Engine Misfire: Meaning, Symptoms Causes, & How to Fix It

The vehicle engine relies on the precise combination of air and fuel, as well as a reliable and powerful spark, to ensure its optimal functioning. Any failure in these essential elements can hinder the proper ignition of the air-fuel mixture in the combustion chamber, resulting in a misfire. When a car engine misfires, it is crucial to promptly identify the underlying cause and address it effectively to restore normal operation. This article explains the engine misfire causes, symptoms, and how to fix it. 

What is a Misfire?

An engine misfire refers to the failure of one or more cylinders in an engine to ignite the air-fuel mixture during the combustion process. The engine misfire can result in various performance problems and, if not addressed, may even cause significant engine damage.

To achieve the proper combustion of the air-fuel mixture within the engine cylinder, three crucial elements are required: a reliable spark, proper compression, and correct air-fuel mixture. If any of these elements are incorrect or missing, the combustion process will be compromised, leading to poor engine power. 

The powertrain control module (PCM) of your vehicle uses a crankshaft position sensor to continuously observes the speed of the crankshaft. If there is no minor improvement in speed from a cylinder after combustion, it indicates an engine misfire, and a diagnostic trouble code will be stored in the memory of your vehicle’s PCM.

Causes Of an Engine Misfire

The engine misfires due to one or more of the below-given causes:

• Bad Spark Plug
• Vacuum Leaks
• Fuel Injectors
• Faulty Ignition Coils
• Bad MAF Sensor
• Bad O2 Sensor
• Intake Manifold Gasket Leaks
• Low Fuel Pressure
• Low Compression

1) Bad Spark Plug

One of the primary causes behind engine misfiring is a worn or damaged spark plug. The spark plug plays a vital role in delivering the spark needed to burn the gasoline in the gasoline engine.

When spark plugs are not functioning properly, the engine fails to fire on all engine cylinders. Symptoms of worn spark plugs include hard starting the engine, power loss, poor engine performance, and decreased fuel economy.

To address these issues, it is important to inspect your spark plug and replace it if it shows signs of wear or dirt accumulation.

2) Vacuum Leaks

Engine misfires can also be caused by a vacuum leak in the vacuum hoses or intake manifold gasket. Such leaks result in a condition called a lean misfire, where there is insufficient fuel and an excess of air in the air/fuel mixture.

Signs of a vacuum leak include the illumination of the check engine light, rough idling, engine stalling, and poor fuel efficiency. A skilled mechanic equipped with the necessary tools can easily resolve vacuum leaks.

Read More: Vacuum Leak Symptoms and Causes

3) Bad Fuel Injectors

The bad fuel injectors may also contribute to engine misfires by failing to deliver the appropriate amount of fuel to a specific cylinder.

Signs of a faulty fuel injector include rough engine operation at idle, poor fuel efficiency, check engine light illumination, and poor engine power.

A skilled mechanic can conduct tests to assess the condition of the fuel injectors and replace any that are found to be bad or clogged.

4) Faulty Ignition Coils

A faulty ignition coil is another one of the major reasons for engine misfires. The ignition coil of your car takes the low-voltage output from the vehicle’s battery and converts it into the high-voltage ignition spark.

When an ignition coil malfunctions, it may lead to a misfire in the corresponding cylinder.

When the ignition coil goes bad, it produces different symptoms, such as the illumination of the check engine light, diminished fuel efficiency, misfiring, and rough idling. It is recommended to contact a professional technician to examine the vehicle and replace any faulty ignition coils.

5) Bad MAF Sensor

The MAF sensor, also known as a mass airflow sensor, is one of the most important parts of your vehicle. It is responsible for measuring the amount of air entering the engine, providing crucial information to the vehicle’s PCM to adjust the air/fuel mixture.

A bad or dirty MAF sensor can disrupt the proper air/fuel ratio, leading to a misfire. A bad MAF sensor produces different signs, such as check engine light illumination, rough operation, failure to start, and poor fuel economy. It is advisable to contact a professional mechanic to clean or replace your faulty sensor as required.

Read More: Bad MAP Sensor Symptoms and Causes

6) Bad O2 Sensor

If the O2 sensor fails or becomes sluggish, it may provide inaccurate readings to the PCM, leading to an incorrect air-fuel mixture. This can result in poor combustion and potentially affect engine performance, including the possibility of misfires.

7) Intake Manifold Gasket Leaks

A leaking intake manifold gasket is also one of the common causes of the engine misfire. This gasket is installed between the intake manifold and the cylinder head.

