Rough Idle – What Are The Main Causes?

by Conner Mckay

Are you experiencing fluctuation in the RPM in your car and do you think that this is a clear representation of a rough idle? Well, if that is the case and you are in this situation, then you are at the right place because there will be a lot to cover on this topic where we will elaborate on everything in detail.

Having the ability to notice something abnormal in your car is key in order not to end up stranded on the side of the road. Frankly, there are a ton of people out there who are really careless and do not think about anything when they drive their cars. They basically drive until the car simply stops and can’t move.

And you don’t want to be one of these people. You need to be a doctor for your car. You need to notice when something is off and needs to be checked and sorted out. And same is the case with the rough idle situation. First, you need to understand the problem and then you need to learn all of the probable causes for this issue and then move on to diagnosing. And that we are going to cover in this article.

First, we will start from the basics and explain to the ones who are not into cars to help them understand more about the rough idle situation. Then we will dive deeper and cover the situations when you will experience a rough idle and then we will cover the causes for rough idle. We will go through each cause briefly and then we will jump into diagnosing the problem and fixing the issue. So, if you want to learn more, please follow along.

What Is A Rough Idle?

Now before we explain the causes for rough idle. Let’s first focus more on the basics and learn what rough idle is in general terms. I bet that there are a ton of people out there who are not into cars and do not understand the basics and this is why we need to clarify these things for them.

So, what is this problem in general? Well, a rough idle is a condition that means that the RPM (revolutions per minute) that your engine produces is not smooth.

Basically, the problem lies in the RPMs. They fluctuate frequently and go up and down. If the normal RPM of your engine is 1,000 RPM when the car starts and 800 RPM at idle when the engine warms up. When you have a rough idle situation, you have problems with the revolutions per minute.

The RPM will go up and down all the time. In this case, you will notice how the needle of the tachometer will go up and down all the time. Another thing that you will notice is the case when the car shakes when idle, or the car struggles to start but runs fine, or even the car idles rough but drives smooth.

Even though the car runs smoothly after you put it in gear and drive, this is not a good situation and indicates a problem. But more on the causes later on in the article.

What is important for you to know for now is the situations that you might come across when you experience a problem of this kind. And in the following chapter, we are going to cover them in detail.

Rough Idle Most Frequent Situations

Now before we cover the rough idle causes, let’s first discuss something more about the rough idle situations. What are the most frequent situations of this problem?

For beginners, there could be instances as we mentioned previously when the car shakes at idle. This is the most frequent situation when you have a problem with the idle. Often caused by bad motor mounts. Bad motor mounts will really make any sound in the engine 10 times louder than it actually is.

Another situation is when the car struggles to start but runs fine. This is a fairly common problem that is usually caused by a weak car battery or spark plugs. This is also associated with rough idle sluggish acceleration. Basically, when the spark plugs and coils do not work right you have these issues.

Rough Idle

Another situation is the case when the car idles rough but drives smooth. This is a common situation that you will experience as well. This is happening because the soon you add pressure to the throttle more fuel is injected and this smoothens up the engine.

Also, a lot of people think that a big problem is when there is rough idle when cold then smooths out. This is completely normal when the RPMs are high when the car is started and then after the engine warms up, they drop.

In addition to this, a lot of people notice rough idle when AC is on. This is the case because when the AC turns on, it takes a bit of power from your engine. So, it is understandable that the RPM will jump quite a bit. Overall, these are the most common situations. Now let’s focus on the causes of this problem.

Rough Idle Causes

Since the rough idle is a really broad topic, we do not have time to separate a chapter for every situation that is out there. In the previous chapter, we listed the most common situations when you will have this problem and their common causes.

In the following chapters, we will focus on the rough idle causes but separately. We will learn all the components and systems that can cause this problem with your car. And frankly, there are a ton of them. So, when you diagnose this problem it is often advised to have an OBD2 scanner tool to scan your car for codes, this will help narrow down the options. Now let’s start with the causes for rough RPM.

1. Filter Related Issues

When it comes to rough idle causes, the first thing that we would like to address is the filter-related issues that could come across and cause you a problem like this.

There are two important filters for you that you need to address when it comes to avoiding problems with the idle RPM speed of your engine.

The first is the air filter that is located in the airbox inside of the engine bay. This filter purifies the air that enters the intake.

The second important filter for you is the fuel filter. The fuel filter is really essential and is usually located along the fuel lines. What can happen is that this filter can get clogged up if not replaced on time and cause you a rough idling situation. Now let’s move to the sensor-related issues.

2. Sensor Related Problems

Now, let’s focus on the sensor-related rough idle causes. There are a few sensors and valves that could ruin the performance of your car under idle. So, what are these sensors?

Well, the first sensor is the MAF sensor, this MAF sensor is measuring the air that gets into the intake. If this sensor does not work properly, you will have trouble with the idle RPM.

