Why Car Shakes at Idle but Smooths Out While Driving?

If you have a car that shakes at idle but smooths out while driving, it may be difficult to diagnose the main issue. This issue is not certainly a hard nut to fix when you understand the causes and how to fix them properly.

You want a smooth ride when your vehicle hits the road, but it doesn’t move smoothly when something goes bad. But it would be strange if your vehicle moves roughly when idling but smoothly while driving. This article mainly explains the causes of the car shakes at idle but smooth out while driving.

Car Shakes at Idle

Causes of Car Shakes at Idle but Smooths Out While Driving

Your car shakes at idle but smooths out while driving due to one of the below-given causes:  

  • Bad spark plugs
  • Broken coil packs
  • Faulty alternator
  • Bad IAC valve or throttle body
  • Faulty intake airflow sensor
  • Clogged fuel injector
  • Bad fuel intake system
  • Worn belts/hoses

1) Engine Mounts

A faulty engine mount is one of the major causes of vehicle shakes at idle. The engine mount is installed to hold the engine in its original place and reduce vibration. A worn or broken mount doesn’t hold the engine in the engine compartment; due to that, you may observe extreme vibration in the engine compartment.

Engine Mounts

This problem causes more frequent vibrations when idling or stationary. If the vibration stops when you stop the vehicle, it means something is wrong with your engine mounts that should be replaced. Also, you may not observe this issue when your car is moving.

2) Bad Spark Plugs

The spark plug plays an important role in the combustion of the air-fuel mixture. It generates a spark that is used to ignite the air-fuel mixture inside the combustion chamber. The spark plug takes power from the ignition coil to generate the spark.

Bad Spark Plug

A worn or dirty spark plug doesn’t generate a spark properly, causing the engine to put more stress on the combustion process. When this happens, your engine may face misfiring and shaking issues. In case of bad spark plugs, your engine starts to consume more fuel than usual.  

3) Broken Coil Packs

The coil pack takes low voltage from the vehicle battery, converts it into high voltage, and transfers it to a spark plug to generate a spark.

If the coil pack goes bad, it will not deliver sufficient voltage to the spark plug, which can lead to poor combustion. This poor combustion may lead to engine shake. According to the vehicle model, the replacement cost of the coil packs is different.  

4) Faulty Alternator

The alternator is responsible for properly charging the battery and delivering power to the electrical system. If your car’s alternator doesn’t work efficiently, your car engine may start vibrating at idle because the battery isn’t properly charged. Fixing a faulty alternator on your car can cost you a fortune.

bad Alternator

5) Bad IAC Valve or Throttle Body

Most passenger cars are premeditated to idle between 600 and 1000 rpm. The engine should not stall at this speed. It also provides sufficient power to run air conditioners and other fittings.

Throttle Body which causes to car vibration at idle

Some types of engines highly depend on the IAC (Idle Air Control) valve. The IAC valve controls the idle speed of your engine. But this valve can be clogged due to dirt and carbon deposits, which can lead to erratic operation.

The throttle body uses to control the air entering the engine. A bad throttle body may cause too little or too much air to enter the engine, causing the engine to vibrate.

6) Faulty Intake Airflow Sensor

The intake air temperature sensor calculates the temperature of the air entering the engine. When this sensor goes bad, it sends wrong information to the powertrain control module (PCM).

When PCM gets wrong information, it doesn’t regulate air according to the engine requirements, which may lead to engine shaking. A faulty intake air temperature sensor may also cause engine overheating.

7) Clogged Fuel Injector

Over time, the fuel injectors of your vehicle may become clogged due to contamination. If you don’t clean them quickly, the contamination affects the engine’s fueling process and makes it difficult to complete the combustion cycle. Improper fuel delivery may also lead to the car shaking at idle.

Leaky Fuel Injector

8) Bad Fuel Intake System

If your vehicle vibrates or shakes a lot at idle, you may have a problem with your fuel intake system. Driving the vehicle for a long time affects the fuel intake system. Over time, the fuel intake system may be clogged with dirt. In some conditions, the valves can clog with all the deposits.

If the fuel intake valve is blocked, the fuel pump cannot deliver fuel properly to the engine. On the contrary, fuel delivery becomes uneven, and vibration appears at idle.

If you suspect your engine is idling poorly, use an OBD-II scanner to check for trouble codes. This method helps you to find the problems faster.

Read More: Bad Fuel Pump Symptoms and Causes

9) Worn Belts/Hoses

The vacuum hoses of your car assist in removing exhaust fumes. Your car has multiple hoses and belts that must be properly aligned to function properly. Worn or damaged hoses and belts may cause excessive vibration and noise.

