What causes a car engine to misfire?

This blog post will answer the question,’ What causes a car engine to misfire’ and cover topics like most common causes, how do engine misfires take place, when can misfires happen, and diagnosing engine misfires.

What causes a car engine to misfire?

The following are the various causes that are usually responsible for an engine misfire:

  • Damaged fuel injector
  • Inadequate fuel pressure
  • Defective spark plugs
  • Defective oxygen sensor
  • Faulty mass airflow sensor
  • Defective catalytic converter
  • Leaking head gasket

A misfiring engine can be identified by the following signs:

  • The check engine light turns on
  • Low fuel pressure
  • Sluggish acceleration
  • Abnormal fuel consumption
  • The vehicle needs to be hard started

Most common causes behind engine misfiring

The following are the most common factors responsible for a misfiring engine.

Defective spark plugs

Spark plugs are responsible for providing the sparks required for the explosions that help the pistons move up and down for completing their strokes.

They are one of the most common causes of a misfiring engine and a faulty one can result in the accumulation of excess fuel in the cylinder that can misfire the engine. However, they are very cheap to replace and must be checked regularly.

Inadequate fuel pressure

This occurs when there is inadequate fuel being sent in the cylinders for mixing and results in a mixture that is way too lean.

There are 3 areas that need to be checked for low fuel pressure.

Fuel pressure regulator

A defective or wrongly adjusted fuel pressure regulator could be the cause behind the adequate buildup of fuel pressure in the cylinders.

Fuel filter

Some vehicles have a fuel filter between the pump and carburetor or another one between the fuel tank and the pump. These filters need to be checked properly as clogging from rusty tanks or any other form of debris can result in low fuel pressure.

Fuel pump

A loud and whining sound can be heard from the gas tank if the fuel pump is defective and can potentially contaminate the fuel inside.

Low engine compression

The most common aspect to be checked in this case is the engine timing. The main culprit is often a faulty engine timing belt. It is advisable to use a timing lamp to check the engine timing and locate the timing indicator on the main pulley of the engine.

If there is a distributor in the vehicle, it can be rotated to adjust the different ignition points.

Leaks in the intake manifold

The intake manifold is the culprit in cases when there are vacuum leaks, however, these are more prone in older vehicles than newer ones. Other signs include thick black carbon buildups in and around the head gasket, indicating that it is leaking.

Faulty distributor

The contact points inside the distributor can be worn out, corroded, or even damaged and can be a big cause behind a misfiring engine. In some cases, the ignition coil that powers the distributor becomes defective.

A defective ignition coil can be identified when it does not provide the optimum voltage needed by the distributor and the spark plugs and this can result in a misfiring engine. They must be replaced as early as possible.

How do engine misfires take place?

Engine misfires take place in the following forms.

Spark plug misfires

These are signs of a defective spark plug, these are the components responsible for ignition or combustion of the air-fuel mixture that is responsible for producing engine power.

They are, however, cheaply replaceable. At times the ignition wires connecting the spark plugs can get worn out and must be replaced with immediate effect.

Misfires due to a defective fuel system

The fuel filters can be clogged and thus prevent adequate fuel from entering the cylinders for combustion and they can also be clogged by rusty fuel tanks or any other form of debris, all the more reason why they should be checked frequently.

In general, fuel system-related misfires can occur more when the car is idling instead of being driven at its regular speed.

Misfires due to mechanical issues

These are the most detrimental and complex problems to fix and can occur due to slipping of the engine timing belts or chains, thus interrupting the combustion process.

Other causes include the valves getting damaged or worn out vacuum lines. In such extreme cases, it is advisable to consult a certified mechanic for a permanent fix instead of applying DIY fixes.

Diagnosing engine misfires

These are the following ways to diagnose a misfiring engine.

Refer to the troubleshooting codes

In case the engine is misfiring and the check engine light is turned on, it is a good idea to refer to the troubleshooting codes that help locate where the exact problem lies.

Most automotive parts stores or mechanics help in providing troubleshooting codes for free.

Analyzing the codes

Most automotive parts stores possess high-end code readers that help in deciphering the meaning of these particular codes. Cheaper code readers will only provide the code and the meaning has to be deciphered online.

These troubleshooting codes look somewhat like this:

  • P0301/302/303/304 indicate misfires on multiple cylinders
  • P0300 indicates a misfire in a random cylinder

Checking ignition

A problem with ignition generally points to the distributor, which is more often present in older cars. We need to check if the contact point inside the distributor is damaged or corroded.

Most of the time, the distributor cap is cheaply replaceable. If the vehicle has multiple ignition coils, listening to the sound and unplugging one of the coils before starting it again can help locate the dead cylinder.

If the ignition cable of the dead cylinder when connected to the new cylinder produces the same result, then the ignition cables are the culprit and need replacing. If it occurs on one cylinder, it is likely the spark plugs that are the culprit.

If the spark plugs are functioning properly, it is likely a leaking intake manifold that is causing the misfiring.

Checking for leaks in the intake manifold

This check should be done by starting the engine and allowing it to idle for about a minute. This should be followed by spraying started fluid around the intake manifold gasket. If the engine starts revving, that is an indication of a leak.

In such cases, the best solution is to replace the intake manifold by consulting a certified mechanic.

Checking cylinder compression

A special compression testing tool called the Innova 3612 can be used to check the cylinder compression.

The spark plug from the dead cylinder needs to be removed first following which the hose from the tool needs to be inserted into it. The other end of the hose should be connected to the pressure gauge.

After unplugging the ignition source, turn the ignition key for about 5-10 seconds and observe the pressure gauge, and noted down the reading. After this spray some motor oil in the hole belonging to the spark plug and insert the compression tool followed by cranking the ignition key.

These steps need to be repeated and the consequent readings need to be noted down. After analysis of the readings, if one of the cylinders has a significantly lower compression rate than the other cylinders, the misfiring is likely due to low compression.

Research shows various troubleshooting codes that are used to diagnose engine misfiring.

Other articles about Mercedes Benz you may be interested in

How to fix a car engine shutting off when idle?

What are the reasons behind my car engine sputtering when starting?

What causes a car engine to misfire?

Conclusion

This blog post answered the question, ‘What causes a car engine to misfire’.

We understood the most common causes responsible for a misfiring engine and came across the ways in which misfiring occurs. Furthermore, the different ways to diagnose this problem have also been discussed. Please feel free to comment on the content or ask any questions in the comments section below.

Frequently Asked Questions (FAQs): What causes a car engine to misfire?

What is the most common cause of a misfire?

The most common causes of a misfiring engine are worn-out spark plugs, defective ignition coils, vacuum leakage, and carbon deposits in and around the head gasket.

What does an engine misfire feel like?

Engine misfires can be identified when the vehicle faces a resistance or load while accelerating, leading to sluggish drive and often a jerking motion when the throttle is pressed.

Can a misfire go away on its own?

Usually, misfires disappear and the engine retains its normal cruising speed. However, this should not be taken lightly as a single misfire is a sign of further misfiring incidents in the future.

References

https://www.repairsmith.com/i/blog/is-your-engine-misfiring-here-are-6-possible-causes/
https://www.jdpower.com/cars/shopping-guides/what-happens-when-a-car-misfires

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