What Causes Car Shakes At Idle But Smooths Out While Driving?

Have you ever heard someone complain that his car shakes at idle but smooths out while driving? Or are you currently experiencing this kind of challenge with your vehicle? It can be unnerving and frustrating, especially when you have no clue what could be wrong.

This problem is not necessarily a hard nut to crack when you understand the causes and how to fix them appropriately. Let’s find out what causes a car to shake when idle and smooth out while on the road.

Car Shakes At Idle But Smooths Out While Driving

What Causes Car Shakes At Idle But Smooths Out While Driving?

The possible reasons why a car shakes when idle but smooths out driving include damaged motor mounts, faulty spark plugs, bad gas caps, faulty position sensors, weak oxygen sensors, loose vacuum hoses, loose battery cables, dirty throttle, or bad throttle sensor.

If you are facing this problem, diagnose the engine compartment thoroughly and fix any malfunctioning components. Contact your mechanic if you can’t track and address the problem yourself.

Defective motor mounts

A vehicle’s motor mounts help to hold the engine in place within its compartment so that it doesn’t move or shake when running.

The motor mounts (sometimes called engine mounts) keep the transmission and engine to the subframe while absorbing shocks and vibrations so that the driver doesn’t feel any of such impacts when driving.

If the motor mount breaks, it will no longer hold the engine properly, resulting in uncontrollable shaking and vibration.

Bad spark plugs

A vehicle’s spark plugs play a significant role in the combustion process; they provide the spark that ignites the air-fuel mixture in the combustion chamber.

If any of the spark plugs become worn out or dirty, they will not be able to function properly, thereby causing the engine to exert more stress in the combustion process. When this happens, the engine will misfire and shake randomly.

Clogged fuel injectors

A car’s fuel injectors or fuel intake system can become blocked with dirt over time. If you fail to unclog the injectors, they will not supply adequate fuel needed in the combustion chamber. This will invariably cause the engine to struggle to complete the combustion process. An uneven or inadequate fuel supply will cause engine vibration.

Loose hoses

Disconnected or loose vacuum hoses happen more often than you can imagine but are usually overlooked. A vacuum hose may disconnect when working in a car or while driving. If a vacuum hose disconnects, it’ll cause vibration or engine stalling in a worse case.

Worn-out belts

A worn-out timing belt is another reason a car would shake when idle. Loose or worn-out belts will cause the engine to malfunction, resulting in strange noise and shaking when a vehicle idles. Depending on the severity of the damage, the noise may become obvious when accelerating.

Missing or loose gas cap

A loose gas cap may be the culprit if your car shakes at idle but smooths out while driving with the check engine light on. A missing or loose gas cap will allow air to enter the fuel system. When this happens, the car will shake when idling but return to normal as you hit the road. A common sign of a loose or missing gas cap is the smell of gas entering the cabin.

Loose battery cables

If one of your battery terminals is not properly connected or is corroded, it may not transmit the required power to idle the vehicle. If this is the culprit, the engine may take longer to start, and the headlights may flicker sometimes. Also, you may hear a clicking noise when starting the engine.

Dirty throttle or faulty throttle position sensor

A car throttle body is an essential component that connects the fuel lines to the engine. It regulates the air-fuel ratio that goes into the combustion chamber and must be in good condition to do so. If the throttle body is dirty or faulty, it’ll affect the engine performance. This will invariably cause the car vibrate when stopped in gear or randomly shake on idle.

Also, if the throttle position sensor is faulty, it’ll send false readings to the engine control unit (ECU). The ECU will make adjustments based on the information received from the throttle position sensor. Of course, this will cause engine vibration.

Catalytic converter problems

The primary function of a catalytic converter is to convert the harmful exhaust fumes from the combustion chamber into a less toxic gas before it goes into the environment. If it’s clogged or not working properly, it will lead to car shakes when idling but not in neutral. Sometimes, the car will shake on idle, whether in neutral or not.

If catalytic converter problems are the root cause, you’ll notice a clattering noise when idling the vehicle. Also, the vehicle will fail emission inspection.

Weak or lousy O2 sensor

Oxygen or o2 sensors measure the amount of air-fuel mixture in the exhaust gas and relate this information to the car computer. The car computer uses this information to adjust the air-fuel ratio entering the combustion chamber.

If the O2 sensor is weak or doesn’t know what levels of air-fuel ratio are in the exhaust line, it’ll send no or inaccurate readings to the powertrain control module (PCM). This will lead to several problems, including rough idling.

Faulty fuel pressure regulator

Another possible reason car shakes when idling and accelerating is a faulty fuel pressure regulator. The fuel pressure regulator regulates the amount of fuel entering the engine. If too little fuel is entering the combustion chamber, the optimal combustion process will be compromised. Of course, this will lead to rough idle.

A common sign that the fuel pressure regulator is bad is when the car runs roughly even though it’s in gear and you’re not pressing the gas pedal.

