For an engine to run correctly, the right amount of air with the right amount of fuel and a good healthy spark are needed. If any one of these components does not works properly, it will not ignite the combustion chamber as it should, and a misfire will occur.
If your car’s engine is misfiring it is important to discover what caused that misfire and to remedy it as soon as you can. The consequences of a misfire can be anything from decreased fuel economy, to complete engine failure. But what is misfiring, and how do you know if you are misfiring, and what can cause misfiring?
In this article, we will discuss what misfiring is, and what possible problems can cause it. We will also discuss how to determine if your engine is misfiring and answer several frequently asked questions.
What is an Engine Misfire?
An engine misfire occurs when one or more of the engine’s cylinders does not ignite the air/fuel mixture during the combustion cycle. Engine misfires result in a variety of performance issues, and can lead to serious engine damage if not resolved in a timely manner.
For complete cylinder combustion to happen, the engine requires three essentials: the correct air/fuel mixture, compression, and spark. If any of those is incorrect or missing, then complete cylinder combustion will not happen, and no power will be made.
Fortunately, the engine control module (ECM) looked at the crankshaft position sensor and monitored the rotation speed of the crankshaft. It knows that the speed of the crankshaft will increase as the combustion takes place in all of the cylinders. So after the combustion occurs on a cylinder, if the speed of the crankshaft does not increase slightly, it has determined that the engine is misfiring and will store a trouble code.
The most common code you will come across is the P0300 code, it will also show a P030X like code. The X is the number of the cylinder where the cylinder misfire occurs, so P0301 is a cylinder 1 misfire.
What Causes An Engine Misfire?
The most common causes of an engine misfire are failed spark plugs or old ignition coils, but there can also be fuel-related problems too, such as a bad fuel pump or malfunctioning fuel injector.
However, unlike the mentioned causes, which account for most of the failures, you can also have engine misfires as a result of engine compression that is too low, which is more rare.
Below is a slightly more complete list of the most common causes of engine misfires:
#1. Bad Ignition Coil.
The first cause of an engine misfire is a bad or failing ignition coil. Some vehicles have one ignition coil for every spark plug, while some vehicles have one ignition coil pack supplying, via ignition wires, every spark plug in the engine.
Older vehicles utilize a distributor to distribute spark to all spark plugs, powered by an ignition coil. If either the distributor or ignition coil is supplying a weak spark, this could create the misfires.
#2. Failed Spark Plug.
The second cause of an engine misfire is number two, bad or failing spark plugs. Spark plugs ignite the air-fuel mixture in the combustion chamber, if the spark plug is having issues, it will not ignite, making a misfire.
Spark plugs should be replaced at a regular schedule for every car model and not replacing them on time can lead to fouled spark plugs, which can cause misfires. If you are not sure when you last changed the spark plugs, you need to look in your service manual and change them if necessary.
#3. Intake Manifold Gasket Leaks.
There exists a gasket located between the cylinder head and intake manifold, known as an intake manifold gasket. This gasket can leak and draws air into the combustion chamber which can lead to a lean air-fuel ratio which can cause misfires.
Also, there are a handful of vacuum lines on the intake manifold that can leak and cause similar symptoms to a bad intake manifold gasket. Finding these leaks is usually fairly easy, and we will cover that further below in the article.
#4. Low Fuel Pressure.
Low fuel pressure is another symptom that can lead to misfires. Low fuel pressure means less fuel gets into the engine’s combustion chambers leading to a lean air-fuel mixture, which is capable of causing misfires on all cylinders.
Low fuel pressure could be due to a bad fuel pump, fuel pressure regulator or clogged fuel filter.
Fortunately for you, checking the fuel pressure in your car is fairly easy with a manual fuel pressure gauge, and is well worth it if you suspect low fuel pressure.
#5. Bad Fuel Injectors.
Fuel injectors inject the combustion chamber with the correct amount of fuel for each cylinder, typically, there is a separate fuel injector for each cylinder. If the fuel injector injects the wrong amount of fuel, the engine will run rich or lean which can cause the engine to misfire.
Fuel injectors fail, but it isn’t common for most modern car models, so it makes sense to check the other possible causes first. But if you can’t find the reason with the other issues, then checking the fuel injectors is worth it.
#6. Bad Engine Sensor.
Engine in modern cars use various sensors to measure everything in the engine so that they can run as efficiently as possible. If any of these sensors fail, it could cause an incorrect air-fuel mix to enter the engine that leads to engine misfires.