In older cars, there are MAP sensors instead of the MAF. This sensor is doing the same thing as the MAF sensor. Some cars for example diesel vehicles are running on both MAF and MAP sensors. And when it comes to poor idling they are the prime suspects.

Another important sensor is the TPS or throttle position sensor. This sensor monitors the throttle response on the throttle body.

Another sensor that you need to pay attention to is the APPS or accelerator pedal position sensor. A really important sensor is located on the gas pedal.

The O2 sensor is also really important and can cause a situation like this. The O2 sensor measures the oxygen flow in the exhaust and is really important when it comes to adjusting the idle RPM.

And the last thing that we are going to cover is the idle air control valve. This valve is in charge of the idle and is one of the prime causes of poor and rough idle. So, check the IAC. Now let’s move to the ignition-related issues.

3. Ignition Related Causes

The third in our list of rough idle causes are the ignition-related causes that can happen in your car and make the car perform in such a way.

The main culprit as always is the spark plugs. These spark plugs are on every gasoline engine. So, when you have rough RPMs you probably have one or two spark plugs that are fouled. The reasons for the spark plug failing can be plenty.

This is why you need to learn how to read spark plugs and notice if they are fouled by gasoline, oil, or another contaminant that should not be on them.

The second most common ignition-related problem is the situation with engine coils. Modern cars implement ignition coils. These coils are basically electric transformers that deliver the power into the spark plug. They can fail and make your car run really rough. So, watch out for them.

Other things that we are going to cover are the distributor and wires. The distributor and wires can cause a problem like this. These are most common in older models. So, whenever you replace the distributor, you also need to learn how to adjust the ignition timing as well in order to avoid rough idle.

4. Vacuum Leaks & Emissions

The next rough idle causes that we are going to cover are concerned with vacuum leaks and the emissions systems on your car.

On each car, there are a ton of vacuum hoses going around and these hoses are doing a job. They know to crack and cause you a ton of issues such as vacuum leaks. Whenever you have a vacuum leak, you allow unmetered air to enter into the intake. And when you are having a situation like this, you have rough idle symptoms.

This is why you need to make sure that all of the hoses are in good condition. Also, the valves that are in charge of these systems.

The EGR valve is the most important one that could cause a ton of issues for you as a car owner. You just don’t want to get with a clogged EGR. In these cases, the car will run really rough.

For the EVAP system, there is the purge valve solenoid that can fail as well as the vent solenoid. These two solenoids are important. Not to mention the charcoal canister for this system.

The PCV system is also important. More precisely the PCV valve, this PCV valve can fail and can cause rough idle symptoms.

And last but not least are the intake snorkel, intake manifold, and intake manifold gaskets. All these need to be in check if you don’t want to experience vacuum leaks and rough work from the engine.

5. Fuel Delivery Issues Can Cause Rough Idle Symptoms

The next rough idle causes that we would like to cover are issues with the fuel delivery. Believe it or not. Fuel delivery problems can really destroy the performance of your engine and cause it to run really rough and poorly.

The first and most common symptom that is connected to this is a bad fuel filter, something that we mentioned previously. This filter should be replaced every 30,000 miles or so if you want to avoid some of the rough idle symptoms.

The next thing that you want to check is the fuel pump that is located in the fuel tank. This fuel pump is the primary fuel pump. Most cars are running only this type of fuel pump.

But direct injection and diesel cars run high-pressure fuel pumps. These fuel pumps are really sensitive and if they do not work correctly, you will experience problems with the rough idle and more.

And last but not least are the injectors and the fuel pressure regulator. The fuel pressure regulator is installed in high-fuel pressure systems. This component basically regulates the fuel pressure inside of the fuel rail so it is always up to spec and if it doesn’t work well, you might have problems like in our case.

The injectors are also one of the most common things that could cause rough engine idling and poor performance. Especially damaged ones. So, they are worth checking out as well.

6. Problems With Engine Timing

The last rough idle causes that we are going to discuss are engine timing-related issues. As you probably know, in the engine everything needs to be timed out perfectly so you don’t experience symptoms like this or something worse such as engine damage.

In order to keep your engine timing right, you have the timing chain or belt. Timing chain tensioner or timing belt tensioner and sprockets.

Sometimes these tensioners can fail or loosen up, allowing the chain or belt to have slack. So, when this happens in your engine, you experience rough engine idle symptoms or something similar.

The timing belt can also get damaged and crack, or even lose some of its teeth. This will cause the belt to skip the timing and this will cause a ton of misfires and issues with running the engine properly.

Be really wary of these components and always make sure that you follow the right intervals to replace them to avoid some of the irreversible damage that could be done to your engine if some of them fail.

