Why My Car Shakes at Idle but Smooths Out While Driving?

If you have a car that shakes at idle but smooths out while driving can be a challenging issue to diagnose. It is a frustrating issue that can cause confusion and safety concerns.

Everyone wants to enjoy a smooth ride while driving, but when something malfunctions, it can lead to an unpleasant experience. But it is very strange when the car shakes at idle but smooths out while driving. It is essential to address this problem promptly to avoid further damage to the car and potential safety hazards. Identifying the cause of the problem is crucial to determine an effective fix.

Reasons Car Shakes at Idle but Smooths Out While Driving

There are various reasons why a car shakes during idle but smooths out while driving. It can be due to simple causes like a faulty idle speed control system, damaged hoses, worn-out timing belts, fouled spark plug, or a faulty motor mount. To identify the root cause of the problem, it is necessary to inspect the fuel intake system as well.

Car Shakes at Idle but Smooths Out While Driving

The following are the most common reasons for a car shaking at idle but smooths out while driving:

  • Faulty Engine Mount
  • Damaged Coil Pack
  • Bad Intake System
  • Faulty Alternator
  • Faulty Spark Plug
  • Damaged Hose
  • Faulty IAC Valve or Throttle Body
  • Damaged Timing Belt
  • Catalytic Converter Problems

1) Faulty Engine Mount

A faulty engine mount could be the culprit when a vehicle shakes while idling. The engine mount of your vehicle engine is responsible for holding the engine in its original place and minimizing vibration.

Faulty Engine Mount

If the mount becomes worn or broken, it fails to keep the engine in position, resulting in excessive vibration in the engine compartment.

This issue leads to increased vibration when your car is stationary or idling. If your car stops shaking once you press the brakes, it indicates a problem with the motor mount that needs to be changed. This problem may not be noticeable when the vehicle is in motion.

Read More: Bad Engine Mount Symptoms and Causes

2) Damaged Coil Pack

The coil pack is responsible for taking low voltage from the car battery, transforming it into high voltage, and then transmitting it to the spark plug to produce a spark according to the requirements.

Damaged Coil Packs, causes of a shaking car at idle

When the coil pack malfunctions, it fails to supply the necessary power to the spark plug, resulting in poor combustion of the air-fuel mixture. This inadequate combustion can cause the engine to shake at idle. The cost of replacing coil packs varies depending on the specific make and model of the vehicle.

Read More: Bad Ignition Coil Symptoms and Causes

3) Bad Fuel Intake System

A faulty fuel intake system is one of the major reasons of your car’s rough shaking or vibration at idle. Over time, the fuel intake system can become dirty and contaminated as the vehicle is driven. In severe cases, the valves may become clogged due to the accumulated debris.

When the vehicle engine valves are blocked, the engine will be unable to receive a consistent supply of fuel. As a result, an irregular amount of fuel is delivered, causing shaking while idling.

Bad fuel Intake System

If you suspect the engine is not functioning properly during idle, use an OBD2 scanner to check for any trouble codes. This can assist you in identifying the problem more quickly and affordably. If you encounter misfire-related trouble codes, it is advisable to inspect the ignition coil and spark plug.

4) Faulty Alternator

Faulty Alternator

The alternator plays a crucial role in charging the battery and supplying power to the vehicle’s electrical system. If the alternator is not functioning properly, the engine may experience vibrations at idle due to an inadequately charged battery. Repairing a malfunctioning alternator can be quite costly.

Read More: Bad Alternator Symptoms and Causes

5) Faulty Spark Plug

A damaged or faulty spark plug is one of the most common reasons a car shakes at idle but smooths out while driving. The spark plug of your vehicle serves a crucial function in igniting the fuel-air mixture within an internal combustion engine.

By producing a spark, the spark plug initiates the combustion process in the engine cylinder. It receives power from the ignition coils in order to produce this spark.

faulty Spark Plug

A deteriorated or contaminated spark plug may struggle to create an adequate spark, leading to increased strain on the combustion process. As a result, the engine may experience different issues, such as misfires and vibrations. If your spark plugs are in poor condition, the engine’s fuel consumption rate may also increase significantly.

6) Damaged Hose

If your car is shaking violently at idle but smooths out while driving, a faulty vacuum hose could be the culprit. This can also cause engine surging or hesitation, resulting in diminished performance.

engine Hose

7) Faulty IAC Valve 

Most passenger vehicles are designed to maintain an idle speed of 600 to 1,000 RPMs. At this pace, the vehicle engine should not stall and should provide sufficient power to operate the air conditioning system and other parts.

The latest engines contain an idle air control (IAC) valve, which is responsible for regulating the engine’s idle speed. However, this valve may become clogged with carbon deposits, leading to erratic behavior.

