Misfire car – symptoms, diagnosis, elimination

 Do you experience misfires at idle or under load? Misfires are one of the most common problems in modern gasoline engines. The ignition spark is crucial for the performance of the engine.

There are many reasons that can cause misfires. They are not easy to fix, especially if you don't know where to look for them. In this guide, we'll go over the symptoms of misfires, their causes, and the best way to fix them.

What is a misfire?

This is a good illustration of how the pistons and crankshaft move in the cylinder when the engine is running. The pistons are pushed down by an explosion in the cylinder. The piston moving downwards rotates the crankshaft. The operation of an engine can be divided into four phases, which is why this type of engine is called a four-stroke engine.

1. The piston moves downwards and fills the cylinder with the air-fuel mixture from the intake.
2. The piston moves upwards and compresses the air-fuel mixture to high pressure.
3. A spark from the spark plug ignites the air-fuel mixture and the explosion pushes the piston down, causing the crankshaft to rotate.
4. The piston lifts and pushes the burnt air-fuel mixture through the outlet.
5. Repeat the process from step 1.

This is a description of how a four-stroke engine works, which is installed in almost all modern cars. There are some older cars that use a two-stroke engine, but we won't cover them.

A misfire occurs when one or more of these steps are incorrect or missing.

• The air-fuel mixture is too lean or too rich.
• Bad ignition spark, wrong ignition timing.
• Low compression, due to the air-fuel mixture.
• Incorrect timing of intake or release of the air-fuel mixture.

Now you know the basics of how a cylinder works in a car engine and the phases at which misfires can occur. With this knowledge, it is much easier to find the cause of misfires.

As you can see, theoretically there are not many reasons. But when you start diagnosing your car, you will realize that it is not always so easy to find the problem. Let's continue to learn how to diagnose and fix dropouts at home. 

Signs of misfire

There are various symptoms that can occur when the engine misfires. Here are some of the most common.

1. Uneven acceleration

If a misfire occurs, you may feel it as a slight or strong jerk from the engine. These misfires often occur when the engine is running under load and is accelerating at high revs and high gears. Acceleration problems are a common sign that your engine has misfires.

2. Unstable idle

Sometimes there are passages when idling. The engine sensors receive incorrect readings and the air-fuel mixture becomes incorrect. This can lead to a very rough idle. The speed can fluctuate up and down until the engine stops.

3. Vibrations

The car engine is balanced in the factory with various balancing machines in order to generate as little vibration as possible. If a cylinder does not ignite properly, the engine will become unbalanced and this can cause strong vibrations in the interior when accelerating or idling.

4. Check if the engine light comes on

Modern cars monitor the status of all engine sensors. If the sensor fails or detects that something is wrong with the engine, it sends information to the ECU.

When the engine control unit (ECU) receives the data, it decides whether the problem is serious or not. If the problem occurs repeatedly, the ECU will turn on the check engine light so that you can take action.

5. Slow acceleration

As mentioned earlier, skipping can cause the oxygen sensors to receive erroneous information and create a mixture that is too rich or too lean.

A mixture that is too lean or too rich can cause slow acceleration and even put your car into limp mode (safety mode). This results in the engine not revving above 3500 rpm and the control unit shutting down the turbo.

6.Change the engine sound

If you know a little about cars, you will have noticed that there are differences in the sound of different engines. V8 engines have a completely different sound than a four-cylinder engine.

If your 4-cylinder engine has a misfire in a cylinder, it may sound like a 3-cylinder engine. If your car sounds unusual, it's most likely a misfire.

Causes of dropouts

Now that you suspect a misfire, where should you start looking for the problem? Here are the most common reasons for absences, sorted from the most common to the rarest.

1. Ignition coil or distributor switch (distributor) defective

If you have an older car, this is the most common reason. Some vehicles have a separate ignition coil for each plug, while others have a single coil with ignition wire for each plug.

Older cars have a breaker and sometimes an ignition coil. If you have separate coils, disconnect each one to find out which cylinder is causing misfires. Replace the faulty ignition coil.

2. Defective spark plugs

The second most common cause of misfires is faulty spark plugs. Spark plugs ignite the mixture in the cylinders and wear out over time. Spark plugs are inexpensive and easy to replace. If you can't remember the last time you changed your spark plugs, maybe it's time to do so.