In those cases, the poor idle would be the least of your concern because you will be paying thousands to get these problems sorted out. But how you can diagnose a problem like this? Let’s elaborate on that in the following chapters.

How To Diagnose Rough Idle Symptoms?

Now as we covered every possible cause for the rough idle, let’s move on to something more interesting and that is diagnosing this problem. How you can diagnose this issue on your car?

Well, the first thing you need to do is to determine if you have this problem. So, what are the conditions that need to be met?

As we said, the engine revolutions per minute need to fluctuate when the car is in idle mode. Meaning that the car is stationary and the engine is running.

When you have a condition like this the RPMs will fluctuate, they could rise above 1,200 and they could drop lower than 600, resulting in stalling of the engine in most cases. This means that you are dealing with rough idle symptoms. So, how you can determine the root cause of this problem?

Well, determining the root cause for this issue is not possible unless you have an OBD2 scanner. With this tool, you will be able to get access to the car computer which is the PCM.

Then based on the codes, you will come to a conclusion of what could be the cause of this situation. Whether it is an ignition problem, fuel-related issue, vacuum leak, or possibly something else.

After you diagnose the problem and make a few assumptions. You will have to start troubleshooting and bench test some components to make sure that they are faulty and then lastly, replace the faulty components from your car.

For this, you will need to step up your game and perfect your troubleshooting skills. Try to narrow down the possibilities and that’s how you will easily diagnose the rough idle problem on your car.

Cost To Fix Rough Idle

So, what is the cost to fix this problem with the rough idle? Well, the cost to fix this problem really depends. It can be cheap as $10 and expensive as $1,500 or more.

The main concern that you will have is to get a scanner for $25 or more than you can spend and make sure that this scanner will work well for your car and detect all the necessary data.

Rough Idle

Then based on the codes, you will determine what could be the problem and then move on to sorting it out. Simple as that.

For starters, if you didn’t do a tune-up lately (and to learn more, check out our explainer on what is a tune-up), you can start with this, change all the filters and spark plugs and see how it will go. You can do this for less than $150.

Rough Idle: What You Need to Know

  • A rough idle is a common symptom of several issues with a car’s engine, including bouncing or shaking during idle, inconsistent/jumping RPMs, skipping/shaking sounds, and idling below 600 RPMs.
  • Rough idling can be caused by several factors, including dirty/faulty fuel injectors, wrong idle speed, faulty fuel pump, clogged fuel filter, faulty engine temperature sensor, faulty airflow sensor, vacuum leak, faulty EGR valve, and failing spark plugs.
  • Different symptoms may occur based on whether the car is cold-starting or hot, and it’s crucial to take note of when the vehicle shakes during idle and any sounds to help the mechanic identify the problem.
  • A rough idle should not be ignored since it is often the first symptom of a deeper problem, and if the car idles roughly consistently or shows related symptoms like slow acceleration, reduced fuel economy, sputtering or a lack of power, difficulty starting, and engine stalling, it’s time to bring it to the mechanic for diagnostics.


In this article, we have covered quite a bit when it comes to the problem with the rough idle. First, we elaborated on the problem and then we covered the situations that you will most often face with poor idle.

Then we covered all the possible causes out there. Starting simple stuff to the most complex things that exist. After that, we covered how you can diagnose the problem and what could be the possible cost.


Now let’s answer some frequently asked questions.

Why Is My Car Shaking When Idle

In most cases, this situation is caused by bad engine mounts. Make sure that the engine mounts are good first. If they are not, replace them, then move on to possible problems within your engine.

What RPM Should My Car Idle At

The Idle RPM for your car should be 800. When you start the car, the RPM goes to 1,000. But after a minute or two when the engine warms up, the RPM will drop to 800. This is the proper working RPM for most cars.

Why Does My Car Idle Rough

There could be a number of things. Starting from the ignition components like spark plugs, coils, fuel filter, and air filter. The options are enormous. This is why you need to perform car diagnostics with an OBD2 scanner and move on from there.

Can A Bad Water Pump Cause Rough Idle

Directly it cannot cause rough idle. But if your engine overheats as a consequence of this situation, then it can. So, when you have a bad water pump, replace it as soon as possible in order to avoid this type of situation.

Can A Bad Belt Tensioner Cause Rough Idle

Yes, it can. This is the case because you can lose the timing of your engine when this tensioner goes bad. It could jump for a few degrees. So, as a consequence, you can expect some rough idling.

Can A Clogged Catalytic Converter Cause Rough Idle

Yes, it can, a clogged catalytic converter can really damage your performance. The car will run extremely poorly when this component is clogged. The only way around it will be to replace it with a new unit.

Can A Leaking Valve Cover Gasket Cause Rough Idle

In most cases, a valve cover gasket will not cause your car to run rough. Mainly because this component does not affect the idle. You can start your engine even with the valve covers off and it will work fine.

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