Some common vehicle sensors that could cause an incorrect air-fuel mix are the MAF, MAP, O2 sensor and the coolant temp sensor.
#7. Low Compression.
As we covered earlier, there needs to be the right air-fuel mix, spark and compression for an engine to fire properly. If you already checked the fuel supply and the spark and cannot find any issues, there is a potential for low compression in the engine.
To know if your engine has low compression, you need to do a compression test and then confirm the results against the manufacturer’s specification. Regrettably, in most situations low compression are caused by worn components inside the engine which could seriously limit the expense of repairs, so you will want to look over the other causes first.
Symptoms Of An Engine Misfire
There are a few symptoms of an engine misfire. They include;
- Rough Acceleration
- Rough Idle
- Vibrations
- Check Engine Light
- Change in Engine Sound
Now a more detail list of the most common symptoms of engine misfires includes;
#1. Rough Acceleration.
When you experience an engine misfire you may feel either a big or small jerk come from the engine. Typically, the vehicle will misfire when there is load applied to the engine. If you choose to accelerate quickly, you’ll likely feel misfires.
The most common place you would expect to feel misfires will be high gear, low RPM, and a wide-open throttle (gas pedal to the floor). Rough acceleration is a common symptom of engine misfires.
You may also just notice other performance issues such as slower than normal acceleration or even the engine stalling while you accelerate.
#2. Rough Idle.
The car engine is most sensitive to minor air-fuel mixture problems while it is idling. So this will likely be the first place you notice that you are experiencing engine misfires.
If you notice that your car engine is idling rougher than normal, it is likely that you are experiencing an engine misfire problem. You may see other problems such as the engine stalling at idle or possibly an uneven idle that is bouncing up and down.
#3. Vibrations.
A car engine is a very balanced mass when it is constructed, and it likely has balance shafts and other various tricks to minimize vibrations.
When one or more cylinders do not fire correctly, the engine will be unbalanced and in turn will likely introduce severe vibrations into your car while you are either accelerating or idling.
You can also open the hood of the vehicle and watch the engine while it’s idling. If you notice that the engine seems to be vibrating more than normal it may be a sign that it is misfiring.
#4. Check Engine Light.
Modern cars tend to have decent monitoring systems for all of the various car sensors on the engine. If those sensors are failing, or if the sensor detects that something is wrong with the engine, it will relay that information to the engine control module.
When the engine control module gets this information it will evaluate if the issue is serious or not. If the issue is occurring repeatedly, the engine control module will illuminate the check engine light to indicate to you that there is a problem and, you should get it repaired.
Another great indicator that you are experiencing engine misfires is if you see a blinking or flashing check engine light on your dash.
#5. Engine Sound Changed.
If you have any experience with automotive repairs you know that each engine has a different sound. For instance, a V8 engine has a completely different tone than a four cylinder engine.
So with that being said, if your engine is a four cylinder engine and you are misfiring on one cylinder, it will sound like a three cylinder engine. If your car engine sounds off or odd or unusual, it is possible it is misfiring on every cycle.
How to Repair Engine Misfires?
If you have an engine misfire, you will need to read the trouble codes from the engine control module and continue your investigation from there. You will want to check the spark plugs and coils, and you will want to look for vacuum leaks and cylinder compression.
Here is a more detailed process to diagnose and troubleshoot engine misfires:
#1. Check Trouble Codes.
Modern car engines are designed with a system that monitors all of the sensors in your car engine, and should there be something wrong with a sensor, it will record it as an engine trouble code in the engine control module.
Since this is the case, when you begin to diagnose misfires, the first thing you will want to do is check for trouble codes with an OBD2 scanner. You may either buy one yourself, or you can take your vehicle to a mechanic and borrow their diagnostic equipment to read the trouble codes from the engine control module.
Reading trouble codes will provide you with very important information. If you see an error code like P0300, or P030X, it means the engine control module is detecting a misfire on that specific cylinder, so you probably start looking at the ignition coil or spark plug on that cylinder.
If you have trouble codes relating to an engine sensor or related to low fuel pressure, you will want to start your diagnosis from there. Either way, always start with looking at your codes, because it will guide you on where to start troubleshooting and isolating the cause of the misfire.
#2. Check Ignition Coils and Spark Plugs.
If you have a misfire trouble code on a cylinder, you will want to check the spark plugs and ignition coil on that cylinder. Remove the spark plug and ignition coil from that cylinder and visually inspect them and replace them if they are bad or dirty.
