Car Shakes At Idle But Smooths Out While Driving [Causes & Fixes]

car shakes at idle but smooths out while driving

Is your car shaking when idle but smooths out while driving? It is one of the most common problems many drivers face, caused by many factors.

If you notice that your car shakes at idle, it could be due to any of the following reasons:

  • Faulty throttle position sensor
  • Faulty idle air control valve
  • Faulty fuel pressure regulator
  • Dirty throttle body or sensor
  • Worn serpentine/timing belts
  • Faulty timing chain tensioner
  • Bad motor mounts
  • Fouled spark plugs
  • Loose/broken vacuum hoses

Rough idle is undesirable since the vibration in the cabin can be uncomfortable. Your car may be safe to drive, but what happens when you stop at the red light or stop sign?

Continue reading for a detailed explanation of why your car shakes at idle but smooths out when driving, including how to fix the problem.

Is It Normal for a Car to Shake When Idle?

Yes. It is normal for your car to shake just a little at idle, but it won’t be noticeable. You won’t even realize your car is idling if everything is okay.

By design, your car or engine will vibrate. Besides, how do you expect the crankshaft, piston, and all other moving parts to do so without causing vibrations?

An engine in good condition will run smoothly at idle. An idling engine is when you start the car and leave it to run without engaging the gear or pressing the gas pedal. The engine in this condition should provide enough electrical power for the accessories.

A smoothly idling engine receives the proper air-fuel mixture to burn. The reading on your tachometer should also be steady, meaning it maintains a constant RPM at idle.

If the tachometer jumps up and down or the car noticeably shakes, the engine idling is rough. It causes discomfort to you and the passengers, and there is a problem you should fix. It can even result in more damage if not addressed on time.

Is it Safe to Drive a Car with a Rough Idle?

Yes. Your car may be safe to drive with a rough idle, but not for long. A rough idle means something is wrong with your engine. Some of its parts may not be functioning or failing.

A rough-running engine should be diagnosed on time and fixed to enhance safety. Do not assume a rough idle. It can lead to poor fuel economy, performance issues, starting issues, and potentially severe engine damage in the future.

Although you can still drive your car because the engine smooths out at higher RPMs, make an appointment with your mechanic at the first opportunity.

What Causes a Car to Shake Idle but Smooths Out While Driving?

As mentioned, a smoothly running engine can only shake a little at idle but should be unnoticeable in the cabin. If you notice a violent shake, the idling is rough, indicating something is not working in your engine and associated components.

An engine can be rough at idle but smooths out when you drive because of the following reasons:  

1. Dirty Throttle Body

Your car can shake at idle when the throttle body is dirty. That is because the dirt constricts or prevents enough air from entering the engine.

If dirt covers the throttle plate, the engine will be starved of oxygen. It will run either too lean or too rich, which causes rough idle.

A dirty throttle body can make the car hard to start. Fortunately, you can fix this problem by cleaning the throttle body to allow more air into the engine.

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2. Faulty Throttle Position Sensor

car shakes at idle

A throttle position sensor is a vital component of your engine. The engine control unit (ECU) cannot know how far you press the gas pedal if the sensor is faulty or dirty.

A defective throttle body sensor sends wrong information to the ECU, which interferes with idle speed control.

The ECU may be tricked that you are pressing the accelerator pedal even at idle, which causes the engine to run too rich. That leads to rough idle and more vibrations in the cabin.

The throttle position sensor can be damaged by heat or when oil gets into it during an oil change. Cleaning it if dirty or replacing it if faulty solves the problem.

3. Faulty Idle Air Control Valve

As the name suggests, the idle air control valve controls the amount of air entering the engine at idle. If it is defective, the car will run rough. Less air means the engine will run too rich, with less oxygen reaching the combustion cylinders.  

The idle air control valve has a small vacuum controlled by the engine’s vacuum. It can fail due to hose leaks or breaks, causing rough idling.

4. Faulty Fuel Pressure Regulator

The fuel pressure regulator controls the amount of fuel that enters the engine at idle. If faulty, you can have too much or too little gasoline delivered to the engine. That means your car will shake at idle because the engine will run too rich or too lean.

