6.7 Cummins Rough Idle No Codes

The 6.7 Cummins engine is a powerful and reliable diesel engine that is widely used in trucks and other heavy-duty vehicles. However, one common problem that many 6.7 Cummins owners experience is rough idling.

This can happen for a variety of reasons, but often there are no codes or error messages to indicate what the problem is. This can make it difficult to diagnose and fix the issue.

The purpose of this guide is to provide a comprehensive solution to this problem. We will explore the most common causes of rough idling in 6.7 Cummins engines, and provide detailed troubleshooting steps and common solutions that can help you fix the problem.

We will also provide tips on how to prevent rough idling from happening in the first place, and how to keep your 6.7 Cummins running smoothly. By the end of this guide, you should have a good understanding of what causes rough idling in 6.7 Cummins engines, and how to fix the problem.

What Should Cummins Idle At?

Cummins is a diesel engine company that produces engines for various vehicles, including trucks, buses, and RVs. The company’s website says that its engines should idle at between 600 and 1,200rpm.

7 Causes of 6.7 Cummins Rough Idling No Codes 

  1. Fuel filter issues.
  2. Fuel pressure problems.
  3. Injector malfunction.
  4. Air filter restriction.
  5. Throttle body buildup.
  6. Vacuum leaks.
  7. Timing issues.

It’s also worth noting that in some cases, rough idling can be caused by a combination of these issues, or by other problems that are not listed here.

The important thing is to understand that there are many possible causes of rough idling in 6.7 Cummins engines and that a thorough diagnosis is required to determine the specific cause of the problem.

Troubleshooting Steps For Rough Idle No Codes

Check and replace the fuel filter:

The first step in troubleshooting rough idling is to check the fuel filter. If it’s clogged or dirty, it should be replaced. Make sure to use a high-quality fuel filter that is designed for use with 6.7 Cummins engines.

Measure fuel pressure and compare to factory specifications:

Next, use a fuel pressure gauge to measure the fuel pressure. Compare the reading to the factory specifications for your engine. If the fuel pressure is too low, it could be causing rough idling.

Inspect and clean injectors:

Inspect the injectors for signs of wear or damage. If they are dirty or clogged, they should be cleaned or replaced.

Replace or clean the air filter:

Check the air filter to see if it’s dirty or clogged. If it is, replace it with a new one or clean it if it’s reusable.

Clean the throttle body:

Use a throttle body cleaner to remove any buildup of carbon and other debris from the throttle body.

Check for vacuum leaks:

Use a vacuum gauge to check for vacuum leaks. If any are found, they should be repaired.

Adjust timing, if necessary:

If the timing is off, it can cause rough idling. Use a timing light to check the timing and adjust it if necessary.

Common Solutions For 6.7 Cummins Rough Idle No Codes

Replacing faulty components:

If a specific component is found to be causing the rough idling, such as a clogged fuel filter or a faulty injector, replacing it is often the best solution.

Cleaning and maintenance of engine parts:

Regular cleaning and maintenance of engine parts such as the air filter, throttle body, and injectors can help prevent rough idling from occurring in the first place.

Adjusting engine settings:

In some cases, adjusting certain engine settings, such as the fuel pressure or timing, may be necessary to fix rough idling.

Consultation with a diesel mechanic:

If the problem persists after trying the above solutions, it is recommended to seek the help of a professional diesel mechanic who has experience with 6.7 Cummins engines. 

They can diagnose the problem and recommend the appropriate course of action.

How Do You Adjust the Idle on a Cummins?

If you need to adjust the idle on your Cummins engine, there are a few things you’ll need to do.

First, locate the adjustment screws on the throttle body. There are usually two screws, one for idle speed and one for the idle mixture. The idle speed screw is located closest to the throttle shaft, while the idle mixture screw is located farther away.

Next, turn off the engine and remove the key so that no one accidentally starts it while you’re working.

Then, use a screwdriver to turn both screws clockwise until they’re snug against the stop pins.

Don’t overtighten them, or you could damage the threads. Now it’s time to start the engine and let it warm up until it reaches operating temperature.

Once it’s warmed up, check the RPM gauge to see what your current idle speed is.

If it’s too low, turn the idle speed screw counterclockwise until it reaches the desired RPM.

If it’s too high, turn the screw clockwise until it drops to where you want it. Finally, adjust the mixture screw until the engine is running smoothly at its new idle speed setting.

That’s all there is to adjust your Cummins’ idling settings!

6.7 Cummins Rough Idle at Startup

If it’s idling rough at startup, there are a few things that could be causing the issue. First, check your air filter to see if it’s dirty. If it is, clean or replaces it as needed.

Second, check your fuel filter and make sure it’s not clogged. If it is, replace it with a new one. Lastly, check your spark plugs and wires to see if they need to be replaced.

If so, do so as soon as possible.

6.7 Cummins Idle Fluctuates

6.7 Cummins Slight Rough Idle

you may have noticed that your 6.7 Cummins truck has a slightly rough idle. This is normal and nothing to worry about! The reason for this is that the 6.7 Cummins engine is equipped with an exhaust brake, which helps to control your speed when going downhill.

When the exhaust brake is engaged, it causes the engine to run slightly rougher than usual. So if you notice your 6.7 Cummins idling Rough Idle, don’t be alarmed, it’s perfectly normal!

6.7 Cummins Rough Idle in Reverse

If it’s idling rough in reverse, there are a few things that could be causing the problem.

It could be an issue with the fuel system, the turbocharger, or even the engine itself.

First, check the fuel system.

Make sure there is enough fuel pressure and that the filters are clean. If the fuel system is not the issue, then it could be the turbocharger. Inspect the turbocharger for any damage or leaks.

If everything looks good there, then it’s probably an issue with the engine itself. There are a few things that can cause an engine to idle roughly in reverse. It could be something as simple as a dirty air filter or a bad spark plug.

Or it could be something more serious like low compression or a blown head gasket. Whatever the case may be, it’s best to take your truck to a qualified mechanic to have it diagnosed and repaired properly.


In conclusion, rough idling in 6.7 Cummins engines can be caused by a variety of issues, such as clogged or dirty fuel filters, low fuel pressure, malfunctioning injectors, restricted airflow, and vacuum leaks.

It’s important to understand that there are many possible causes of rough idling and that a thorough diagnosis is required to determine the specific cause of the problem.

The troubleshooting steps outlined in this guide can help you identify the cause of the rough idling and provide a comprehensive solution to the problem. 

Common solutions include replacing faulty components, cleaning, and maintenance of engine parts, adjusting engine settings, and consulting with a diesel mechanic.

It’s also important to keep in mind that regular maintenance and timely diagnosis of problems can help prevent rough idling from occurring in the first place.

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  • MoparProud

    MoparProud brings a rich background in automotive mechanics to the Truckguider team. With over 20 years of hands-on experience, he is an expert in engine mechanics, particularly the 5.7 Hemi and 6.7 Cummins engines. His insights have also made valuable contributions to forums like Cummins Forum, solidifying his reputation in the automotive community.

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