Can spark plugs cause rough idle? What to know

The role of the spark plugs is to ignite the working mixture in the cylinder. If the plugs are worn, this affects the operation of the engine in the same way. A faulty spark plug causes incomplete combustion of the fuel, and in the worst case, the working mixture does not ignite at all.

The service manual of the car specifies how to check and replace spark plugs. The spark plugs are often not even checked – they are simply replaced as soon as a certain mileage is reached.

In order for the fuel system and engine to function correctly, faulty plugs must be identified and replaced.

One of the most common issues of concern today is dealing with a rough idling engine. To address this type of problem, you need to find the root cause as soon as possible to eliminate the potential for other unnecessary malfunctions and damages.

Having said this, here is one common question that many car owners may ask early on, and that is, can spark plugs cause rough idle?

Based on the car and the circumstances that are occurring, the answer to this question can vary from one vehicle malfunctioning to another.

So, when you are doing your research, here are a few factors that need to be taken into account in order to find the right answer to this question.

First of all, there are many different reasons why a car can have a rough idle. And, of course, the spark plugs can be the root cause of this type of engine problem. So, here’s a few things you need to know about the following:

What should you check first when you have a rough idle?

As mentioned above, there are many different reasons why your vehicle engine can have a rough idle. Some of the more commonly known include the causes that have been listed below.

  • Idle speed needs adjusting
  • Malfunctioning fuel pump
  • Loose or dysfunctional distributor cap
  • Worn out spark plugs
  • Dirty fuel filter
  • Clogged fuel injectors

Also, as you can see, there are quite a few commonly known causes of a car idling rough. Therefore, you will need to isolate your issues first before you can say the rough idle is actually caused by your spark plugs.

So, let’s get started by looking at what is going on, and then let’s check out the actual state of your spark plugs. This article will also discuss a few other associated factors that are related to a rough idling engine.

Note the conditions when your rough idling occurs

You may want to take a few notes first. These notes can be used for more information when diagnosing specific issues. For instance, you need to know the actual circumstances that are occurring when you hear the rough idle.

Is it early in the morning (cold start)? Or, does the car run rough after you restart a car that is already warmed up? Whatever the situation, if it is a cold start, the problem could be related to a number of different things, including repairing a vacuum leak or bad spark plugs. Also, if it is a newer model car, you should make note of the computer code that is displayed.

The notes that you make can provide clues for your own troubleshooting purposes. Or, these observations can be expressed later on to a mechanic that needs to dig deeper to resolve your idling problems completely.

Check Your Spark Plugs: Manually or with the Use of a Multimeter

If you think that the rough idling of your vehicle is due to your spark plugs, you can investigate these problems as one of the first options in troubleshooting. Also, here is what to look for.

Do a manual Inspection of your spark plugs to see if they are old, worn out, dirty, bad, or failing.

The first step is to remove each spark plugs one by one to see if their conditions are visually bad. To make this determination, however, you will need to know exactly what a normal spark plug looks like. For instance, a normal spark plug will have grayish tan or brown deposits on the sides of the electrode.

On the other hand, if the spark plug has dry black soot on the electrodes and its insulator tip, this normally indicates a wide variety of concerns, including driving the vehicle excessively at lower speeds or simply idling the vehicle’s engine too long.

In either event, this normally indicates that your spark plugs need to be replaced right away.

Use a digital or analog multimeter (or ohmmeter)

Fortunately, there is more than one way to check the state of your spark plugs. Aside from looking at the spark plugs manually, you can use a multimeter or an ohmmeter to see if the resistance is within the manufacturers’ guidelines.

If not, the old worn-out sparks will need to be replaced.

What Happens if the Spark Plugs are failing and going bad?

The spark plugs are an essential part of the engine’s electrical combustion system. One of its primary functions is to generate enough to keep the engine running smoothly instead of roughly.

Therefore, if the spark plugs are bad or too dirty to do this job, they may need to be replaced to correct the problem with a rough idling car.

It is also important to note that the average cost of parts and labor for spark plug replacement is around $195 and $251. AS for the prices, the cost of the parts is about $65 to $95 and the labor can range from $125 to $160.


Can Spark plugs cause rough idle? The answer to this question is yes, especially if the sparks in the engine have gone bad. Therefore, if you think that this is the source of your car’s idling problems, you may want to start troubleshooting with these issues in mind. And, if this is the main source of your car running rough, you can install new spark plugs, and your car will begin to run smoothly again.

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