Troubleshoot Engine

Diagnosing Car Shakes: Could It Be a Bad Spark Plug?

spark plug replacement
Bad Spark plugs

Modern spark plugs can last for up to 100,000 miles before they need replacing. Sometimes they are overlooked when it comes to routine servicing and this can lead to problems with how well your engine runs.

A very common cause of a car shaking when running is bad spark plugs. Bad spark plugs can cause symptoms like car shaking because of poor combustion, misfire, and unburnt fuel finding its way into the exhaust fumes.

As spark plugs age, they can lose their ability to generate a sufficient spark that is powerful enough to ignite all of the fuel in the engine cylinder. This can cause an engine to misfire and lose some of its performance capabilities.

If you suspect that the spark plugs in your car are bad and need changing then this is easy to confirm by simply removing and inspecting them.

Can Bad Spark Plugs Cause Car Shaking?

Yes, one or more bad spark plugs will cause an engine to run badly and this will lead to a car shaking especially when idling or under hard acceleration.

Spark plugs are a serviceable part of the engine that will wear out over time. It’s not uncommon for spark plugs as they age to become fouled with oil or carbon build-up on the electrode.

Oil on spark plugs can be a major contributing factor when it comes to engine misfires and subsequent car shaking, especially in older vehicles with high mileage engines.

Spark plug contamination by oil can commonly be caused by bad valve cover gaskets, worn spark plug o-rings or valve guides, a faulty head gasket, or a broken piston ring.

If the electrode tip of a spark plug becomes coated in oil, then this inhibits its ability to create a powerful, well-timed spark during the combustion phase of the engine cycle. This will inevitably lead to a reduction in engine performance along with misfires and exhaust backfires (caused by oil and unburnt fuel in the exhaust). These symptoms along with an uneven delivery of power lead to the car shaking.

Spark plugs can also become fouled because of carbon buildup on the spark plug tip or electrode. Much like oil contamination, this will affect the ability of the spark plug to operate to its full capacity and this will cause symptoms such as misfires, shaking, and a lack of engine power.

What Are The Bad spark plug symptoms that can cause a car to shake?

Engine misfires, rough idling, uneven power delivery, poor fuel economy, exhaust popping, and exhaust backfires are some of the symptoms of bad spark plugs that can cause your car to shake when driving.

When it comes to the smooth performance of your car, spark plugs are one of the most important components involved in the fuel combustion process. They are designed to operate under extreme temperatures, in a cramped high-pressure environment.

Most spark plugs will fire twice for every rotation of the crankshaft, meaning they are often creating a spark over 500 times a minute when driving.

So it’s not surprising that if they are not operating as well as they should, then they will cause a range of symptoms that will lead to your car shaking when you are driving. Here are a few of these symptoms in more detail.

Symptom 1: Engine Misfires

A common symptom associated with bad spark plugs is engine misfire. An engine misfire occurs when at least one of the engine cylinders does not produce enough power to sufficiently drive the piston and crankshaft rotation. This lack of power leads to the common car shaking or stuttering sensation.

When it comes to bad spark plugs causing misfires, it’s usually caused by the incomplete combustion of the fuel/air mixture inside an engine cylinder. If there is oil in the spark plug well or on the electrode tip of a spark plug, this will hinder its ability to generate a spark that is hot enough and powerful enough to completely burn the fuel vapor in the cylinder.

The same is true for carbonized spark plugs. Carbon-fouled spark plug tips are no longer able to produce a clean spark, resulting in misfires and subsequent lack of power and shaking.

Symptom 2: Exhaust Backfires and Popping

Another symptom of bad spark plugs that can cause car shaking is known as engine or exhaust backfires.

As explained above, bad spark plugs will result in fuel in the combustion chamber not burning up completely, and this unburnt fuel will escape via the exhaust valves to the exhaust pipe.

An engine backfire occurs when the fuel mixture combusts in the exhaust manifold after it leaves the engine cylinder. When fuel vapor doesn’t burn completely in the affected cylinder chamber it will escape into the exhaust and can ignite there because of the high temperature and high pressure creating mini-explosions or backfires. 

Not only do exhaust backfires cause a car to shake but they will also lead to damage to your car’s exhaust if the cause is not fixed.

