poor engine performance. You’ll need to inspect and adjust accordingly.

Before you diagnose and fix your 5.9L Cummins misfire, discover some of the most common causes. 

Faulty Ignition System

A faulty ignition system can cause your engine to hesitate or idle rough at startup. It might be due to failed or weak injectors. When injectors don’t deliver the proper amount of fuel, your engine can misfire. Test your injectors and clean or replace the problematic ones. 

Air-to-Fuel Ratio Problems

If your engine is idling rough, backfiring or accelerating poorly, it might be an air-to-fuel ratio problem. Two possible causes include:

• Leaking fuel injector O-rings: If fuel leaks around the injector, it can cause a misfire. Inspect your O-rings for leaks and replace them if needed.
• Faulty fuel pressure regulator: 5.9L Cummins engines can misfire due to a defective regulator causing an improper fuel mixture. Testing the fuel pressure and replacing the regulator should resolve the issue.  

Wiring and Electrical Issues

If you notice intermittent misfires or rough running, especially when the engine is under load, your Cummins engine may have an electrical issue. This problem may be due to loose or corroded connections preventing proper signal transmission to vital components. Inspect all wiring and connectors and replace worn or corroded ones. 

Faulty or Poorly Tuned ECU

Another cause of engine misfires is a faulty or improperly tuned electronic control unit (ECU). If your ECU is corrupt or faulty, it can mismanage fuel delivery and affect timing. To address this issue, reflash the ECU with the latest software or replace it if necessary.

Crankshaft or Camshaft Position Sensor Failure

Misfires, stalling and difficulty starting are symptoms of crankshaft or camshaft position sensor failure. If these sensors malfunction, they will not provide the correct data to the engine control module (ECM). Test these sensors and replace them if they are faulty.

Protect your engine, improve fuel efficiency and maintain a smooth-running engine with Hot Shot’s Secret advanced transmission oils:

Hot Shot’s Secret Diesel Extreme

Diesel Extreme helps prevent rough running and misfiring in a 5.9L Cummins engine by cleaning and restoring injectors, improving fuel system efficiency and reducing carbon buildup in the combustion chamber. It addresses rough idle and engine hesitation caused by dirty injectors or poor fuel quality. This powerful six-in-one formula features the most concentrated detergent formula on the market. It cleans fuel lines, increases fuel lubricity and boosts power for improved truck performance. 

Hot Shot’s Secret Everyday Diesel Treatment (EDT)

Hot Shot’s Secret EDT provides daily fuel system protection by cleaning and lubricating injectors, increasing fuel lubricity up to 26% with an infusion of LX4 Lubricity Extreme. It removes water and stabilizes fuel while improving cetane up to seven points for better power and efficiency. This powerful formula is a must for maintaining a smooth-running engine and improving fuel economy. Add it to the fuel tank each time you fill it up to protect and preserve the fuel system.