Rough Idle Causes – Why Your Car Is Idling Rough?

by Conner Mckay

Have you noticed that your car recently started to idle really rough? And you are looking to learn more about the possible rough idle causes? If that is the case, then you are at the right place because there will be a lot to cover on this topic where we will learn everything when it comes to this problem.

Having a problem like this when the car runs roughly can be frustrating to deal with. The engine will simply not going to be happy no matter what you do. The RPM will fluctuate all the time and you will think that the engine is nearing its end but this isn’t always the case.

Sometimes, the causes of a rough idle can be mundane and simple to fix. This is why you need to study them and learn more about how you can fix this issue on your vehicle. And that’s why we are here to help you out when it comes to this problem with your car and we are here to offer you the ways of how you can tackle this problem with your car.

First, we are going to elaborate more on the problem and learn more about rough idle in general and how serious it is. Then we will take a look at the situations where you get rough idle and then we will move on to the rough idle causes. After we elaborate on the causes, we will jump into the diagnosing and fixing process. So, if you want to learn more about solving this problem, keep up with us.

What Is Rough Idle?

Now before we dive into the rough idle causes, let’s first cover some of the basics and that is what is rough idle in general. I bet that there are a ton of people who are not into cars and want to familiarize themselves with the problem first. So, what is a rough idle?

Well, a rough idle in simple words is a situation when the idle RPM is not normal. But what is a normal idle RPM?

The normal idle RPM is adjusted from the factory. The computer is programmed to run the car to a certain RPM when the car is not in gear. This means that the engine is running and the RPMs should be steady at one level. But what about the numbers? Which are the real numbers for a good idle RPM?

Usually, when you start your engine the RPMs are set to 1,000 revolutions per minute. This is necessary because the engine when started is cold and demands a bit more fuel to keep running. But after 30 seconds or a minute, this RPM should drop to 800.

800 RPM or revolutions per minute is the gold standard on most cars and this number should be steady until you press the gas pedal. If the needle is moving up and down, this means that you are in a situation when you are experiencing rough idle on your car.

So, what are the rough idle causes? More on that later on, let’s first learn more about whether or not you should be concerned when you are dealing with an issue with rough idle speed.

Should You Be Concerned?

We learned what is rough engine idle. And now before we cover the rough idle causes, let’s cover another topic and that is whether or not you should be concerned when it comes to this problem?

And the short answer is yes, you should definitely be concerned with the issue. This is an issue that affects the reliability of your car big time.

It couldn’t damage the engine or cause issues with the mechanical components. But it could leave you stranded in the middle of nowhere without the ability to start your car.

Rough Idle Causes

This is why you should consider resolving this problem as soon as possible if you want to avoid a situation like this. Nobody wants their car to break down and call roadside assistance to pick them up.

Not only that it will cost you a ton of money but also the frustration and anger will be there that you left in the middle of nowhere unable to start your car.

So, you need to understand first the situations when you could experience rough idle in your car first and then learn the rough idle causes. And we are going to do precisely that. In the next chapter, we are going to cover the situations, and after we will learn the causes of rough idle. So, follow along.

Situations When You Experience Rough Idle

It is worth noting that rough idle is not only one state but several different ones. There are a ton of situations when the drivers can face a rough idle and based on these situations you will be able to understand the rough idle causes that we are going to cover later on.

The first situation is when the car idles rough but drives smoothly. Why is this the case? Well, it could be a number of things but the most common thing for this problem are bad spark plugs or bad ignition coils (which you can diagnose by learning how to test ignition coil).

Another situation is when the RPM goes up and down at idle. This situation could indicate a problem with the throttle position sensor as well as problems with the throttle body itself. The throttle body is the component that has the butterfly flap on it that opens and closes when you press on the gas.

Rough Idle Causes

Another situation is when the car shakes when starting and then dies. This is another situation that could indicate problems with the spark plugs and ignition coils.

Also, a situation when the car struggles to accelerate and shakes. This is a big problem that could indicate problems with the fuel delivery system. If there is not enough fuel in the engine, the engine will shake and will struggle to accelerate.

And the last situation that we are going to cover is when the car shakes during idle and acceleration. Another indication of faulty spark plugs that are causing issues or a clogged fuel filter in some instances.

But it is worth that when the engine is shaking, the motor mounts could also cause an issue like this. Especially if they are worn out. But more on the rough idle causes in the next chapters.

Rough Idle Causes

As we elaborated more on the problem and the situations where you could face this issue, we can now move and cover the rough idle causes.

Knowing the causes will help you tackle this problem in no time. This is why we separated the causes into categories so it would be easier for you to understand this information and learn what could possibly cause this issue with the rough idle to you.

1. Filter Related Causes

The first type of rough idle causes that we are going to cover are the filter-related causes. The filter-related causes can make your car run rough and cause you a ton of trouble. But what are the main filters that could cause this problem?