3. Intake manifold leaking

Leaks near cylinder heads are also very common in spark plugs. This problem often occurred in older vehicles without steel intake manifold gaskets.

If you have an old engine, you can check this. If you have a new car, check the intake manifold gasket for other signs of leaks. Check for damaged vacuum hoses.

4. Low fuel pressure

May be caused by a faulty fuel pressure regulator, a faulty fuel pump or a clogged fuel filter. Low fuel pressure causes the engine to become emaciated, causing misfires in all cylinders. If there are ignition fault codes on all cylinders, check the fuel pressure.

5. Injectors

Another problem that used to be common was problems with the injectors. A defective injector can lead to misfires.

This is quite difficult to diagnose without checking the function of the injector. Injector problems are not common in new cars. Therefore, first look for other possible causes.

6. Low compression/damage in the motor

If you have checked everything else, there may be low compression or damage in the engine. A worn timing belt can also lead to low compression – check this first.

Is it safe to drive with a misfire?

If a leak occurs, the air-fuel mixture entering the engine comes out unburned. Modern cars have a catalytic converter that reduces the car's emissions. The catalytic converter can heat up to 600 degrees during operation.

And what happens when the unburned air-fuel mixture gets in? That's right, it's going to explode in the catalytic converter. Explosions inside the catalytic converter can damage it. And a new catalytic converter is not a cheap pleasure.

In addition, misfires can also damage other sensors in the engine, such as oxygen sensors. We would never recommend driving a vehicle with misfires, especially on long distances. Short distances to the petrol station without loading the engine are normal, but you should not ignore gaps and continue driving.

How does the control unit know when dropouts occur?

The ECM can detect misfires differently depending on the vehicle model and engine. The ECU uses many sensors to know when to ignite the spark plug, when to inject fuel into the cylinder and how to change the air-fuel mixture. To detect misfires, the control unit often uses a crankshaft sensor.

The crankshaft sensor measures the position of the crankshaft and calculates its revolutions per minute. The crankshaft sensor uses the camshaft sensor to determine which cylinder is in the upper dead center and is ready for ignition.

Pressing down on the pistons results in a slight increase in crankshaft speed. If the crankshaft sensor does not detect this increase in speed, the engine control unit stores a fault code for the cylinder where the misfire occurred.

Sometimes the ECU cannot detect which cylinder has a misfire and stores the error code P0300 (random misfire).

Some control units use the resistance of the ignition coil. If there is no ignition, the control unit detects this from the electrical wiring, which leads to a fault. This method is not as common as detection using the crankshaft sensor.

How to diagnose and fix omissions

If you've read the article from the beginning, you now know what can cause gaps. If you have arrived directly at this point, we will briefly write down the possible reasons. Omissions occur when one of the following is missing or occurs at the wrong time:

1. Ignition.
2. Fuel-air mixture.
3. Compression.

To diagnose omissions, we need to check everything. But some mistakes are more common than others. If you start in the right direction, you can save a lot of time. Let's write a quick guide on what to do when trying to find the cause of a misfire.

1. Reading errors

The first thing to do is to read out the errors from the control unit. You can do this yourself using a diagnostic scanner or an ELM327 adapter using the Torque or ScanMaster software.

If you find errors in the controller code memory, make a note of them and delete them. We will need them in the next step.

2. Use the information you receive

Now that you know the error code, you need to understand its meaning. Further diagnostics depend on this.

Case 1: You've counted a lot of misfire DTCs on different cylinders. In addition, there are faults related to the air-fuel mixture.

For instance. If you have two or more of these error codes:

• P0171 – lean mixture;
• P0300 – Accidental Misfire;
• P0301, P0302, P0303, P0304 – Misfires in cylinders 1-4.

In this case, if the engine runs unsteady or unstable, you should start repairing the air-fuel mixture. Most likely, problems with this are caused by omissions. If you were looking for an air-fuel mixture code and a skip code, always start your search with the mixture. There may be times when mixing errors occur due to omissions, but this is rare.

Common causes should be checked. Start with air leaks around the intake manifold and check the hoses for cracks. You can use brake cleaner or "quick start" spray to gently spray the intake while the engine is idling. If the speed increases, there is an air leak. Keep in mind that these sprays burn very easily. Therefore, have a fire extinguisher ready just in case.