If the vacuum hoses are damaged or loose, the leak may affect the process of removing gasoline byproducts and cause the engine to shake. Therefore, worn hoses are one of the major causes of car shakes at idle but smooths out while driving. 

How to Fix Shaking Car at Idle?

The following repairs can fix the shaking car at idle:

1) Replace Motor Mounts

If the engine idles a lot but runs normally, one or two of the engine mounts may be broken. You may push and move the motor in different directions to see if it’s loose.

You need to replace the engine mounts to fix the car shaking at idle. The replacement cost of the engine mounts is from $190 to $210.

2) Change Spark Plugs

If your car is stalling at idle, you may have a bad spark plug. If the spark plug is clogged, you should clean it as soon as possible. Use a spark plug spray cleaner and wire brush to remove deposits from the electrodes.

However, if your spark plug is damaged, replace it. The average replacement cost of the spark plug is from $5 to $39.

3) Repair Fuel Intake System

Check the fuel intake system to check what maintenance is required. It is advisable to also inspect the fuel pump during this inspection.

Fuel induction services change according to your vehicle requirements. In some cases, only a fuel induction cleaning may be needed. Service is also required for the intake manifold, throttle body, and intake valves.

4) Replace IAC Valve

A bad or clogged IAC valve is also a major cause of car vibration. If your IAC valve is clogged, properly clean it. The IAC is located in the intake manifold just behind the throttle body.

If your IAC valve is damaged, replace it. The average replacement cost of the IAC valve is from $90 to $520.

5) Replace Worn Belts/Hoses

Your car may start shaking at idle due to worn belts or hoses. The cost of a bad belt or hose replacement cost varies according to the vehicle model, labor cost, and the type of brand.

FAQ Section

What repairs can fix the car shakes at idle but smooths out while driving?

  • Replacing the bad fuel pump
  • Replacing or cleaning the throttle body
  • Replacing the bad IAC valve
  • Replacing the bad throttle valve
  • Replacing the engine mounts
  • Cleaning or replacing the spark plugs
  • Replacing the bad fuel injectors
  • Replacing the damaged or worn timing belts

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

It’s very rare for a car to shake while idling, no matter how small it is. If your vehicle shakes at idle, you may have a problem with your engine mount.

Defective engine parts such as fuel injectors, spark plugs, or timing belts can also cause the vehicle to idle unevenly.

If the vehicle shakes when parked with the air conditioning on, it’s a good idea to take the time to identify any potential problems and evade further risk or damage. If you cannot fix the issue yourself, seek professional help.

Why does my car vibrate so much when idling?

If the vehicle shakes a lot at idle, you may have a problem with your engine mounts. The vehicle engine mounts use to dampen vibrations, especially when the vehicle is idling.

Damaged engine mounts can no longer automatically dampen vehicle vibrations, so you will feel just as much vibration when idling.

If the vehicle vibrates at idle with the A/C on, shift into neutral (N) and watch for a while to see if the engine mounts are bad. If the vibration decreases, most likely, the motor mount is damaged.

Can a bad spark plug cause the car to shake?

A faulty spark plug may cause your car to shake when idling. A bad spark plug can cause uneven fuel consumption during the combustion process. In such a situation, your vehicle’s RPM will fluctuate, and the engine will make a lot of noise.

In addition to fluctuating RPM and loud noise, the force the engine is exerting to complete the combustion cycle can cause the car’s engine to start vibrating due to damaged spark plugs. The vibrations can spread from the engine to any part of the vehicle.

Is it safe to drive when your car is shaking?

Driving a shaky car is dangerous and may damage your car further. If you’re not sure what caused the vibration, it’s a good idea to be cautious and have your car checked by a mechanic. Once you find the reason for the vibration, you can move further to fix the issue and get back on the road safely.

Is it safe to drive a car with rough idle?

If the engine tensioner or timing belt is damaged, the valves and pistons will not move properly, and the engine may not run properly. In such conditions, do not continue driving until the problem is resolved.

Similarly, a vacuum leak in the engine should be repaired before driving. This is because a large enough port can create too much pressure in the cylinder and prevent air from being released once combustion starts.

Why is my car shaking when idle but not when driving?

  • Bad spark plugs
  • Worn belts/hoses
  • Broken coil packs
  • Bad fuel intake system
  • Faulty alternator
  • Bad IAC valve or throttle body
  • Clogged fuel injector
  • Faulty intake airflow sensor

Why is my car idling weird but driving fine?

You have a rough idle if you feel the roughness when your car idles but drives smooth. This is generally caused by a change in the amount of air entering the engine, vacuum lines, the regularity of the ignition, and many other factors.

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