Is it better to leave a car engine running when changing the spark plugs?

One of the main reasons why you should never change spark plugs with the engine running is safety. When you change spark plugs, you are working on the ignition system of the engine, which involves disconnecting and reconnecting electrical components.

Spark plugs are essential components of your car's engine that produce sparks to ignite the air-fuel mixture in the combustion chambers. They need to be replaced periodically to ensure optimal engine performance and fuel efficiency. However, changing spark plugs is not a simple task that can be done while the engine is running. In fact, doing so can be very dangerous and damaging to your car and yourself. Here are some reasons why you should never change spark plugs with the engine running.

Safety Concerns

Leaving the engine running during this process can be dangerous for several reasons:

• Electric Shocks: Open spark plug wires and exposed ignition components can lead to electric shocks if touched while the engine is running. This can result in injuries to you or damage to the vehicle's electrical system .
• Fire Hazard: The engine generates heat and can accumulate fuel vapors, especially if the fuel injectors are still functioning. This combination of heat and fuel vapor increases the risk of a fire or explosion when working near open spark plug holes . 
• Moving Parts: The engine's moving parts can be hazardous when exposed. If you accidentally drop a tool or a part into the engine bay while it's running, it can cause serious damage to the engine or other components .

Important Tip: Spark Plugs: A Comprehensive Guide to Types, Features, and Performance

Incorrect Timing

Another reason why you should never change spark plugs with the engine running is incorrect timing. Changing spark plugs can sometimes involve removing ignition components like spark plug wires or ignition coils. If these are not reconnected properly, it can lead to misfires, poor engine performance, and potentially damage the ignition system .

For example, if you remove a spark plug wire from a cylinder while the engine is running, that cylinder will stop firing and cause the engine to run rough. This will also trigger the check engine light and set codes in the engine control computer. You would need to reset the engine controller after changing the spark plugs.

Moreover, if you try to screw in a new spark plug while the engine is running, it will be very difficult to do so because of the air going in and out of the spark plug hole. The air pressure can also force the spark plug out of the hole like a missile and damage the threads in the head or injure you .

Read: Troubleshooting No Spark Issues in Modern Cars: A Comprehensive Guide

How to Safely Change Spark Plugs

To safely change spark plugs, follow these steps:

1. Turn Off the Engine: Make sure the engine is off and cool before you start working on it. This will prevent burns, shocks, fires, and other hazards.
2. Disconnect the Battery: To avoid any electrical issues, disconnect the negative terminal of the battery before you touch any ignition components.
3. Locate the Spark Plugs: Depending on your car model, you may need to remove some parts like air intake, hoses, or covers to access the spark plugs. Refer to your owner's manual or a repair guide for specific instructions.
4. Remove the Spark Plug Wires or Coils: Carefully disconnect the spark plug wires or coils from each spark plug. Label them or mark them with tape to remember their order. Do not pull on the wires, but twist and pull on the boots.
5. Remove the Old Spark Plugs: Use a spark plug socket and a ratchet to unscrew each spark plug from its hole. Inspect them for signs of wear or damage like cracks, corrosion, or deposits.
6. Install the New Spark Plugs: Before installing new spark plugs, check their gap with a feeler gauge and adjust it if necessary. Apply some anti-seize compound on the threads of each spark plug and screw them into their holes by hand. Then use a torque wrench to tighten them according to the manufacturer's specifications.
7. Reconnect the Spark Plug Wires or Coils: Reattach each spark plug wire or coil to its corresponding spark plug. Make sure they are securely connected and in their original order.
8. Reinstall Any Removed Parts: Replace any parts that you removed to access the spark plugs, such as air intake, hoses, or covers.
9. Reconnect the Battery: Reconnect the negative terminal of the battery and start your car. Check for any leaks, noises, or warning lights.

Don’t forget to share it