Car Misfire At Idle Only: Causes And How to Fix It

Jos Fallon
Car Misfire At Idle Only

If you’re experiencing a sudden jerk or shake when your car is idling, it indicates that your vehicle is experiencing engine misfiring. This could mean a more serious underlying issue with your vehicle, which should not be ignored.

A faulty spark plug or ignition coil is one of the primary reasons why a car misfires at idle. Other potential causes include vacuum leaks, faulty fuel injectors, malfunctioning air control valves, and low compression.

Further, in this article, we’ll discuss what engine misfiring is, the symptoms of engine misfiring, the five causes of why a car misfires at idle only, and how to fix a car that misfires at idle.

Car Misfire At Idle Only

What is Engine Misfiring?

Engine misfiring is a common problem in vehicles where the engine does not run smoothly. It occurs when one or more cylinders in your car’s engine fail to ignite fuel in the combustion chamber at the right time, which disrupts the engine’s power delivery.

This can lead to poor acceleration, reduced fuel efficiency, and other issues. If the misfiring is not fixed promptly, it will severely damage your whole engine.

Symptoms of Engine Misfiring

The most noticeable symptom of engine misfiring is a sudden jerk or vibration when the car is idling.

Other signs of engine misfiring include decreased power and acceleration, rough idle, engine hesitation or stumbling reduced fuel efficiency, and loud exhaust noise.

5 Reasons Why a Car Misfires at Idle only

To fix the issue, you must be fully aware of the sources that may have caused the misfiring. So, here’s a list of 5 major components that have contributed to car misfiring in an idle state:

Dirty or clogged fuel injectors

1. Dirty or clogged fuel injectors

Fuel injectors play a crucial role in the engine’s combustion process, designed to deliver atomized fuel to the engine’s cylinders.

If the fuel injectors are clogged, they don’t deliver the right amount of fuel to the combustion chamber, which can cause incomplete combustion, leading to an engine misfire.

How to clean injectors?

Injectors need to be removed for cleaning. However, eliminating fuel injectors is daunting, and we don’t recommend it for the amateur car owner. The place of injectors varies greatly based on the vehicle models.

However, if you want to try it, watch a DIY video where an expert carefully removes and cleans them.

Fuel injection kits are available in the market for cleaning fuel injectors. The injectors are pumped with a solution that dissolves and carries out the inner dirt and debris.

2. Faulty spark plugs

Spark plugs play a crucial role in the engine’s ignition system by creating the spark that ignites the fuel in the combustion chamber.

If the spark plugs are damaged or burnt, they may not be able to create the right amount of spark, which can cause engine misfiring. In some cases, spark plugs may become fouled with oil or carbon, leading to engine misfiring.

The error can identify the faulty spark plugs codes P0301 to P012. Here, the last two digits represent the engine cylinder where the misfire occurs.

How to Replace a Spark Plug?

  1. Begin by disconnecting the battery and ensuring that the engine has cooled down.
  2. Then, remove the spark plug wires or coil packs from the spark plugs.
  3. Use a spark plug socket to remove the spark plugs from their sockets carefully.
  4. Thoroughly inspect each plug for any visible issues based on the Champions auto chart, and replace any burnt or damaged ones.
  5. Once you have inspected the plugs, reinstall the spark plug wires or coil packs back onto the spark plugs.

3. Vacuum leaks

Vacuum leaks occur when there is a hole or opening in the engine’s intake system, allowing air to enter the engine without passing through the throttle body. This can cause an imbalance in the air-to-fuel ratio, leading to the engine misfiring at idle.

This is probably one of the common reasons the engine misfires in an idle state. Apart from misfiring, vacuum leaks can also result in decreased fuel efficiency.

Vacuum leaks are diagnosed with error code P0300, indicating a vacuum leak near the intake manifold. Sometimes, the codes P0171 and P0174 appear, which show a lean mixture causing a misfire due to vacuum leaks.

How to fix vacuum leaks?

To see the exact location of vacuum leaks, use a propane cylinder.

However, that may be dangerous to try it yourself. So, you must visit a professional to replace the intake manifold or other leaking components.

driving car
Photo by Oleksandr Pidvalnyi

4. Malfunctioning idle air control valve

The idle air control valve regulates the engine’s idle speed by controlling the air entering the engine. If the idle air control valve is not functioning correctly, it may cause the engine to misfire at idle.

In some cases, the idle air control valve may become stuck or clogged with dirt, preventing it from properly regulating the idle speed.

How to fix a malfunctioning air control Valve?

  • Locate the IAC valve, which is situated adjacent to the throttle body on your engine.
  • Ensure the car is switched off before detaching any battery cables connected to the valve.
  • Using a fine cloth, carefully remove any dirt, carbon deposits, or other debris impeding the valve’s proper functioning.
  • Thoroughly inspect the valve for any signs of damage or wear, and replace it with a new one if it appears damaged.
man in black jacket and black knit cap inspecting car engine

5. Low compression

Low compression in one or more cylinders can cause the engine to misfire at idle. Compression is essential for the engine to function properly, as it creates the pressure needed to ignite the fuel in the combustion chamber.

Low compression can occur for various reasons, including worn or damaged piston rings, faulty valves, or a damaged cylinder wall. A low compression usually throws a misfire code, as we mentioned earlier, along with check engine light illumination.

How to fix it?

Low compression is likely caused by internal wear and tear of engine cylinders. Probably, you won’t be able to take that apart with your basic DIY skills. So, better consult a skilled technician.


A car misfire at idle is a common problem that can lead to severe engine damage if not addressed promptly. This misfire is usually caused by one of the reasons mentioned in the article.

You can identify the source and fix the issue by keeping yourself aware of misfiring symptoms and error codes. In this case, most of the fixes require technical knowledge and expertise.

So, we recommend getting your car inspected by a professional mechanic if any of the misfiring symptoms appear.

How useful was this post?

Click on a trophy to rate it!

Average rating 5 / 5. Vote count: 2

No votes so far! Be the first to rate this post.

We are sorry that this post was not useful for you!

Help us improve it for you and others.

Tell us how we can improve this post? Any and all details appreciated.

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Posts