It is worth that there are two filters that could cause problems like these. The first filter is the air filter that is located in the airbox. This air filter can get clogged and not allow enough air to enter into the intake and cause issues like these.

The second important filter for you is the fuel filter, the fuel filter is the one that filtrates the gas before it goes into the engine. And whenever this filter is clogged up, you will face rough idle symptoms and the inability to run the engine the right way. Now let’s move to the next rough idle causes.

2. Ignition Related Causes

The next set of rough idle causes is ignition-related issues. The ignition components as you probably know are the spark plugs and coils if you have a modern car. Or if you have an older vehicle you probably have spark plugs, distributor, and wires.

If you have a modern car, you can diagnose bad ignition coils with an OBD2 scanner. If you are running an older car, you need to see if the wires are delivering spark and adjust the timing if necessary.

Spark plugs are the same for both old vehicles and new ones. If the spark plug electrodes are all black, then you have a problem with either the car burning oil or running rich. If they are blistered, then it means that the engine runs lean.

Learn how to read spark plugs, there are a ton of resources online. Spark plugs will tell you a ton of stories about your car.

3. Vacuum Leak Related Causes

Now let’s cover the next set of rough idle causes and that is the vacuum leaks. The vacuum leaks are a notorious cause for you having issues like these. So, why is the case?

Well, this is the case because having a vacuum leak means that you have unmetered air that is entering the intake manifold. And this unmetered air will ruin the air to fuel mixture attributing to rough engine idle.

Common components that fail and cause issues like this are the intake manifolds, the intake manifold gaskets, the EGR valve, the PCV valve, and the EVAP system. Along with all the rubber hoses that all these systems are running. And trust me, there are a lot of hoses.

Vacuum leaks mainly occur at the fittings or the valves themselves. Finding these leaks is really difficult because you will need to use a smoke machine or possibly soapy water and a sprayer. Sometimes these leaks are identifiable by the hissing noises. It really depends. The vacuum leak rough idle causes are the most difficult ones when it comes to diagnosing.

4. Sensor Related Causes

The next set of rough idle causes that we are going to cover is the sensor-related issues. There are a ton of sensors on a car that can fail and cause problems like these.

The main sensors that can do some damage to your engine performance are the MAF sensor on newer cars or the MAP sensor on older vehicles. These sensors are basically measuring the airflow that goes inside of the engine.

The other sensors that can cause problems like these are the O2 sensors. Every car has at least two of them. One upstream and one downstream O2 sensor.

The upstream is located before the catalytic converter while the downstream sensor is located after the catalytic converter. These sensors are essential because they measure the oxygen in the exhaust gases and help adjust the air to fuel mixture in the car.

The next set of sensors is the ones that are located on the throttle body. And they are one of the most important when it comes to issues like these. The first is the idle air control valve. This valve controls the flow of idle air into the system. So, in most cases, it is the main cause of rough idle.

While the second sensor is the TPS sensor also known as a throttle position sensor. This sensor observes the position of the flap in the throttle body and determines how much air to enter into the engine. Now let’s move to the next set of rough idle causes.

5. Throttle Body Causes

The next rough idle causes that we are going to cover are the issues with the throttle body itself. If you don’t know, the throttle body is the component on which the TPS sensor and the idle air control valve are mounted. So, you understand why is so important. But what could go wrong with it, it’s only a butterfly flap, right?

Well, not quite, even though this is only a flap, there is much going around this throttle body that you make your car run really bad.

Modern cars include a ton of emission systems. For example, the EGR system, and the EVAP system. And what is bad is that these systems are pushing a ton of dirt inside of the intake.

So, this dirt in these fumes over a ton of miles will create a ton of gunk of carbon deposits around the throttle body. This will prevent the throttle body to operate as it should and will cause you a ton of headaches. So, if you have a really dirty throttle body, your best bet would be to replace it with a new one. Now let’s move to the next set of rough idle causes.

6. Engine Timing Related Causes

The next set of rough idle causes that we would like to cover is the situation with the engine timing. Believe it or not, engine timing can affect your engine performance and could cause the engine to run really poorly.

It is a key thing to inspect your timing belt and see its condition and check if there is cracking on it as well as missing teeth. If there are some problems like these on the timing belt, then your belt has probably skipped some timing and is making the engine misfire and run rough. Also, do not forget to check the tensioner.

On the other hand, if you have a timing chain, the same thing can happen as well. The tensioners on the chain could break and some engines are notorious for these issues. So, if you hear rattles from the timing cover, then your chain is probably loose and needs replacing. Now let’s move to the last set of rough idle causes.

7. Fuel Delivery Related Causes

And the last set of rough idle causes that we would like to cover is the issues with fuel delivery components.

Believe it or not, the fuel delivery components are one of the most important ones when it comes to tackling this problem in some cases.

If you run an older car, then you probably have a carburetor (you can learn more in our explainer on how to clean carburetor). These carburetors are notorious for breaking and causing problems like these and nowadays is a real pain to find a mechanic who wants to work on them.

