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8 Main Causes of Rough Idle

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

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Rough idle common causes may include:

  • Bad ignition system components
  • Fuel system components in need of service
  • Faulty Engine grounds
  • Defective sensors

Within these categories, some components are more common to cause trouble, especially maintenance items; others just are bound to fail after many miles of service because of wear or the conditions under which they need to operate.

Depending on your particular model, a failed or troublesome part may trigger the Check Engine Light (CEL). If you see this light on, download trouble codes (DTCs) from your computer’s memory. Knowing these codes and their definition will save you time and money in your diagnostic process.

In many cases, though, you won’t see the CEL come on when your engine starts to shake or vibrate at idle. There’s where the following sections will help you diagnose the problem.

These sections list the most common parts that may cause an engine to idle rough when they start to fail. Furthermore, a Resources section at the end gives you a list to other posts to help you zero in on the potential problem, if necessary.

One more thing. Having the repair manual for your particular vehicle make and model will help you locate and identify components, and assist you in parts or system diagnostics when necessary.

If you don’t have this manual yet, you can buy a relatively inexpensive copy from Amazon. Haynes manuals include step-by-step troubleshooting procedures accompanied with illustrations and images. It helps you identify components and gives you how-to instructions to replace parts and service the different systems. So you’ll recoup your small investment in a short period of time.

Index

1. Clogged Air Filter

2. Worn or Faulty Spark Plugs or Wires

3. Clogging Fuel Filter

4. Dirty or Faulty Fuel Injectors

5. Vacuum Leaks

6. Stuck PCV Valve

7. Stuck EGR Valve

8. Faulty Oxygen Sensor

Video: Fixing a Common BMW Rough Idle

Other Potential Problems

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1. Clogged Air Filter

The air filter is a common maintenance component that many drivers tend to forget about. Depending on your particular model, you may need to replace the filter every 12 or 24 months.

Dust, dirt and other foreign particles accumulate around the filter element preventing free air flow and choke the engine. Besides a rough idle, a restricted air filter may cause the following symptoms:

  • reduced engine power
  • poor acceleration
  • lack of engine power
  • hard to start

Inspect the filter for particles accumulation. Place the filter against the sun or another strong source of light. If light can't go through the filter element, replace it.

Also, consult your vehicle repair manual for the service schedule.

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2. Worn or Faulty Spark Plugs or Wires

Several components within the ignition system can give you trouble, specially when they are in need of service.

Depending on your particular model, you many need to inspect one or more components for trouble.

Within this system, spark plugs and wires are bound to cause more trouble because they wear more quickly than the other ones.

Older vehicles models use a single ignition coil, a distributor, rotor, spark plug wires and spark plugs. Any of these components is a potential source of trouble.

Newer ignition systems have eliminated the distributor, rotor, and, on some models, the spark plug wires altogether. However, it’s not uncommon for an ignition coil to go bad. So keep this in mind.

Check:

  • Spark plugs’ gap; regap or replace plugs as necessary.
  • Spark plug wires’ resistance.
  • The distributor cap and rotor for carbon tracks, wear, cracks and other potential damage.
  • The ignition coil(s), if necessary.

Consult the repair manual for your particular vehicle model to look up the service schedule and available system tests for your particular model. Also, check the Resources at the bottom of this post for more help.

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3. Clogging Fuel Filter

Fuel filters need to be replaced at regular intervals. Most manufacturers suggest installing a new fuel filter every 12 months.

Usually, a fuel filter that is becoming restricted gives you a warning. You may notice:

  • Your car doesn’t have the same power at highway speeds.
  • It may hesitate at times during acceleration.
  • You notice occasional misfires.
  • Becomes rough at idle.

Look up the service schedule in your repair manual for the fuel filter in your particular model.

On some models, replacing the fuel filter takes a few minutes; on other models, it may require some time and more tools to complete the job. Consult your repair manual.

4. Dirty or Faulty Fuel Injectors

This used to be a more common source of trouble in the past. With cleaner fuel available and additives included in today's gasoline, a dirty fuel injector is not so much the source of trouble it used to be.

Still, foreign particles in the fuel tank may end up restricting one or more fuel injectors. Besides, an internal valve, coil or circuit may malfunction. So, don’t discard this possibility.

Also, keep in mind that some vehicle models incorporate a cold-start fuel injector to add extra fuel to a cold engine. If this injector fails, you’ll notice hard starting and a rough idle. Consult your vehicle repair manual to see if your particular model comes with this special injector.

The post(s) on fuel injectors linked at the Resources section will help you clean or check the fuel injectors using a few simple tests.

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5. Vacuum Leaks

Vacuum leaks lean the air-fuel ratio causing all kinds of performance trouble. And this problem is more common than you may think.