If there are no intake leaks, the next step is to check the air-fuel mixture fault code. You can use Google to find the possible causes of this error code and fix it.

Case 2: You keep reading a fault code on a certain cylinder despite deletion

If a particular cylinder is constantly malfunctioning, or it just misfires that do not have any other fault code associated with it, just keep reading. This problem is much easier to solve. We will cover this in the next step.

3. Check the ignition

It's time to check the ignition. Because in most cases, the problem is due to a faulty ignition. This is where we start. If you have an older car with an ignition distributor, always check the inside of the distributor first. Often this is worn.

If the engine is running out of round and you have an engine with separate ignition coils. You need to start it, run it at XX, and then disconnect a coil in series. Remember that ignition coils have a very high voltage and you need to be very careful.

If you disconnect the ignition coil and feel no difference in engine power, you have found a cylinder that has misfires.

You can also use the same procedure for older vehicles that have multi-point ignition. Disconnect the spark plug wires until you know which cylinder is not firing.

When working, you need to use insulated tools or protective equipment. If you use your hands to cut wires without insulation, you run the risk of being exposed to tension. Always use the right tools or disconnect the cables with the motor off.

Now that you know exactly which cylinder is not working, you can swap the ignition coil (cable) for the working cylinder. If the problem spreads to another cylinder, replace the ignition coil (cable).

If the problem persists with the same cylinder, you can do the same with the spark plugs. Change the spark plugs between the two cylinders. If the problem persists, replace the spark plugs.

If you have tried to replace the spark plugs and ignition coils but the problem is still with the same cylinder, you will have to check the ignition spark. If there is no spark in this cylinder even if you replace the coils and spark plugs, you will need to look at the performance graph of your engine. You need to check the voltage, grounding, etc. This step is more complicated and we will not write about it here.

If the spark is fine, but the cylinder is not working, proceed to the next step.

4. Check the tightness of the intake tract

To do this, let the engine idle. Listen carefully near the intake manifold gasket. You may hear unnatural noises due to leaks. Additionally, you can use a "quick start" spray or brake cleaner. Spray it around the intake gasket. If the engine speed increases, there is an air leak that needs to be repaired.

Always take a fire extinguisher with you. Because if the spray or brake cleaner ignites, it will burn very well. Believe me, this is really true.

Leaks in the intake tract do not always cause the engine in this cylinder to not run at all. Instead, it runs irregularly. However, this is easy to check and could be a possible reason. If you do not notice any leaks, proceed to the next step.

5. Check compression

Now that you know that the spark is fine, but it doesn't work on this cylinder, it's time to check the compression on all cylinders. Use a compression gauge to check. You can buy it for 500 – 1000 rubles.

If the compression of a cylinder is too low and misfires occur, put some oil in it and check again. If the compression is still low, there is nothing else to do. You need to disassemble the engine.

You should check the valves, valve gaskets, and piston rings. You could also check the camshaft timing, but in that case you would see low compression on all cylinders. If the compression is fine, it's time to move on to the next step.

6. Check the fuel

Now there is only one possible reason: there is no fuel injection in these cylinders. This means that you have a car with an injection engine. Because if you had an old engine with a carburetor, it would not be able to turn off just one cylinder.

First of all, disconnect the plug from the injector on the cylinder that does not ignite. With the ignition switched on, check if there is 12 V power on any of the cables. This is checked with a multimeter. The other cable is grounded via the control unit, which is not so easy to check.

With the help of an LED, you can check whether there is ground on this cable while the engine is running. However, we recommend using an oscilloscope. You can also check the fuel pressure, but in this case the problem would most likely be with multiple cylinders.

If there are 12 volts on the injector and the problem is not resolved, you will need to find an oscilloscope and check the mass of the engine control unit.

We hope you have found your problem. If not, you need to go through this guide again step by step.

Graduation

• Ignition coils and defective spark plugs are the most common misfire problem.
• Intake leaks are a common problem when you have a fault code related to the air-fuel mixture.
• If you have an air mix fault code, always start checking there.

I hope you have learned something and solved your misfire problems.

source : Misfire: symptoms, causes, step-by-step diagnosis