Car Doctor Q&A: How Did My Sparkplug Fall Out?

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Nobody expects their sparkplug to just fall out, but it happens from time to time, even in normally running vehicles. John Paul provides some advice on how to fix it and how to keep it from happening again.

Q. I have an old Dodge Caravan that I use for work. It is a V-6 and it ran great until the muffler rotted off. I kept driving it and then it got louder and lost power.

I have it at a repair shop, where I thought they were going to fix the muffler. The mechanic told me that in addition to needing an exhaust system, a sparkplug fell out. How does this happen?

I have changed many sparkplugs and never found a loose plug. In fact it is usually just the opposite, I have trouble getting the sparkplugs out of the engine. The mechanic hasn’t had time to fix the problem yet, any idea what I’m in for?

A. The sparkplug could have come out because it was either over tightened or not tight enough.

It is possible that if the sparkplug were over-tightened, the treads were damaged and stripped, allowing the plug to work its way out. If this is the case, the solution is to Heli-Coil the sparkplug hole. This is a dedicated kit that uses a threaded insert to the to repair the damaged plug hole in the cylinder head. This is generally a permanent repair.

This repair happens to be on a Yamaha SR500, but the process of Heli-Coiling an automotive head is exactly the same. The motorcycle video just provides a better view of what’s happening:

The other possibility is the threads on the cylinder-head were coated with carbon and caused the sparkplug to bind up. When this happens, the plug never fully seats and doesn’t get tightened sufficiently.

As a general rule if a sparkplug can’t be threaded in by hand it is a good idea to clean the threads with a tool called a thread chaser. Although hardly ever performed, using a torque wrench is the preferred method to tighten any sparkplug so this doesn’t happen.

John Paul is senior manager of public affairs for AAA Northeast. A certified mechanic, Paul tests dozens of new cars each year and also hosts a radio show on AM 950