A trick mechanics often use on car engines with separate ignition coils is to switch the spark plug and ignition coil to another cylinder. Then clear the codes and drive for a while to see if the problem has moved to another cylinder.
If it did, you know it is a spark plug or ignition coil problem and will want to replace all spark plugs and the ignition coil on the cylinder that is misfiring or has the trouble code.
#3. Look for vacuum leaks.
If you don’t find anything on the spark side, it’s time to look at the air-fuel mixture. Vacuum leaks can easily cause an incorrect air-fuel mixture and are also pretty easy to check, so you want to start there.
Checking vacuum leaks is done by spraying a flammable spray around the engine’s intake manifold while the engine is idling and watching for engine speed increases.
If the engine speed increases, you have a leak and are injecting the flammable spray into the engine. But be very careful while doing this and make sure you have a fire extinguisher within close reach.
Another option is to use an EVAP smoke machine to easily identify vacuum leaks, but often these machines are rather expensive and likely not something most folks have lying around their garage.
#4. Inspect fuel pressure.
Next, check fuel pressure. While most newer vehicles use an electrical fuel sensor, you should verify the information with a manual fuel pressure gauge, as electrical sensors can and do fail.
Attach a fuel pressure gauge to the fuel rail and start the engine. Check what the fuel pressure is and verify against manufacturer specifications, and if it’s too high, or too low, you need to check the component levels in the fuel system i.e fuel pump, fuel filter, fuel pressure regulator.
#5. Inspect compression.
You should also verify if the engine’s compression is proper, so you want to carry out a compression test on the all cylinders. You can accomplish this with either a compression tester or a leak down tester.
Compare the results to manufacturer specifications, and if the compression is low on a cylinder you will need to carry out inspection of the engine parts i.e., timing belt, piston rings, valves, or pistons.
All of these parts are often extremely expensive to replace so you would want to ensure you do the correct diagnosis before proceeding to replace any.
#6. Take it to a mechanic.
If you have checked all of the above, but still can’t pinpoint the problem causing the misfires, it might just be worth your time and money to take it to a professional mechanic. Engine misfires can be very, very difficult to diagnose for professionals in the industry.
Even if you think it would be prohibitively expensive to take it to a mechanic, in many cases getting a professional diagnosis can in fact be cheaper than replacing the parts that are functioning perfectly.
FAQs.
Is Engine Misfire Serious?
Yes, it is. An engine misfire is serious because it can impact fuel economy, cause engine damage, and decrease your power. Misfire can stall your car or cause it to become difficult to steer due to the loss of power steering, which puts you into a dangerous situation. It can also cause serious damage to your engine itself.
What Does an Engine Misfire Feel Like?
If your engine is misfiring, it will feel like it is shaking or vibrating, shaking and possibly making a knocking noise. You will most likely notice that your engine doesn’t sound like it used to and that your car has decreased power and fuel efficiency while you are driving.
If you believe you are having trouble with misfires, then you should take it to the mechanic to be checked.
How Much Does It Cost to Fix Engine Misfire?
You can expect to pay between $200 to $500 to fix a misfiring engine in most scenarios, but that does vary depending on how serious the fault is, and the specific repairs that need to be made.
Two of the most common reasons for engine misfires are bad spark plugs or bad ignition coil, both of which can be somewhat cheap. If you’re lucky, you can fix this relatively cheap, but if you’re unfortunate it can be due to a lack of compression, which can be very costly.
How long can you drive with a misfiring engine?
It is not recommended to drive at all when your car engine is misfiring. If you suspect that your engine could be misfiring, the best thing to do is to take care of it as soon as possible.
Continuing to drive with a misfiring engine can cause further serious damage to your car’s engine.
Can I drive with a misfiring engine?
No, it’s highly not recommended to drive with misfires. Misfires can ultimately cause serious damage to the internal parts of your engine and to the catalytic converter of the vehicle, which can also be very expensive. So, it’s good practice to diagnose and repair the issue as soon as possible.
So now you can see that a misfiring engine can be a serious issue that needs to be addressed as soon as possible. By understanding the causes and symptoms, and how to fix them, you are able to keep your car smoothly running and avoid expensive repairs that are not necessary.
Sometimes fixing a misfire could be difficult without the correct knowledge and tools, considering the multitude of possible causes. So if you feel you have checked the basics, then maybe you should consider taking the car to a qualified mechanic to seek expert advice.
I truly hope you enjoyed this article, and now have the knowledge to understand how to diagnose car engine issues such as misfires and can therefore fix the issue with your own car thereby saving money!