Faulty IAC Valve, reasons of car shaking while driving

It is worth noting that not all vehicles are equipped with an IAC valve. Many latest car models employ electronic control systems on the throttle body, rendering the IAC valve irrelevant for a significant number of drivers.

8) Damaged Timing Belt

Various belts and hoses within the vehicle must be in optimal condition to ensure a smooth driving experience.

bad Timing Belt

A damaged belt or hose can lead to shaking and create extreme noise. Take, for example, the serpentine belt. If it becomes worn or loose, it may result in increased shaking at idle, and a damaged belt may also damage other important parts of your engine.

9) Catalytic Converter Problems

bad Catalytic Converter, car shaking at idle

The primary function of a catalytic converter is to remove harmful pollutants from your vehicle’s exhaust by converting them into less toxic substances. If it fails to perform efficiently, it can lead to the accumulation of deposits, which in turn can cause your car to vibrate or rattle when idling.

How to Fix a Shaking Car at Idle

The replacement or repair of one of the below-given parts can fix a car that shakes at idle but smooths out while driving:

  • Replace engine mount
  • Replace worn timing belts
  • Replace IAC valve
  • Replace spark plugs
  • Repair the fuel intake system

1) Replace Engine Mount

If your engine shakes violently while idling but smooths out while driving, it is likely that one or more engine mounts are damaged. You can test this by attempting to move the engine in various directions to check for looseness.

To eliminate shaking caused by a faulty motor mount, you need to replace it. The cost of replacing an engine mount typically ranges from $180 to $720, with the price varying depending on the accessibility of the mounts.

2) Replace Worn Timing Belts

The damaged or worn timing belts may lead to the car shaking at idle. The cost of replacing a damaged belt or hose depends on labor expenses, vehicle model, and the specific brand.

3) Replace IAC Valve

Another common cause of vehicle shaking at idle is a faulty or clogged IAC valve. If the IAC valve of your vehicle is obstructed, ensure that it is cleaned thoroughly. You can find the IAC valve in the intake manifold, situated just behind the throttle body.

4) Replace Spark Plugs

If your car experiences a misfire during idle, it is possible that your spark plug is damaged or faulty. If they are dirty, you can clean them using a spray-on cleaner specifically designed for spark plugs and a wire brush to remove contaminants from the electrode.

However, it may be more economical to replace dirty spark plugs altogether. For most vehicle models, spark plugs can be found at prices ranging from $4 to $25 each. The most significant expense in replacing spark plugs is the labor, particularly if they are difficult to access.

5) Repair the Fuel Intake System

Examine the fuel intake system to find the necessary services. During this inspection, it is also a good idea to check the fuel pump.

Fuel induction services differ based on the vehicle’s requirements. In some instances, a simple fuel induction cleaning may suffice. However, in other cases, maintenance may be needed for the intake valves, throttle plate, and intake manifold as well.

Frequently Asked Questions 

What causes a car to vibrate when idling?

  • Faulty engine mount
  • Damaged coil packs
  • Bad intake system
  • Faulty alternator
  • Faulty spark plug
  • Damaged hose
  • Faulty IAC valve
  • Damaged timing belt
  • Catalytic converter problems

What causes rough idle at low RPM?

Rough idle at low RPM can be caused by different reasons, such as an imbalanced air-fuel mixture, dirty or malfunctioning fuel injectors, bad ignition coils, worn spark plugs, a faulty IAC valve, or vacuum leaks. Proper diagnosis is important to find the main issue and apply the appropriate fix.

Why does the car shakes when idling but not in neutral?

If the car shakes when idling in gear but not in neutral, it may be due to a problem with the engine mounts or transmission mounts. When the car is in gear, there is more stress on the mount, which can increase shaking if they are damaged or worn. In neutral, the stress on the mount reduces, which leads to very low noticeable shaking. Inspect the mounts for wear or damage and replace them if needed.

What repairs can fix the car vibration during idling?

  • Replace the damaged engine mount
  • Replace worn timing belts
  • Replace bad IAC valve
  • Replace bad spark plugs
  • Repair the damaged fuel intake system
  • Replace the faulty catalytic converter

Can a bad spark plug cause the car to shake at idle?

Yes, a faulty spark plug can cause a car to shake at idle. When your spark plugs don’t work efficiently, they lead to inefficient or poor combustion in the affected cylinder. Poor combustion leads to uneven engine operation, which manifests as vibrations or shaking while the car is idling.

Can a bad PCV valve cause rough idle?

Yes, a bad PCV valve may cause a rough idle. If your PCV valve is stuck open or clogged, it may disturb the air-fuel mixture, leading to poor combustion and rough engine operation, which can be noticed as a rough idle.

Is it normal for the engine to shake slightly?

It is fairly typical for a car’s engine to exhibit some degree of vibration during normal operation. This is due to the rotation of essential components like the crankshaft, transmission gears, and other internal engine parts. These movements can generate mild vibrations that are considered normal.

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