How Tight Should Spark Plugs Be? (& Effects Of A Loose Plug)

• Brian Onyango

Updated on March 29, 2024

Spark plugs are one of the most important parts of your car. They create the spark that ignites the air/fuel mixture in the engine, and if they’re not working properly, your car won’t run.

To determine how tight a spark plug should be, it is recommended that you follow the torque specifications provided in your car’s owner’s manual or the spark plug torque chart. These usually range from 15 to 25 ft-lbs (20 to 34 Nm).

If no specific value is given, a good rule of thumb is to turn them until they are snug, then give them an extra quarter-turn. A loose plug could cause misfires, engine knocks, rough idle and run, as well as stalling.

Spark Plugs

Spark plugs refer to the part of the engine that ignites the air/fuel mixture. The plug is a metal rod with a tapered, threaded tip that protrudes into the combustion chamber. The spark plug has a gap between the electrode and the ground strap where electrical current jumps to create a spark.

The type of spark plug used in an engine depends on the engine design. Older engines typically used a carburetor to mix air and fuel, while newer engines use fuel injectors. The correct spark plug must be used for the application to ensure the proper operation of the engine.

Rule of Thumb and Spark Plug Torque Chart

A spark plug torque chart refers to the amount of torque, or rotational force, required to properly install a spark plug in an engine. This chart is usually provided in the owner’s manual with specifications on how tight the spark plug is to be tightened.

Without a doubt, the best method to tighten a spark plug to specifications is using a torque wrench. A torque wrench is a calibrated tool that will help you apply the correct amount of torque to the spark plug.

If a torque wrench is not available, there is a rule of thumb that can be followed. Tightening a spark plug too much can cause damage to the threads in the cylinder head and potentially strip them. If this happens, the spark plug will need to be replaced.

What Will Happen If Spark Plugs Are Too Loose

Spark plugs need to be tightened properly, or they will cause all sorts of problems. If they are too loose they can cause;

1. Misfiring

The most common problem is that the engine will misfire. Misfiring can cause damage to the catalytic converter, and if left unchecked, can lead to engine failure.

A loose plug can also cause the engine to run lean, which means there is too much air and not enough fuel. This can cause the engine to overheat and damage other components.

2. Pre-ignition/Knocking

A loose spark plug can make the air/fuel mixture ignite too early. This can cause what is known as “pre-ignition” or “knocking.” Pre-ignition is uncontrolled combustion that happens before the spark plug fires. It can damage the engine by causing piston and valve damage.

3. Fouled Plug

A fouled plug is a plug that has been damaged by too much heat or deposits. A loose plug can cause the spark to jump erratically, which can damage the electrode. This will cause the engine to run poorly and may eventually lead to engine failure.

4. Damaged Threads

Threads refer to the part of the spark plug that is screwed into the engine. If the threads are damaged, it can cause an oil or coolant leak. It can also cause the plug to fall out, which will damage the cylinder heads.

5. Poor Engine Performance

The engine will not run as efficiently if the spark plugs are loose. This can cause a decrease in fuel economy and power due to the engine not firing properly. The engine may also run rough or stall.

How to Tighten Spark Plugs

Now that we already know how tight the spark plug should be. Let’s take a step-by-step look at the process of tightening spark plugs.

Step One: Allow Engine to Cool Down

When you have been driving your car you need to start by letting it cool before opening the hood. This is to prevent you from being burned by the engine.

Step Two: Disconnect the Battery

You will need to disconnect the battery while working to prevent any electrical issues.

Step Three: Locate the Spark Plugs

First, you need to locate the spark plugs. In most cars, they are located on the top of the engine near the firewall. Once you have located them, you need to remove the spark plug wires. Be sure to label them so you can put them back in the correct order.

Step Four: Remove the Old Spark Plug

Use a spark plug socket and ratchet to remove the old spark plug. Be sure not to drop the old plug as it can damage the threads.

Step Five: Inspect the Threads

Once the old spark plug is out, take a look at the threads in the cylinder head. If they are damaged, you will need to have them repaired before proceeding.

Step Six: Insert the New Spark Plug

Now it is time to insert the new spark plug. Be sure to hand-tighten it until it is snug. Do not over-tighten as this can damage the threads.

Step Seven: Reconnect the Battery

Now that the new spark plug is in, you can reconnect the battery.

Step Eight: Start the Engine

Once you have reconnected the battery, start the engine and check if the engine is running properly.

Frequently Asked Questions

Can You Over-tighten Spark Plugs?

Yes, you can over-tighten spark plugs. This can damage the threads and cause a coolant or oil leak. It can also cause the plug to fall out, which will damage the cylinder heads.

How Do You Know If a Spark Plug Is Loose?

The best way to tell if a spark plug is loose is to remove it and inspect the threads. If they are damaged, then the plug is loose. Further, you can also look out for symptoms such as poor engine performance, decreased fuel economy, and a rough running engine.

How Often Should Spark Plugs Be Replaced?

Spark plugs should be replaced every 30,000 miles or so. However, this may vary depending on the make and model of your vehicle. Be sure to check your owner’s manual for specifications.

What Happens If a Spark Plug Falls Out?

When a spark plug falls out, it can damage the cylinder heads. It can also cause an oil or coolant leak. Further, the engine will not run as efficiently and may experience a decrease in power.

What Causes a Spark Plug to Come Loose?

There are a few things that can cause a spark plug to come loose. These include:

• Incorrect torque when installing the plugs
• Vibration from the engine
• Thermal cycling of the engine
• Corrosion of the threads

Each of these factors can cause the spark plug to loosen over time. It is important to check the plugs regularly to ensure they are tight.

How Can I Prevent My Spark Plugs From Coming Loose?

There are a few things you can do to prevent your spark plugs from coming loose:

• Use an anti-seize compound when installing the plugs
• Check the plugs regularly and retighten them if necessary
• Use quality spark plugs that are designed for your vehicle

Conclusion

It is recommended that you follow the torque specifications provided in your car’s owner’s manual or spark plug torque chart which usually ranges from 15 to 25 ft-lbs (20 to 34 Nm).

If no specific value is given, a good rule of thumb is to turn them until they are snug, then give them an extra quarter-turn. A loose plug would cause all sorts of problems and even damage to the engine.

It is important to make sure that spark plugs are tightened to the correct specification. This will ensure that the engine runs properly and avoid damage to other components.

Find more relevant information with these articles:

Carbon Fouled Spark Plug: What Causes The Buildup?

What Causes Spark Plugs To Go Bad Fast?

Changed Spark Plugs And Coils Still Misfiring – What To Do Next