What Are Boots for Spark Plugs?

When you replace your vehicle’s spark plugs, do you also need to replace the spark plug boots, and what do these boots even do?

What Is a Spark Plug Boot?

Spark plug boots insulate the coil or spark plug wire connection. They are often made of rubber or silicone, which wears down and breaks over time.

If the voltage arcs due to a bad spark plug boot, the arcing voltage can cause engine misfires because the voltage isn’t going to the spark plugs as it should. Rough idling and stalling are also symptoms of a bad spark plug boot. Preventing voltage arcing is the main reason spark plug boots are necessary.

Modern vehicles have coil-on-plug (COP) insulator boots, which are part of COP ignition systems. COP boots are the modern version of spark plug boots. They serve the same purpose, but they’re part of a system where the ignition coil is mounted directly to the spark plug. A good spark plug replacement will include replacing the COP coil boots as well.

If you have a misfire “bite” under load, spray a bit of mildly soapy water on the spark plug boots with the engine running. If you see lightning pop out of the plug boot and a misfire happens at the same instant you see it, that boot is done for.

– Richard McCuistian, ASE Certified Master Automobile Technician

When To Replace Spark Plug Boots

It’s typically a good idea to replace the spark plug boots whenever you replace your vehicle’s spark plugs if you have COP coils.

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Pro Tip: If your vehicle has spark plug wires, the boots will come with new wires. Simply replacing the boot on an existing wire is not recommended, and spark plug wire boots aren’t typically available without buying the wires anyway.

If you see a streak of carbon on your spark plug, it’s likely from a bad spark plug boot. If you replace the spark plug but not the boot, the boot can cause more carbon tracking on the new spark plug.

Oil-soaked boots also need to be replaced, but you’ll need to locate and eliminate the oil leak source as well.

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Tips for How To Change Spark Plug Boots

Replacing COP boots can be pretty simple, but a lot depends on your vehicle’s exact year, make, and model. We’ve compiled some good tips for changing spark plug boots on a COP system, but keep in mind that these tips might not be applicable for every vehicle.

To prevent bad burns, remember to wait for the engine to cool down completely and disconnect the battery before starting this project.

Recommended Tools

You’ll need a screwdriver or wrench to remove the coils or coil block when working with COP boots. If you also plan to replace the spark plugs, you’ll need a plug wrench and gapping tool. If you can, pick up a set of spark plug wire boot pliers as well.

Reaching the Boots

Before you examine the spark plug boots, you’ll need to reach them. Remove the engine cover to reach the spark plugs and coils. In a COP system, the boot is integrated into the spark plug and ignition coil design. You can remove the whole thing from the spark plug well and inspect it for damage.

Spark Plug Boot Removal and Replacement

Once you’ve removed the COP unit from the spark plug well, place it on a flat surface before twisting the boot off the coil. Examine the boot and the coils for any damage. Check the coils and take note of any resistors or connecting springs. Their position and orientation will need to align again when you install a new spark plug boot.

Compare the new spark plug boot with the old one, and check to make sure the designs match. Slight variations should be ok, but don’t proceed if there are major differences. Reassemble everything, making sure all the resistors and connecting springs are in the correct position. Lubricate the boot on the spark plug end using a bit of dielectric grease before reinstalling it. Avoid over tightening any fasteners in the system when reassembling everything.

Additional Steps

If your COP unit has a gasket, you might want to replace it as well. It’s only a matter of sliding it off the coil and sliding a new one on. While the COP unit is disassembled, you might also want to check the ignition coils and spark plugs for damage.

Boots for spark plugs are essential for keeping your vehicle’s spark plugs firing correctly. Whenever you change your spark plugs, you can also change the spark plug boots. It doesn’t take much extra effort, and doing so will help your brand new spark plug last as long as possible.