Large vacuum leaks are easier to detect. But small leaks can go undetected because are hard to hear and find, and usually cause only trouble at idle.

In this case, you may be dealing with a vacuum hose that has become loose or suffered some damaged. Vibration and high temperatures are the main culprits here. Also, you may accidentally disconnect or leave a vacuum hose loose while doing some work around the engine.

Once a vacuum hose begins to leak, you end up with a rough idle or other performance problems.

What you can do:

Trace every hose from end to end using your fingers to make sure they are properly connected and to feel for rough spots that might indicate damage. High temperatures will turn a soft rubber hose brittle and vibration will break it.

Also, you can use a length of rubber hose to listen for the characteristic hissing sound of a leaking vacuum hose. Put one end of the hose against your ear and use the other end to trace hoses to locate a potential hissing sound.

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6. Stuck PCV Valve

Although a positive crankcase ventilation (PCV) valve can work without problems for years, they do require some attention. Often, engine performance issues that might go unnoticed may hinder the proper operation of an otherwise perfectly working PCV valve.

The PCV valve routes toxic gasses from the combustion process back into the intake manifold to be reburn. This system works through engine vacuum, and the flow of gases is controlled by the valve.

Normally, the valve remains closed until enough intake pressure forces the valve to open. However, if the valve sticks open when it should be closed, you will notice a rough idle.

It’s not uncommon for combustion byproducts to restrict and get the valve stuck.

What you can do:

Manufacturers recommend checking the PCV valve at least once a year. Also, check hoses and connectors in the system. A loose or damaged hose or connector can also cause a vacuum leak and lead to a rough idle.

Consult your vehicle repair manual to locate the valve in your particular model. Also, check the Resources section at the bottom of this post.

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7. Stuck EGR Valve

Just like a PCV valve, an exhaust gas recirculation (EGR) valve can become stuck in the open position.

The EGR valve allows small amounts of exhaust gases back into the combustion chambers to reduce harmful emissions.

Throughout the years, mechanical and electronic system designs have been introduced, each one with its own set of issues. Regardless of the particular configuration, an EGR valve may become stuck in the open position, letting exhaust gases to flow continuously, and causing a rough idle.

Sometimes, all you need is to remove the valve and clear passages of carbon buildup.

To test your EGR valve, consult your vehicle repair manual, if necessary. Also, check the Resources section at the bottom.

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8. Faulty Oxygen Sensor

A bad oxygen (O2) sensor is not as common as the previous components. It’s more common for other engine issues to affect the proper operation of this particular sensor.

O2 sensors measure oxygen content in the exhaust stream and send this information to the car’s computer using a voltage signal. The computer uses this information to adjust the air-fuel ratio and control emissions.

However, faults in the fuel, ignition and other emission systems may go unnoticed for months at a time or simply ignored. Over time, though, this will inevitably affect the oxygen sensor’s job; the sensor may end up coated in soot or simply become “lazy” and send false information back to the computer.

Today, car computers are good at detecting an O2 sensor that is producing false readings. You’ll see the check engine light (CEL) come on. However, even if the oxygen sensor in your car needs to be replaced, you need to make sure there are no other issues affecting the sensor operation; otherwise, you’ll end up with the same old problem.

If necessary, consult your vehicle repair manual.

The following video shows how a faulty ignition coil can cause a rough idle in a BMW due to a misfire.

Fixing a Common BMW Rough Idle

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Other Potential Problems

Many other components can cause your engine to idle rough when they go bad. However, they are not as common or require frequent attention. Still, it’s worth to take a quick look at the most common in this category, specially if you can't seem to find the culprit for the rough idle among the most common ones.

If necessary, check for a:

  • Faulty engine coolant temperature (ECT) sensor
  • Loose or corroded engine grounds
  • Bad manifold absolute pressure (MAP) sensor
  • Faulty camshaft position (CMP) sensor
  • Inoperative idle air control (IAC) valve
  • Bad throttle position sensor (TPS)
  • Intake manifold gasket leak
  • Misfire problems

Also, these other system components may need a look into.

Fuel Delivery System

The fuel delivery system incorporates key components that can go bad either because maintenance has been overlooked or a component has failed.

Check for a:

  • Faulty fuel pressure regulator (FPR)
  • Worn or faulty fuel pump

Ignition System

Besides worn or bad spark plugs, plug wires, distributor and rotor, you may want to check the:

  • ignition coil, coil pack or coil on plug (COP), depending on model.
  • pickup coil
  • ignition module

Your vehicle repair manual gives you more information about service, troubleshooting and replacement procedures for these components and systems.

Resources

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Dan Ferrell

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