Technically, a misfire is the result of incomplete combustion (or zero combustion) inside one or more of an engine’s cylinders. But to you, the driver, the problem will usually feel like hesitation or shaking when the car is running. On modern vehicles, the check engine light will also pop on when there’s a misfire.
When the check engine light illuminates, your car’s primary computer, which is often referred to as the powertrain control module (PCM), will store a diagnostic trouble code (DTC) in its memory. Codes P0300 to P0312 are the primary DTCs associated with an engine misfire.
Why is My Engine Misfiring?
There are many reasons why your engine might be misfiring. It can be a sensor that can cause the engine to misfire or there can be other causes. It is important to get diagnosed and repaired as soon as possible to prevent from damaging other components.
Most Common Causes of an Engine Misfire
Okay – let’s say your car has an engine misfire. The question is: Why is the problem happening? Unfortunately, there are many potential causes, so answering that question isn’t always easy. If your car is suffering from a misfire, it’s best to let a professional figure out why.
Here is what a mechanic might find when diagnosing the issue:
1. Ignition system problems
When most people hear the term misfire, they think of worn out spark plugs. What they don’t realize is the spark plugs are just one part of the ignition system. A typical modern ignition system contains a variety of components, including the control module, crankshaft position sensor, coil packs, wiring and, of course, the spark plugs. Issues with any of these parts can result in an engine misfire.
2. Air and fuel delivery problems
Air and fuel mix together inside the engine, then the mixture is ignited by the spark plug. The explosion sets the engine in motion, creating the rotational force needed to propel your car down the road. Any issue that throws off the air/fuel mixture – ranging from a failed fuel injector to a vacuum leak – can cause a misfire.
3. Emissions equipment problems
Late-model cars have an array of emissions equipment that helps minimize the amount of pollution released into the atmosphere. A couple of examples include the exhaust gas recirculation (EGR) system and the positive crankcase ventilation (PCV) system. In some cases, issues with emissions equipment can alter the engine’s air/fuel mixture enough to create a misfire.
4. Engine mechanical problems
Many people also don’t realize that an engine mechanical problem can cause a misfire. Each cylinder inside the engine contains a piston that must compress the air/fuel mixture for complete combustion. And when the piston is moving upward, the cylinder must remain completely sealed off to create adequate compression. Internal engine problems can prevent the cylinder from sealing properly, leading to a loss of compression and an engine misfire.
5. Sensor and module problems
Today’s vehicles contain a plethora of sensors, many of which the PCM uses to determine control of critical functions, such as fuel delivery and spark timing. A such, sensor problems can easily contribute to an engine misfire. Although relatively rare, a problem with the PCM itself can also cause a misfire.
6. Control circuit problems
All of the input and output engine management devices (i.e., sensors, ignition coil packs, etc.) are connected where needed via electrical circuits. Problems within these circuits, such as damaged wiring or a loose connection, can cause an engine misfire.
What Causes A 4 Cylinder Misfire?
When your mechanic talks about a 4 cylinder misfire, or a cylinder 4 misfire, it means their OBD2 diagnostic scanner is showing error code P0304. When this occurs, it’s typically caused by your vehicle’s Powertrain Control Module identifying an engine misfire in cylinder #4. The P0304 code can affect how your vehicle performs and drives so the cause needs to be resolved by your mechanic as soon as possible.
Diagnosing the exact problem is not exactly straightforward and there is no ‘magic fix’. Other issues that your car is showing need to be considered in order to eliminate, and track down the potential cause.
Possible causes of a cylinder 4 misfire include:
- A damaged fuel injector
- Sticking valves
- Faulty spark plugs
- Worn coil packs
- Defective injector circuit
- Bad catalytic converter
- Low fuel pressure
- Leaking head gasket
- Faulty oxygen sensor
- Defective mass airflow sensor
- Faulty throttle position sensor
Any of these components can cause a cylinder 4 to misfire and be the root cause of a P0304 engine code. However, a misfire will be considered in conjunction with the following signs:
- Check engine light
- Engine stalling
- Rough idle
- Slow acceleration
- Excessive fuel consumption
- Hard starting the vehicle
- Low fuel pressure
- Low compression
When Does My Engine Misfire?
