Can Bad Motor Mounts Or Spark Plugs Cause A Car To Shake When Idle?


All cars vibrate, at least a little. However, hard shaking is never normal, and it’s typically a sign of a problem. That shaking becomes particularly noticeable when your car is idling, and you should take note of it.

Yes, bad motor mounts and bad spark plugs can both cause your car to shake when idle. Motor mounts secure your engine to the car’s frame and dampen its vibrations. Those vibrations will travel through the frame when it wears out, causing the vehicle to shake. Bad spark plugs cause engine misfiring, where only some chambers combust. The imbalance also causes the car to shake.

Read through this guide to understand more about motor mounts and spark plugs and how they can cause your car to shake when idling. Plus, you’ll also learn how to fix those two parts.

Let’s get started.

What Are Motor Mounts, And How Do They Work?

Motor mounts, also called engine mounts, are small components that your engine rests on inside your engine bay. They’re made of metal and rubber and are bolted to your car’s frame.

As the name suggests, your engine attaches and sits on the other side of the engine mounts. In other words, the engine mounts are the connecting points between the engine and your car’s frame.

Motor or engine mounts are small and straightforward components. They’re not complex at all, and they don’t move.

Despite that, these simple components provide many crucial benefits to your engine and the vehicle as a whole. Those benefits include the following:

  • Reducing vibrations: As you read earlier, engine mounts are mostly made of rubber. That’s a crucial part of the design, as it helps the mounts absorb and reduce the engine’s vibrations. That will prevent most engine vibrations from spreading throughout the frame.
  • Reducing noise: Wherever there are vibrations, there’s noise. So, those mounts also reduce noise by reducing the spread of the engine’s vibrations. Any noise that the engine naturally creates will stay contained inside the engine bay.
  • Protecting the engine: Mounts also keep the engine secured to the car frame at all times. In doing so, they protect the engine from damage by ensuring it doesn’t come off and hit other parts like the car’s frame or other components in the engine bay.
  • Protecting other components: Of course, by securing the engine to the car’s frame, the engine mounts also protect other components in the engine bay. Even in a collision, the engine will remain in its place instead of slamming into other parts like, say, the radiator.

Generally, when the motor mounts are in good condition, and the engine is secured to them, the car will not vibrate excessively or shake much.

Read: Denso vs NGK – Which Spark Plugs Should You Use?

What Are Spark Plugs, And How Do They Work?

Spark plugs have nothing to do with the engine mounts directly. Instead, each engine has several spark plugs attached to it, particularly inside the combustion chambers.

When the car supplies electricity to the spark plugs, they generate a high-voltage spark to combust the air and fuel mixtures inside the engine’s chambers. This process happens continuously, with sparks being delivered non-stop for as long as the engine runs.

Spark plugs that are in excellent working condition will prevent the engine from shaking, despite all the moving engine components. 

That’s because their sparks will ensure that the combustion process in all of the engine’s chambers is happening correctly and smoothly.

When all chambers are synchronized and running perfectly, their movements will cancel each other out, minimizing any shaking by the engine itself.

So, as long as you maintain and replace your vehicle’s spark plugs as needed, they will not cause the engine to shake or vibrate excessively.

Read: Why Is There Oil On My Spark Plug Threads?

How Do Bad Motor Mounts And Spark Plugs Cause Cars To Shake When Idle?

Motor mounts and spark plugs are two very different components, serving different systems inside your car. They aren’t closely related, and yet both of them can cause your car to shake especially when it’s idling.

But how does that happen?

Firstly, let’s talk about bad motor mounts. Earlier, you read that engine mounts are typically made from rubber. That material works excellently to reduce vibrations, but they’ll eventually break down with long-term wear.

When engine mounts break down, the engine’s vibrations will no longer be buffered by the rubber. As a result, those vibrations will spread throughout the engine’s frame.

You and your passengers will feel those vibrations and likely interpret them as shaking, which becomes particularly noticeable when the car is idling and not moving.

Simultaneously, the worn-out engine mounts will also fail to keep the engine secured in its place. As such, the engine will move and shake in its place, adding to the movements that you’re already feeling while the car is idle.

Bad spark plugs also cause similar shaking but through a different process. When spark plugs get bad, they lose their ability to produce consistent sparks inside their engine chambers.

As a result, the fuel/air mixture in the affected chambers will not combust.

Remember: an engine doesn’t normally shake despite its many moving parts because they move in harmony and cancel out each other’s vibrations.

But when only some chambers combust and not others, that harmony is lost. So, the engine starts to shake erratically.

Again, that shaking becomes most noticeable when the car is idling.

Regardless if the root cause is bad motor mounts or spark plugs, you’ll feel similar shaking while the car is idling.

Read: 5 Ways to Improve Poor Fuel Economy of Your Car

How Do You Fix Bad Motor Mounts Or Spark Plugs?

Your car should never shake when idling under normal conditions. So, you must troubleshoot the car immediately once the shaking becomes noticeable.

Unfortunately, you can’t fix bad motor mounts. Once they go bad, the only solution is to replace them with new ones. Thankfully, new engine mounts last very long, so you won’t have to worry about buying any more of them anytime soon.

However, you have a bit more flexibility when it comes to dealing with your bad spark plugs.

Firstly, you’ll have to remove and inspect your spark plugs under a clear light. That will tell you just how bad each of them is.

You can salvage some spark plugs by cleaning them thoroughly using a wire brush, some sandpaper, and a spark plug cleaning product. Those will remove most of the carbon buildups to restore the plug’s functionality.

However, spark plugs that are too far gone can’t be saved. Instead, you’ll have to replace them with new ones. That’s the only way to resolve the problem and stop the car from shaking when idling.

Even if only some of your spark plugs are bad, you should consider replacing all of them together as a form of preventive maintenance. If some have already gone bad, the others will likely go bad soon as well.

Read: What Happens if You Put Diesel in a Gas Car?

Final Thoughts

As you can see, a shaking car is never a normal condition. It’s a sign that there’s a problem, though that problem can be caused by several different factors.

A car that shakes while idling is typically caused by either bad engine mounts or bad spark plugs. When engine mounts go bad, they can’t dampen the engine’s vibrations from spreading throughout the vehicle’s frame. As a result, the car shakes.

Meanwhile, bad spark plugs lead to misfiring, and that, too, will cause the car to shake.

Replacing the affected part will solve the problem and prevent shaking for a long time to come.

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