When this gasket is damaged, it allows additional air to enter the engine cylinder, resulting in a lean fuel-air mixture and potential misfires.

Similarly, vacuum lines connected to the intake manifold can also develop leaks, leading to similar signs as a faulty intake manifold gasket.

8) Low Fuel Pressure

Another factor that may lead to misfire is low fuel pressure. Insufficient fuel pressure results in a reduced amount of fuel entering the engine cylinder, leading to a lean fuel-air mixture and potential misfires across all cylinders.

Low fuel pressure may be attributed to issues such as a damaged fuel filter, a faulty fuel pump, or a malfunctioning fuel pressure regulator.

Checking the fuel pressure using a manual fuel pressure gauge can help determine if low fuel pressure is the cause.

9) Low Compression

To ensure proper combustion of the air-fuel mixture, your engine requires the correct fuel-air mixture, compression, and spark. If you have properly inspected your vehicle’s spark plugs and fuel system and found no issues, it is necessary to consider the possibility of low compression in the engine.

Conducting a compression test and comparing the results to the manufacturer’s recommendations can help determine if low compression is present.

It’s important to note that low compression issues are most commonly associated with worn engine components, which can be costly to repair. Therefore, it is recommended to properly inspect other potential causes before exploring low compression as the primary issue.

Symptoms of the Engine Misfire

When your engine misfires, it produces one or more of the following symptoms:

• Rough Idling
• Poor Acceleration
• Black Smoke
• Stuttering Idle
• Illuminated Check Engine Light
• Engine Noise
• Excessive Vibration

1) Rough Idling

The rough idle is one of the most common symptoms of the engine misfiring. This is commonly felt when your car is at low speed, in high gear, and the accelerator pedal is depressed.

In case of a misfiring, your car may exhibit a jumpy or erratic movement instead of experiencing smooth and consistent acceleration.

2) Poor Acceleration

Poor acceleration is one of the most noticeable signs of engine misfire. Misfires tend to happen most regularly as the car is under load and during continuous acceleration.

As a result, the car’s response to acceleration may feel slow. Otherwise, there may be a vibrating motion while pressing the gas pedal. It is important to note that these symptoms can pose threats to both the driver and other road users.

Finding and addressing engine misfires as soon as possible is important for maintaining optimal engine performance and ensuring safe driving situations.

3) Black Smoke

Another symptom of engine misfires can be the presence of black, thick smoke from the exhaust pipe. This may also indicate that the engine is not effectively combusting the fuel-air mixture.

If you observe such exhaust emissions from your car, it could be a sign of a misfire.

4) Stuttering Idle

When the sensors responsible for maintaining the correct air/fuel mixture in the engine encounter difficulties, it can lead to erratic behavior and misfires, particularly during idle. In some cases, the engine may completely stop the vehicle.

Since idle conditions are crucial for proper air-fuel mixture control, experiencing misfires during idle can serve as an early warning sign.

5) Illuminated Check Engine Light

The latest vehicle models are equipped with advanced monitoring systems that oversee the performance of various engine sensors. Whenever an engine sensor malfunctions or monitors an issue, it relays the data to the powertrain control module (PCM).

Upon receiving the data, the PCM determines the severity of the issue. If the issue persists or is deemed significant, the PCM will activate the check engine light, indicating that there is a problem that requires attention and repair.

An illuminating check engine light is one of the clear symptoms of an engine misfire. It signals that the misfires are occurring repeatedly and should be addressed promptly.

Read More: Methods To Reset Check Engine Light

6) Engine Noise

One prominent sign of engine misfire is an abnormal noise emanating from the engine. If you are aware with the usual noises of your vehicle, you will likely notice when an unusual noise occurs.

While the engine is misfiring, your engine can produce an unexpected noise described as backfiring, sneezing, or popping.

Backfiring happens when non-combusted fuel leaves the combustion chamber during the exhaust stroke, and then it ignites further in the exhaust system due to the spark of the next cylinder, resulting in a loud explosion through the exhaust system.

Additionally, if one cylinder is not functioning properly and not contributing to the engine’s operation, you may observe a noticeable change in the overall engine sound. For instance, in a four-cylinder vehicle, the absence of one working cylinder will alter the engine’s sound.

7) Excessive Vibration

When a car’s cylinders experience misfiring, it may disrupt the effectiveness of vibration-dampening systems, including the balance axle. As a result, heavy vibrations may be felt in the cabin during acceleration or at idle.

These vibrations can be particularly pronounced and disruptive to the overall driving experience.