Engine Misfire When Accelerating
When an engine misfires under acceleration/when the throttle is pressed, it’s not only bad for the engine, but it can be particularly dangerous for everyone on the road. Misfires can happen when a vehicle is under load while accelerating. This causes slow or sluggish acceleration and your vehicle may have trouble getting up to speed. You might feel a jerking motion when pressing down on the throttle.
The most common cause of an engine misfire when accelerating is worn-out spark plugs. When spark plugs are suffering from excessive wear, they don’t ignite the fuel in the piston cylinder when they are supposed to. This can also be caused by fouled spark plugs, a cracked distributor cap, or bad spark plug wires. All these problems lead to the same outcome to prevent coil voltage from jumping the spark plug gap, causing the engine to misfire when the car is accelerating.
We also see a lot of cars with a failing throttle position sensor (TPS) and dirty fuel injectors being reported as having a misfire by their owners. A lot of other systems such as the fuel injection and air intake system rely on the TPS for accurate data, and if it isn’t functioning correctly, you can experience an engine misfire when accelerating, an illuminated check engine light, and the vehicle may even go into ‘limp home mode’.
Misfire At Idle Only
It’s not uncommon for a car to drive perfectly fine, but then display signs of little hiccups or small misfires at idle. This can, at the surface, seem difficult to determine the cause of, as it doesn’t always log a diagnostic code. Some mechanics may be reluctant to look into the problem further, stating if there’s no code, there is no problem – but this is not the case. Sometimes, as a precaution, mechanics may want to replace the fuel pump, injectors, and spark plugs if they can’t track down the exact cause of the misfire at idle only.
What we find is generally the cause of a misfire at idle is an incorrect air/fuel mixture. This can be caused by a faulty O2 (oxygen) sensor or a single injector that needs cleaning, or even a vacuum leak. Other physical symptoms will need to be considered to track down the problem such as if the car is suffering from minor backfires, if the engine revs are not consistent, or if any unusual noises are coming from the engine bay such as a hissing sound which indicates a vacuum leak.
An engine misfire can cause a lot of stress for a vehicle owner as it makes their car more difficult to drive. This stress can be amplified if your mechanic tells you they’re not entirely sure what is causing the problem. Gathering as much information about any problem you’ve experienced or signs from your engine that something is wrong can help your mechanic track down the cause of your misfire.
What Does An Engine Misfire Feel Like?
It’s worth knowing what an engine misfire feels like as it will help you identify the problem quickly. Keep in mind that you could be driving at any speed when a misfire kicks in, and what your engine misfire feels like depends on what’s causing it.
As you’re driving a misfire can cause the engine to lose power infrequently, or a brief hesitation in acceleration when the throttle is pressed. Acceleration may be rough or the car feels like it’s lost power and accelerates slower than usual. The engine may feel like it’s stumbling for a few seconds before regaining pace. This can be caused by an incorrect air/fuel mixture due to a faulty O2 sensor. Alternately, a misfiring cylinder can unbalance the engine, causing a shaking sensation.
As the engine misfires and loses power, it may jerk or vibrate aggressively. The car may seem to run normally most of the time, but when you stop at a stoplight or immediately after starting your car, it can seem like it’s struggling to idle. Stalling will occur more frequently if you are also causing a high accessory load such as running the air conditioner or headlights. Some misfires will allow you to keep driving (albeit with a fair degree of difficulty) while others will cause your engine to stall completely. Any sign of a rough idle is a fairly good indication that your vehicle’s fuel system is causing a misfire.
What Does An Engine Misfire Sound Like?
An engine misfire causes a very unique and noticeable sound from your engine. Even if you’re not an expert on automotive sounds, you’ll notice when this happens. You will be able to hear it from the engine, either inside or outside the vehicle, or you may notice a sound coming from the exhaust. So what does an engine misfire sound like?
