Why Does Car Runs Worse After Changing Spark Plugs?

Replacing spark plug can increase the fuel efficiency by at least 30%. It also fixes problems such as rough idle. But is the opposite happening to you? You probably are wondering, why does car runs worse after changing spark plug

Some of the most common reasons why your car runs worse after changing spark plugs are inappropriate spark plug gaps, using wrong plugs, or messing up the wirings. Follow the recommendations in the user manuals to know the appropriate measurements or processes to fix these new spark plug issues.

A few more mistakes during the spark plug installation can lead your car to perform poorly. Stay tuned till the end to know all the reasons and how to fix them.

7 Reason Why Car Runs Worse After Changing Spark Plugs (With Solution)

You have changed the spark plug to get a better engine performance, but now your car is performing rough. Some of the reasons why a car runs worse after changing spark plugs are:

  1. Incorrect gap
  2. Wrong spark plug
  3. Inappropriate torque (leak)
  4. Damaged spark plug wire
  5. Improper plug boot seating
  6. Mixing up spark plug wire
  7. Reverse spark plug wires

It is clear that you did something wrong that caused this mess. Now the question arises, how to solve these spark plug problems and get optimum performance from your car?

The following is a detailed guideline on fixing new spark plug problems:

1. Incorrect Gap

The first and most common reason for a car performing poorly after installing a new spark plug is an incorrect gap. You have gapped beyond the recommendation.

Different engines have different gap requirements. For instance, some cars run fine on 27 or 28 gaps, while others need 32.

You can use a feeler gauge such as OEMTOOLS 25025 26 Blade Master Feeler Gauge to measure the existing gap before replacement and follow it. 

If you already have changed the plug, follow the user manual to find the recommended gap. Now, remove the spark plug and install it with the appropriate gap.

2. Wrong Spark Plug

Even after installing the spark plug with the correct gap, you may not get the expected results. It may happen due to using the wrong spark plugs.

Ensure that you use the exact cross-reference spark plugs replacement for the original FoMoCo plugs. 

Some spark plug companies claim their products fit all engines, but personally, I don’t use those models unless I have no alternatives.

3. Inappropriate Torque (leak)

Installing spark plugs with appropriate torque is essential. If you do not follow the recommended torque, it will cause a leak, and the engine will underperform.

The only solution to this problem is reinstalling the screws with appropriate torque.

4. Damaged Spark Plug Wire

A lot of wires are involved in the installation of spark plugs. Many people often end up damaging spark plug wires during the process.

Use a digital multimeter to check if all the spark plugs are ok. The Greenlee – Dmm, 600V Ac/Dc Elec Test Instruments could be one of the best digital multimeters to opt for checking spark plugs condition.  

If you find any faulty spark plug wire, replace it with a new one.

5. Improper Gasket Seating

The head gasket or plug boot seatings are complicated, and you may have a feeling they are seated well when they are not. 

Check the gasket seating again and ensure that you hear the metal clicking sound that indicates a perfect fit.

6. Mixing Up Spark Plug Wire

You may also mistakenly mix up the wires during the installation. Now, a faulty connection between the ignition coil and the spark plug will cause poor engine combustion.

Recheck the wires properly to ensure they are connected correctly. Take the help of expert friends if you are confused.

7. Reverse Spark Plug Wires

Mistakenly setting the spark plug wires reversely can change the firing order and cause an engine to misfire. If such problems happen to your car, check the wires and set them properly.

Can New Spark Plugs Cause Rough Idle?

A rough idle is a sign of a faulty spark plug or damaged spark plug wires. Installing a new spark plug can solve this problem. New spark plugs can also cause rough idling, if not installed properly.

In fact, replacement of spark plugs in the wrong way can cause several other problems such as engine misfires, poor fuel economy, bad starting, and trouble accelerating. 

What Should I do After Changing Spark Plugs?

After you change the spark plug, recheck all the connections. Once you are satisfied, start the car. 

You will find some noticeable differences such as optimum combustion, improved fuel economy, and smooth driving experience.

If your car performs the opposite, there must be something wrong. Check the discussion above to find out the problem and fix it.

Do Spark Plugs Increase Speed?

Spark plugs will not necessarily add speed to your car. However, a new spark plug can increase the horsepower of the vehicle and improve overall performance. If your vehicle is lacking speed due to bad spark plugs, a new one will help it to gain that speed and make your car faster.

What Spark Plugs Make Your Car Faster?

It is essential to use the best spark plug to get optimum performance and make your car faster. 

For instance, NGK 6619 Iridium IX Spark Plugs is one of the best spark plugs, in my opinion. That’s because it features an iridium construction that provides better sparks and a longer lifespan.


How Often Do Spark Plugs Need Changing?

Spark plugs are made to perform long, and they usually require changing after every 80,000 to 100,000 miles. However, they can sometimes get damaged earlier than expected and require replacement.

Does Changing Spark Plugs Make Your Car Run Better?

Changing spark plugs improve your car’s combustion and fuel economy. As a result, you will see an improvement in the performance, which will make your car run better and smooth.

Do Spark Plugs Affect Engine Performance?

Faulty spark plugs can worsen the car’s combustion and damage the car’s catalytic converter, leading to an engine misfire. Installing a new spark plug can solve this problem, meaning the spark plugs affect the engine performance. 


Bad spark plugs can cause engines to be weak and perform poorly. They can interrupt the combustion process, which will cause the engine performance to suffer or even misfire. In that sense, changing or replacing the spark plugs could be a solution. 

But you need to make sure that you can do the changing or installation process accurately. If you don’t have enough confidence, you should get professional help. 

Install the spark plugs appropriately to get the best performance from your car.

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