After new spark plugs are installed, what causes them to come loose?

Dear Car Talk

Dear Car Talk | Feb 01, 2003

Dear Tom and Ray:

I recently had new plugs and wires installed in my '93 Chevy Cavalier V-6. After about 2,000 miles, I was driving home from my local grocery store when the car started making a horrible noise, followed by running extremely roughly and a loss of power. I finished driving the less than 1 mile to my house. When I opened the hood, I found that one of the spark plugs had come COMPLETELY out of the engine. It was still attached to the wire. The plug appeared dry, but there is a black ring around the white coating at the top half of the plug. I need to know what kind of damage this might have done to my engine. -- Darrell

RAY: Probably none, Darrell. But I bet it scared the hell out of you, huh?

TOM: Here's what happened. You had the spark plugs replaced, and the mechanic forgot to tighten one of them down completely. He left it "hand-tight."

RAY: Then, as the engine ran and vibrated, the plug slowly unscrewed itself. When it was most of the way out, carbon from the cylinder was blowing past it, which is what left that dark ring. If you could have heard the exhaust escaping over all the other noises your engine makes, you would have a heard a "pffft, pffft, pffft," or a "tick, tick, tick" sound at that point.

TOM: Finally, the plug unscrewed itself the rest of the way and popped out of its hole. That's when it got real loud! And what you were hearing was compressed air trying to escape through the spark-plug hole.

RAY: It sounds like a "Gatling gun."

TOM: But most likely, there was no damage done. And all you have to do is screw the spark plug back in and go on your merry way. But I'd check the other plugs, too, to make sure they're tight, given the circumstances.

RAY: Sometimes the spark-plug hole gets stripped, and then your mechanic has to rethread it or install an insert before you can screw the plug back in. But given the timing of your problem, I'd put money on the "forgot to tighten that one" theory, Darrell.

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