The most common descriptions of an engine misfire are a sound like a popping, sneezing, banging, chuffing, or a backfire, usually when the engine is anywhere between 1,500 – 2,500 rpm. The sound occurs when unburnt fuel exits the cylinder and is pushed out during the exhaust stroke before being ignited by the spark of the next cylinder, causing it to explode out through the exhaust system.
You can also tell that you likely have an engine misfire if it sounds like your car is struggling. You may need to switch off your radio and close your windows to hear it from inside your car. If you listen closely to your engine, you will notice that it sounds different than normal. An overall change in engine sound can be an indication that one cylinder is not working. This can be confirmed by other symptoms of an engine misfire such as the vehicle lacking power under full throttle.
Can My Engine Be Ruined by a Misfire?
A misfire can be caused by a myriad of issues. However continuing to run an engine that is misfiring can cause catastrophic damage – and the longer you put off fixing the cause of the misfire, the more damage you’ll cause to the engine. In the worst-case scenario, a continuing misfire could cause some costly problems that could effectively destroy your engine.
One of the worst culprits, a cylinder misfire, can cause a host of problems starting with your catalytic convertor. When a cylinder misfires, it can cause a cylinder’s worth of hot, unburnt to fuel enter the catalytic converter, melt the ceramic material inside. As the inside of the catalytic converter heats up it can start to break apart, plugging the exhaust, which causes problems to keep cascading.
But catalytic converters aren’t the only concern. When an engine runs lean (too much air/too little fuel), it generates excess heat, which can cause damage to the engine itself. Excessive oil consumption can heat the cylinder up, creating preignition which in turn slows the engine down and damages crankshaft bearings. The heat that is generated can warp or crack valves and the cylinder head. Even if a misfire is caused by an engine running rich (too much fuel/too little air), it will cause the engine to run hot, carbon fouling of values, and low compression conditions.
Diagnosis of Common Engine Misfire Codes
One of the first things you mechanic will do when you present a car with a misfire is to check for diagnostic trouble codes (DTCs). These codes won’t tell the mechanic exactly what is wrong with the vehicle, but they are one tool that can be used to diagnose what is causing an engine to misfire.
An engine misfire code might indicate a problem with a specific cylinder, or that the engine is running lean. Depending on the diagnostic tool being used, it may show how many misfires occurred within a certain number of cycles, or the engine RPM when the misfire occurs. Not all misfires will cause a DTC to be logged, however, particularly if the misfire is intermittent.
The following codes may indicate a potential misfire:
- P0100 – P0104: Mass airflow sensor
- P0171 – P0172: Lean or rich fuel mixture
- P0200: Fuel injector circuit malfunction
- P0300: Random misfire that is not isolated to one or two cylinders.
- P0301: Misfire in cylinder 1
- P0302: Misfire in cylinder 2
- P0303: Misfire in cylinder 3
- P0304: Misfire in cylinder 4
- P0305: Misfire in cylinder 5
- P0306: Misfire in cylinder 6
- P0307: Misfire in cylinder 7
- P0308: Misfire in cylinder 8
Vehicles showing any of the above DTC codes should not be driven due to the potential for unsafe vehicle operation and driveability concerns.
What Should I Do If My Engine Misfires?
In short, if you suspect your engine is misfiring, make an appointment with a mobile technician from RepairSmith as soon as possible to have your vehicle inspected and repaired and to prevent further damage. But before you call to make an appointment, collect as much information as you can, including any peculiar sounds, to assist your technician in diagnosing the problem.
As you’re driving, pay close attention to how your vehicle behaves. Take note of any unusual sounds or behavior, and under what circumstances the engine is misfiring, such as if it occurs immediately after the car has started if the misfire occurs under acceleration or at idle, and how frequently you notice the misfire occurring. The more information you have, the easier it will be for your mechanic to find the cause of the misfire.
Is It Safe to Continue Driving with an Engine Misfire?
If your engine misfires as you are driving your vehicle, this can present a potential safety hazard, especially if you’re driving in heavy traffic or on a busy road. You should not continue driving with an engine misfire. Although the car may run well enough to get you where you’re going, you risk potentially damaging costly components, such as the catalytic converter and the engine itself. That’s why you should have a professional diagnose and repair a misfire right away.