Symptom 3: Engine Power That is Reduced and Erratic

Another telltale sign of a bad spark plug is a reduction in engine power and an uneven power delivery that leads to shaking and stuttering.

A bad spark plug generally won’t fail completely, but will often perform unpredictably because of oil contamination or because of dirt or carbon buildup on the electrode tip. The number of sparks it can generate per minute may be significantly reduced. Or the precision of the spark may be compromised, leading to an overall reduction in the ability of the spark plug to ignite and fully burn the fuel mixture in the cylinder.

This reduction in spark plug efficiency will inevitably lead to a reduction in power output along with a less predictable supply of power that matches the needs of the engine.

What Next: How Do You Fix Bad Spark Plugs That Cause Car Shaking?

Fixing bad spark plugs is a two-step process that involves removing the old spark plugs and then examining and interpreting the wear or damage to the plug.

Spark plugs can go bad because of wear and tear due to the effects of mileage on the engine, or they can prematurely wear because of problems with the engine that result in contaminant buildup on the spark plug.

When it comes to fixing bad spark plugs that are causing the car to shake, changing the spark plugs is the obvious first step. However, it’s important to inspect the removed spark plugs to decide whether the spark plug failure is simply caused by usage and age, or if something else is the cause.

Step 1: Remove the spark plugs

Start by removing the spark plugs one at a time. It’s usually a good idea to refer to a relevant service handbook for the engine of your car. This will give you exact details of how to locate and remove the spark plugs and any engine covers or electrical connections.

Some spark plugs can prove difficult to remove, especially if they have been damaged by heat or leaking oil. It’s important to use the correct spark plug removal socket so as to ensure no further damage occurs.

If you are having trouble removing a spark plug because it is damaged or stripped, then read our article on how to remove a stripped spark plug for guidance.

Step 2: Examine The Spark plugs

Once you have removed each spark plug, you should examine them to see if they have worn abnormally or have been damaged or contaminated. Any oil or wetness on the threaded part of the plug, or on the electrode tip will need to be investigated further before fitting new spark plugs.

If you are changing the spark plugs because of symptoms like misfires or shaking then it’s advisable to carry out a cylinder compression test to determine if there are any leaks.

Cleaning a spark plug can improve its performance

If you find that the compression of a cylinder is within the normal range and the spark plug is not too worn, then I suggest cleaning the spark plug with a wire brush and refitting it. If the engine continues to run rough then try swapping out the spark plugs for new ones.

FAQ: Can A Bad spark plug Cause Car shaking?

1. Can Bad Sparks Cause a Car To Shut Off?

Yes if a spark plug is worn out to the point that it no longer produces a sufficient spark, then this can cause an engine to cut out or not start at all.
A bad spark plug will usually cause a misfire in the affected cylinder, and depending on the engine and how fast you are driving, this can cause a car to shut off. Sometimes this can be due to the misfire itself, or it might be because the ECU has detected the fault and shuts down the engine to prevent damage to the cylinder and the exhaust system.

2. Will New Spark Plugs Make A Car Run Smoother?

Yes, new spark plugs can make a car run smoother if the old spark plugs were not operating as well as they should. New spark plugs should create a cleaner spark that will improve engine performance and will increase fuel economy too. This is because a new spark plug will create stronger sparks with a consistent quality that will help to burn more of the fuel in the cylinder. This provides a greater return on the energy output of the fuel.

About the author

The Motor Guy

The Motor Guy is a passionate car enthusiast with a love for troubleshooting and diagnosing all sorts of vehicle problems.

With years of experience in OBD diagnostics, he has become an expert in identifying and solving complex automotive issues.

Through, he shares his knowledge and expertise with others, providing valuable insights and tips on how to keep your vehicle running smoothly.

- 12 years experience in the automotive industry
- ASE Master Automobile Technician
- A Series: Automobile and Light Truck Certification, A9 Light Vehicle Diesel Engine Certification
- Bachelor's Degree in Information Systems


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  • I’ve recently noticed my car shaking more than usual, especially when I’m driving at low speeds or when it’s idling. I’m starting to suspect it might be the spark plugs since I haven’t changed them for a while. How can I check if my spark plugs are the issue, and what signs should I look for when inspecting them?