How To Fix Engine Misfires?

You can use one or more of the following methods to fix the misfire:

1) Check Trouble Codes

The latest vehicle models are equipped with a comprehensive sensor monitoring system that detects any abnormalities and stores them as fault codes in the powertrain control module (PCM). Therefore, when diagnosing a misfire, it is crucial to begin by checking the trouble codes using an OBD-II scanner.

You have the option of obtaining an OBD-II scanner yourself or visiting a mechanic who can utilize their diagnostic tool to retrieve the fault codes stored in the PCM. These codes provide valuable information for the diagnosis process.

For instance, if you come across a trouble code like P0300 or P030X, it indicates that the PCM of your car has detected a misfire in a particular cylinder, prompting you to focus on examining the ignition coil or spark plug associated with that cylinder.

When error codes related to low fuel pressure or engine sensors are detected, it becomes essential to initiate the diagnosis from those specific areas. Starting by reading the fault codes provides a critical starting point for troubleshooting and enables a systematic approach to identifying and resolving the root cause of the misfires.

2) Inspect Spark Plug and Ignition Coil

If you encounter a misfire trouble code associated with a specific cylinder, it is advisable to focus your initial attention on the ignition coils and spark plugs for that cylinder.

Start by disconnecting the plug wire connected to the spark plug of the affected cylinder. Remove the plug by using a spark plug socket, allowing you to closely inspect its condition.

The appearance of the spark plug can provide valuable insights into the cause of the misfire. If your spark plug appears worn out or old, changing it can resolve the issue. Ensure that you install new spark plugs with the correct gap, as specified in the vehicle’s repair manual.

The spark plugs with a carbon-fouled or black end indicate that the engine has been running rich. On the other hand, a plug that appears wet with gasoline or oil suggests potential fuel regulator failure or internal engine block issues.

If the spark plug appears to be in good condition, check the gap between the metal electrode and the base of the plug. Compare this gap to the specified gap mentioned in the manufacturer’s repair manual. If the gap is too large, it can impede the proper ignition of the air-fuel mixture.

Since the ignition coil is responsible for transmitting current to ignite the air-fuel mixture, but a bad coil can lead to misfires. Some cars may provide definite trouble codes for a malfunctioning coil.

However, you can test your vehicle’s ignition coil by disconnecting the spark plug wires and using a multimeter to measure the resistance between the top two pins of the coil. Compare the measured resistance with the specified value for your particular car. If they do not match, it indicates that the coil pack needs replacement.

By examining the spark plugs and ignition coil, you can gather valuable information to diagnose and address the underlying causes of the misfire.

3) Check Vacuum Leaks

If no issues are found with the spark plugs and ignition coil, it is necessary to investigate the air-fuel mixture. Vacuum leaks are a common cause of an improper air-fuel mixture and should be examined as a starting point.

To check for vacuum leaks, you can gently spray a flammable spray around the engine’s intake manifold while the engine is idling.

If the engine speed increases, it indicates the presence of a leak, as the flammable spray is being drawn into the engine. However, it is crucial to exercise caution during this process and have a fire extinguisher readily available.

Alternatively, an EVAP smoke machine can be used to identify vacuum leaks effectively. However, these machines are often expensive and not readily available in most home garages.

4) Check Fuel Pressure

It is important to assess the fuel pressure. But many latest car models utilize an electric fuel pressure sensor; it is advisable to verify the readings with a manual fuel pressure gauge, as electronic sensors can fail.

• Attach your fuel pressure gauge to the fuel rail and turn on the engine.
• Note the fuel pressure reading and compare it to the vehicle manufacturer’s recommendations.
• If the pressure is too low or too high, further inspection of fuel system components such as the fuel pressure regulator, fuel filter, and fuel pump is necessary.

5) Check Head Gasket

If the misfire occurs in adjacent cylinders, it is likely due to a blown head gasket.

A blown head gasket produces different symptoms, including poor engine performance, poor fuel economy, bluish exhaust smoke, the presence of coolant in the oil, and an oil leak at the junction of the cylinder head and engine block.

Repairing a blown head gasket is a complex task that typically needs professional tools.  

6) Check For Compression

In severe conditions, engine misfires may be occurred due to bad connecting rods, damaged cylinders, or piston rings. If your spark plugs are coated in oil, they may suggest that the piston rings are bad, permitting oil to pass freely into the cylinder, and resulting in reduced compression.

In such instances, the engine cylinders, connecting rods, and crankshafts may need to be disassembled to replace the faulty parts.

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