But how do you know your fuel pressure regulator is faulty? If your car shakes after shifting to gear and not pressing the gas pedal, the pressure regulator is the likely culprit.

However, the problem can also occur due to a faulty ignition control module. If your car dies at a stop sign or stops at idle, the ignition system is the problem. It prevents fuel injectors not to fire, which causes the engine to stall.

5. Faulty Oxygen Sensor

The oxygen sensor is often overlooked because of its small size. Don’t let the size deceive you because this component is as crucial as all other engine parts.

An oxygen sensor measures the oxygen level in the exhaust and transmits the data to the ECU. After processing, the ECU knows how to get the appropriate air-fuel mixture to deliver to the combustion cylinder.

If the oxygen sensor is faulty, the ECU gets confused and may not get the air-to-fuel ratio right. That will cause all sorts of problems, including rough idle.

A faulty oxygen sensor will make the car have to start. You will also realize a drop in fuel efficiency and an increase in emissions. Fix your oxygen sensor as soon as it fails to keep your engine in the best running condition.

6. Loose Battery Cables

Loose Battery Cables

Loose battery cables are the unlikely cause of rough idling or car shaking at idle on this list. As odd as it may sound, this problem causes rough idling or prevents the engine from starting.

The loose cables deliver electrical power intermittently or can cut it off. Electricity is required to precisely time ignition, deliver fuel, and control the air that enters the engine. So, everything must be connected tightly for a continuous current flow through all electrical systems in the car.

The same applies to your car’s alternator. A defective alternator will cause rough idling because it does not generate enough power for the electrical systems. The battery will struggle to keep up since it is not being charged.

7. Worn Serpentine and Timing Belts

Your car engine has two belt drives: serpentine and timing belts. These are vital for engine operations.

A serpentine belt drives engine accessories such as the alternator, water pump, air conditioning compressor, power steering pump, and many others. If it is worn out or loose, it may cause the engine to shake at idle.

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The same applies to your timing belt. This component connects the crankshaft and camshaft, keeping the two synchronized. If damaged or defective, the piston and valves fall out of synchronism. This can cause misfiring and violent engine vibrations.  

These belt drives rely on tensioners to keep them under appropriate tension. But the tensioner can become faulty over the engine’s life, which will slacken the belt.

Slackened belt drives cause vibrations as they wobble when turning. Checking the belts and tensioners and replacing the defective parts can solve the problem.

8. Bad Motor Mounts

Motor mounts secure the engine to the vehicle frame. Keeping the engine from shaking at idle becomes nearly impossible with weak mountings.

Bad motor mounts will cause vibrations which become more pronounced when the engine is idling or stopped. But how do you know the motor mounts are the problem?

If the vehicle stops shaking when you shift to neutral gear, the motor mounts are the possible culprits. They are the likely culprits.  

9. Fouled Spark Plugs

Spark plugs fire the air-fuel mixture in the combustion cylinder. If they are dirty or fouled, they can misfire or fire inappropriately, causing vibrations.

Diesel engines do not have spark plugs because the air-fuel mixture ignites when compressed. So, this does not apply if you are driving a car powered by a diesel engine.

10. Loose/Broken Vacuum Hoses and Gas Cap

Broken Vacuum Hoses

The vacuum hoses help to remove exhaust gasses from the vehicle. After combustion, the gasoline by-products have to be removed from the engine immediately. Clearing the exhaust becomes compromised if there is a vacuum leak due to broken hoses.

Gas caps are often overlooked, but they are some of the most common causes of rough idle. If they are loose, they allow air to enter the fuel system. That causes the engine to work harder to keep running.

Check your gas cap and replace it if it cannot fit as recommended. Also, if the gas cap remains too tight for too long, it can cause some fuel to evaporate into carbon particles.

11. Blocked Fuel Injector

Your fuel injector can be blocked over time, especially when the fuel quality is below the standard. This restricts the flow of fuel to the engine.