Faulty idle air control valve

The idle air control valve regulates how much air goes into the engine on idle and relates this information to the ECM. If the valve fails, it’ll send false or no readings to the ECM. In turn, the ECM will work with the information it receives, leading to too much or too little air in the combustion chamber.

The car will run rough since it is either receiving too little or too much air in the combustion chamber. A bad idle air control valve will cause other problems, like a delayed response when you depress the gas pedal, meaning the car lurches backward or forward instead of speeding up smoothly.

Lousy Alternator

The alternator is responsible for recharging the battery while the engine runs and supplying power to the electrical system components. If the alternator becomes weak, it’ll lead to the car vibrating when stopped at a red light or at idle because the battery is not properly charged. If this is the root cause, you have to repair or replace the alternator.

what does it mean when your car shakes while idling

What Does It Mean When Your Car Shakes While Sitting Still?

When your car shakes while sitting still, it is most likely that the motor mounts are disconnected from the engine or broken. The motor mounts are responsible for keeping your car’s engine attached in place within the engine compartment.

Therefore, if the motor mounts are broken or disconnected, they can no longer hold the engine in place, resulting in sporadic shaking. The shaking can be excessive to the extent of impacting the entire car.

To diagnose this challenge, put your gear on neutral (N) and observe the shaking for a moment. If the shaking reduces, it is a challenge from broken motor mounts. It is crucial to fix the issue immediately to avoid further damage.

How To Fix Car Shakes At Idle But Smooths Out While Driving

Once you diagnose your car’s engine and other components to ascertain the specific cause of the shaking, it is essential to verify the faults that you can rectify, especially as a DIYer, and contact an expert auto mechanic in cases that require professional service.

Fixing the problems outlined above will not cost a fortune. However, if you neglect them, they may result in further damage. The following recommendations will help you handle the issues stated above.

  • Contact a mechanic to fix the broken motor mounts.
  • Clean or replace defective spark plugs.
  • Replace clogged fuel injectors.
  • Reconnect loose hoses and replace any broken vacuum hose.
  • Replace the worn timing belt.
  • Fix lousy alternator
  • Change or clean the idle air control valve
  • Change lousy O2 sensor
  • Replace clogged or defective fuel pressure regulator

Most of the replacements outlined above will require the service of a professional auto mechanic. However, some are DIY fixes that anyone can do.


Is it normal for a car to shake a little when idle?

No matter how little it shakes, it is quite abnormal for a car to shake when idle. If a car shakes when idle, it is a sign that something could be wrong with the engine mounts.

Also, a shaky idling vehicle may result from faulty engine components such as timing belts, spark plugs, or fuel injectors.

If your car shakes when stopped with AC on, it is best to take a moment to find out what could be wrong to avoid further damage. Contact a mechanic to address the underlying issues if you can’t figure out the problem.

Why does my car vibrate so much when idling?

You likely have defective motor mounts if your car vibrates so much when idling. A car’s motor mounts are responsible for dampening the vibrations, especially when the car idles.

Broken motor mounts will automatically be unable to dampen a vehicle’s vibration; hence you will eventually feel so much vibration when your car is idling.

If your car vibrates when idle and AC is on, kindly take a moment to ascertain whether the car’s motor mounts are defective; to do that, switch the gear to neutral (N) and observe it for a moment. If it reduces the shaking, you most likely have broken motor mounts.

Can clogged fuel injectors cause vibration?

Of course, clogged fuel injectors can cause vibration in a vehicle. It would be best to do everything possible to prevent your car’s fuel injectors from getting clogged with dirt.

A clogged fuel injector will automatically hinder the corresponding cylinder from its ability to fire adequately.

When a cylinder fails to fire correctly, the engine will find it challenging to finish each combustion cycle due to insufficient fuel, resulting in engine vibration. If you experience this kind of situation, ensure to contact an expert auto mechanic immediately.

Can a bad spark plug cause the car to shake?

A bad or defective spark plug can be a culprit to a car shaking when idle. Faulty spark plugs result in uneven burning of fuel during the combustion process in an engine. When this happens, the RPMs will fluctuate, and the engine will produce a loud noise.

Besides the loud noise and fluctuating RPMs, the car’s engine will start to vibrate due to the force the engine exerts to finish a combustion cycle due to a bad spark plug. The vibration can extend from the engine to every part of the car.

Can a bad battery cause my car to shake?

A defective battery cannot generate sufficient power to keep a vehicle’s engine running, thereby making the alternator work harder for the engine to keep running.

The stress that the engine exerts to stay running due to insufficient power from the battery will eventually cause the car to shake, stall, and sometimes stop.

A car battery is an essential component that enables a car to work efficiently; otherwise, the consequence of having a defective battery can be frustrating, especially when it leaves you stranded.

Final Words

Most of the reasons why your car shakes at idle but smooths out while driving and the solutions to the problems have been addressed above in this article. If you have a friend who says, “My car shakes when I drive,” you may want to refer them to read this article.

It is crucial to fix mechanical faults early enough before they escalate into more complicated problems. So, if you are facing this challenge, follow the recommendations above or contact an expert auto mechanic to diagnose and fix the problem.

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Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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