Car misfires often caused by spark plugs, ignition boot

Jul 26, 2023

Question: I have a 2006 Kia Sedona, and it randomly misfires roughly once for every week of normal driving and once after each carwash. When it happens, I pull over and shut off the car. Then, I turn it back on, and the misfire stops. What could be the problem?

Answer: Misfires can be caused by a number of issues, but the most common are bad spark plugs or an ignition boot failure.

Misfires aren’t just disruptive to the driver; they also affect fuel economy and emissions quality. An engine that misfires will often fail an emissions test because it produces a high level of hydrocarbons in the exhaust.

A bad spark plug can usually be isolated to see if it’s cracked, fouled or has wiring that’s gone bad. An ignition boot is also called a coil boot or a spark plug boot, and a lot of vehicles now have one ignition coil, which creates voltage, that mounts on top of each spark plug. The “boot” is what connects the ignition coil to the spark plug. It is similar to a spark plug wire but isn’t visible.

Over time, the rubber deteriorates and loses its insulation quality. The spark from the coil changes direction; instead of traveling through the boot, from the coil to the spark plug, it will shoot out the side of the bad boot and spark to the cylinder head. This causes that cylinder to misfire.

It may happen more in wet weather or after a carwash, as water is a conductor and creates an easier path for the spark to travel. I would recommend you take the car to a reputable repair shop to inspect the car’s secondary ignition components.

While there, a code check for any history codes may also help to aid in determining which cylinder is at fault.

Chris Garman is President of Wilhelm Automotive (www.wilhelmautomotive.com). To ask a Car Care question, please email Chris at [email protected].