Can a loose spark plug really cause $2,000 in damage?

Dear Car Talk

Dear Car Talk | May 01, 2009

Dear Tom and Ray:

Last night, my car, a 2001 Volkswagen Jetta, was in perfect working order when I parked it at the movie theater. When I came out two hours later and started the car, I heard a "pop" sound, and the engine sounded like a really loud lawnmower. It was also shaking violently. I had it towed to the Volkswagen dealership. They called me back with a quote of more than $2,000, saying that one of the spark plugs is loose and the entire head needs to be replaced. What do you think happened? Does this sound right to you?

-- Micah

TOM: It sounded like a lawnmower, and was shaking violently? And what alerted you that something was wrong, Micah?

RAY: It sure sounds like one of your spark plugs popped out. The loud bang you heard was the spark plug launching itself into the underside of the hood. And then, because that spark plug was missing, the engine shook violently, because it was running on only three cylinders.

TOM: When you remove a spark plug, you hear the unmuffled noise of the combustion chamber. That was the lawnmower sound.

RAY: So the diagnosis sounds correct. The treatment, however, is another matter.

TOM: Sometimes, spark plugs just work their way out over time. If a plug wasn't tightened properly when it was replaced, it could have just loosened up over time until it finally popped out. If that's what happened, all you need to do is screw it back in. That's two bucks, not 2,000.

RAY: More likely, when someone installed that spark plug, he stripped the threads in the head that hold it in. It held for a while, but because it's under extremely high pressure, eventually it gave way.

TOM: That's why the dealer wants to replace your head. We've wanted to replace my brother's head for years, but every time we try it, he wakes up and makes us stop.

RAY: Replacing the head might not be necessary in your case, Micah. There's a thing called an "insert" that's designed for exactly your situation -- when the threads in a cylinder head are stripped but the head is otherwise fine.

TOM: The insert is basically a new set of threads that screw into the head. If you can fix this with an insert, that'll cost you more like one hundred dollars.

RAY: Now, if the head is badly damaged for some reason, they may have no choice but to replace the whole thing.

TOM: But inserts often work just fine, and if the insert is secure enough that they can tighten the spark plug to the required torque, the head will be as good as new. So, find another mechanic who specializes in VWs, and get a second opinion, Micah. Ask him if an insert might work for you. Good luck.

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