More modern cars on the other hand are having different problems. A problem with the fuel pump, injectors, fuel filter, and high-pressure fuel pump in direct-injection engines. All these components can fail and be considered rough idle causes. But how you can diagnose a problem based on these rough idle causes? Well, that’s what we are going to elaborate next.

How To Diagnose & Fix The Rough Idle?

Now let’s get to business and start learning how we can diagnose the rough idle based on these rough idle causes?

If you have an older classic motor, the first thing you should do is check the spark plugs, distributor, and wires. This should be your main concern. The next most important component will be the carburetor. And finding the problem could be really simple since there is not a lot going on in these engines.

On the other hand in modern engines, things are different. In order for you to diagnose a modern engine, you should have an OBD2 scanner and see if you are getting some codes. If there are codes, then based on this data you should look for the issue. As a preventive, you should replace the air filter and the fuel filter if they are old.

If the problem persists, components that you need to check are the spark plugs, ignition coils, throttle body, idle air control valve, and TPS sensor. Then check for vacuum leaks. These are the most important. If you can’t find the issue in the ignition and the throttle body, you can move to fuel-related stuff. Check the fuel pump and injectors.

For most of these components mentioned you will get a trouble code probably. You can also read live data on some OBD2 scanners and see how each of these sensors performs and if they are meeting the demanded requirements. Then bench test some of the components with a multimeter tool to see if they are any good. It can be a bit tricky but you can do it.

Cost To Fix The Rough Idle

So, what is the cost to fix the rough idle? Well, that really depends on the rough idle cause. As you probably are aware, we elaborated on a ton of different rough idle causes.

Rough Idle Causes

So, it could be any of these causes that is the prime reason why your engine is not running well. But in most cases, this is due to bad spark plugs. So, you can expect to pay somewhere in the range of $150 to $350 to sort out this issue.

Car Engine Rough Idling: Facts You Need to Know

  1. Rough idling is a common problem in vehicles, and it can be caused by several factors, making diagnosis challenging.
  2. The way your engine idles can indicate its overall health, and it’s essential to address the problem promptly and check all potential causes.
  3. Rough idling is usually identifiable by a shaking and bouncing sensation in the vehicle, along with odd sounds and inconsistent RPM counts.
  4. Four common reasons for rough idling are vacuum leaks, damaged or incorrectly installed spark plugs, dirty fuel injectors, and carburetor problems.
  5. Vacuum leaks can cause misfiring due to too much air mixing with fuel, while spark plugs burning fuel at an inconsistent rate can cause engine stuttering and jerking.
  6. Dirty fuel injectors can cause poor gas mileage and reduce engine performance, and using an injector cleaner additive can prevent this issue.
  7. Carburetor problems can cause excessive black exhaust smoke, and using a carburetor cleaner can help dissolve carbon deposits and prevent rough idling.
  8. Promptly addressing rough idling is important, as the underlying issue can affect your vehicle’s overall performance in the long term.
  9. If you’re unsure what’s causing the rough idling, it’s best to call in professionals for a diagnosis.
  10. A properly functioning engine should run smoothly without excess noise, and addressing rough idling promptly and checking all potential causes is crucial to maintain engine health.


In this article, we have covered quite a bit when it comes to the rough idle. First, we learned what is this problem in the first place and the situations that you will come across.

Then we covered the rough idle causes and how to diagnose a rough idle on your car and the cost to sort out this problem.


Now let’s answer some frequently asked questions.

What RPM Should My Car Idle At

The idle RPM on most cars is about 800. When the car starts the RPM is higher. But will eventually drop and settle down at 800 RPM. Anything above or below can be considered a rough idle speed of the engine.

Is Idling Bad For Your Car

Idling is not bad for your car. The engine simply runs at the minimum optimum temperature and could run like this until you decide to turn it off. No risk for engine life when doing this practice. So, you shouldn’t be worried at all.

What Should RPM Be When Starting Car

Usually when the car starts the RPM goes at about 1,200. This is a cold start situation. When the engine warms up, the RPM will drop to 800. On some cars, the cold start RPM is lower at 1,000 RPM. So, it really depends on the manufacturer.

What Causes Rough Idle

Rough idle can be caused by a ton of different reasons. So, it is hard to tell right off the bat what could be causing it. Even though the most common reasons are problems with the spark plugs, coils, idler control valve, and TPS sensor among the many. These are the things that you want to focus on first.

What Causes Rough Idle And Stalling

Rough idle and stall can be caused by a bad MAF sensor. This is the sensor that measures the air that enters the engine. So, if it’s not working, the car could stall. Other reasons include clogged air and fuel filters. A weak alternator is also one possibility.

What Causes Rough Idle On Startup

Rough engine idle on startup can be caused by a bad heater element in the O2 sensor, bad spark plugs and ignition coils, broken injectors, and clogged air and fuel filters. There could be a ton of reasons. So, you need to diagnose this stuff professionally.

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