A malfunctioning fuel intake system will cause your engine to run rough at idle. That is because the engine does not get a steady fuel supply. The unregulated amounts reaching it will cause vibrations, which become more pronounced at idle.

How to Fix a Car Shaking at Idle but Smooths Out While Driving

Diagnosing the problem can be one of the most time-consuming processes because of the many possible causes. It involves checking them one at a time to find the culprit.

After a complete diagnosis, fixing or stopping your car from shaking at idle can involve doing any of the following:

  • Replacing Motor Mounts
  • Cleaning/Changing Spark Plugs
  • Replacing Serpentine/Timing Belts
  • Fixing Idle Air Control Valve
  • Cleaning Throttle Body or Sensor
  • Replacing or cleaning the oxygen sensor
  • Replacing or cleaning the throttle position sensor
  • Snug-fitting loose battery terminal connections
  • Replacing faulty timing belt tensioner
  • Fixing loose gas caps and vacuum hoses

Cost of Fixing a Car That Shakes When Idle

The cost of fixing the problem varies, depending on the cause. It also depends on the make and model of your car.

The labor cost will also vary depending on your location. As a result, quoting an actual figure as the cost of fixing the problem is impossible. Consider the values below as estimates, which can change over time.

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Parts Replacement cost
Motor mounts replacement $200 to $600
Spark plug replacement $50 to $100 and $400 for high-performance engines
Fuel injector replacement $600 to $12,000
Fixing broken or loose vacuum hoses $200 to $500
Timing belt replacement $300 to $500
Intake air flow sensor replacement $150 to $400
Throttle body replacement $200 to $400
Fixing fuel intake system $100 to $300
Alternator replacement $500 to $2500
Throttle position sensor replacement $150 to $200
Oxygen sensor replacement $50 to $300

FAQs

Is it safe to drive when your car is shaking?

If the car is only shaking at idle and smooths out, it may be safe to drive. But if it shakes even when driving, the transmission or wheels may be the problem, which can be dangerous. Generally, contact a mechanic to check your car when it shakes before your trip.

Is it normal for the engine to shake a little?

Yes. Your car’s engine is designed to shale a little when idle or running at high speeds. However, the vibrations will not be noticeable in the cabin. If the vibrations cause discomfort to you and the passenger, something is not working in the engine. Call a mechanic to check it.

Can low oil cause cars shake?

Yes. Low oil can cause your car to shake. Apart from lubrication and cooling, oil pressure is used to retard and advance the camshaft for proper timing of the ignition. If the oil level is low, it cannot generate enough pressure or effectively lube the engine parts. The result will be high vibrations as the engine runs.

Can wrong gear oil cause vibration?

Yes. Car engines are sensitive to oil viscosity. If the wrong oil is used, it can be too thick or thin. Such oils fail to lubricate the engine parts, leading to vibrations and shaking of the car.

Can transmission cause the car to shake?

Yes. Low transmission fluid will cause shaking when you accelerate. However, transmission problems will not cause your car to shake at idle because it is not engaged. Do not ignore any car shake. 

Can a bad fan clutch cause vibration?

Yes. Fan clutch naturally causes vibrations, but these are unnoticeable in the cabin. However, if it is faulty, the vibrations can intensify and cause discomfort to car occupants.

Final Remarks

If your car shakes at idle but smooths out when driving, there is a problem with fuel and air delivery to the engine. Most likely, the throttle body is dirty, the throttle position sensor is damaged, the timing belt and serpentine belts are worn-out, and many other causes.

Diagnose the problem and fix it as soon as possible. Do not continue driving your car with a rough idle, as that can cause more severe engine damage. Contact your mechanic or try to solve the issue yourself. It involves fixing or replacing defective parts.

Depending on the cause of the rough idle, fixing it can be a DIY task or require a professional mechanic. Do not handle it yourself if you doubt your DIY skills!

DrillandDriver

I’m Brian, a car fanatic, and the founder of DrillandDriver.com, I’ve been into cars my whole life. Actually, not just into them but obsessed with them. I have been working as a mechanic for over 15 years and have been deeper into the motor and electric vehicles for some years.

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