It can be difficult to diagnose the reason for an engine misfire, but context gives major clues. Misfires are often intermittent and will sometimes only manifest with a specific type of fuel, environment, or engine load (how deep you’re into the gas pedal).
Engine misfire symptoms should not be ignored because misfiring cylinders strain the engine and decrease fuel economy over time. Here are the most common signs of an engine misfire.
What Is an Engine Misfire?
A healthy four stroke combustion engine performs a series of actions in the right order and at the right time to burn fuel and make power.
- As the piston travels downward, a precise air fuel mixture enters the combustion chamber as the intake valves open. After the valves close, this mixture is compressed as the piston travels upward.
- On the power stroke, this air fuel mixture is ignited by the spark plug, further increasing cylinder pressure. This high pressure forces the piston down very quickly, turning the crankshaft in the process.
- The exhaust valves open on the exhaust stroke, expelling the burnt gases from the cylinder, and the cycle repeats.
Misfires are caused by a problem with one or more of the components necessary for combustion (fuel, oxygen, and a spark) or the mechanical components in the system, such as the cylinder itself.
Incorrect timing or one or more cylinders that fail to complete all of the steps both result in an engine misfire. If the air fuel ratio is incorrect, the mixture may not combust at all or may prematurely detonate (colloquially referred to as knock).
See Also: Symptoms of a Bad Knock Sensor
Common Symptoms of an Engine Misfire
Engine misfires can, like many other vehicle problems, manifest with the same symptoms as other issues. The first step in diagnosing the problem is having the vehicle’s computer checked for codes.
Any or multiple of these symptoms should make you suspicious of a misfire.
#1 – Loss of Power, Stumbling, and Vibration
You may be wondering, “what does an engine misfire feel like?” There may be a palpable loss of power resulting from a vehicle that may be running on fewer cylinders than normal, or a brief hesitation in the delivery of power when gas is applied.
You may also feel intermittent jerking, bucking, or stumbling from the engine. You’ll notice rough and/or slow acceleration.
Vibration while the vehicle is running is common especially if the misfire is caused by a mechanical problem. The severity varies depending on the RPM and is often worse at idle.
#2 – Abnormal Sounds
So what does a misfire sound like? During a misfire, the engine will make a sudden sound that can be described as popping, sneezing, or backfiring.
Backfiring occurs when unburned fuel exits the cylinder on the exhaust stroke and is then ignited farther in the system by the spark of the next cylinder.
You may notice an overall change in engine sound if one cylinder is not working at all, since a four-cylinder car will now only have three working cylinders. It’s the same as if a spark plug blew out of the cylinder head.
In addition, the sound of a vacuum leak may also be present in the engine.
#3 – Unusual Smells
Because a misfire can lead to incomplete combustion of the fuel-air mixture, unusual smells can result. When a cylinder misfires, the unburned fuel can be dumped into the exhaust system, producing a strong gasoline smell.
Additionally, if this excess unburned fuel reaches the catalytic converter, it can cause an overload, resulting in a sulfur or rotten egg smell due to the converter’s inability to process all the hydrocarbons.
#4 – Check Engine Light
After 1995, vehicles came equipped with OBD2 systems to aid in diagnosing engine issues. When any of the vehicle’s sensors detect readings outside their expected values, a check engine light is illuminated and the code is stored in the engine control unit (ECU).
Most OBD2 vehicles will have a code for “Cylinder X Misfire” that will help you locate the problem (where X is the cylinder number that misfired). Sometimes the check engine light will remain illuminated until cleared, but in some cars the light will flash when the misfire occurs.
The check engine light has a variety of potential triggers, so it’s important to get the code read to see what exactly went wrong. Diagnostic trouble codes P0301 through P0312 are common codes that show up. The last 2 numbers indicate which cylinder is having the misfire. For example, P0306 would indicate a cylinder 6 misfire issue.
Code P0300 is another code that may appear as it relates to multiple cylinders misfiring.
Many automotive stores such as Auto Zone can do this for you for free. Write down the code and description of the problem to aid in troubleshooting the problem.
Related Trouble Codes:
#5 – Struggling Engine
Because a misfire disrupts the engine’s normal operation, this can lead to a lack of smoothness and consistency in power delivery. When one or more cylinders do not fire properly, the engine loses balance in its power output, causing it to struggle, shake, or hesitate.
This can be felt as a jerking motion when accelerating or a general loss of power, as the engine cannot produce the expected level of performance. The struggle is often most noticeable when the engine is under load, such as during acceleration or when climbing hills, where all cylinders need to work efficiently to maintain speed and power.
On top of that, misfires can sometimes cause difficulty starting the vehicle or stalling of the engine when stopped. The later is even more likely with a high accessory load (such as when using the air conditioner, headlights, and radio).
#6 – Excessive Exhaust Smoke
Seeing black, blue-tinged, or very white and sweet-smelling exhaust is a sign that something is not right. Dark, sooty exhaust could indicate a rich air fuel mixture or carbon buildup. Blue-tinged exhaust is indicative of burning oil.
White, sweet-smelling exhaust may point to a head gasket leak. Anything in the combustion chamber that is not air or gasoline will affect the vehicle’s ability to burn the mixture as intended.
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