Dear Car Talk:
I have a 2001 Ford Expedition 5.4-liter V8. I'm having an odd problem: This week marks the third time that a spark plug has been ejected from my engine.
The first time this happened, my mechanic said he "tapped" it. The second time, he assured me that it would never happen again.
I'm not sure if it's the same cylinder, but it just happened for the third time.
Do you know what is causing this? Thanks. -- Frank
This is a well-known problem in this engine, Frank.
Apparently, the aluminum cylinder head doesn't have sufficient threads to keep the spark plugs in place. Those spark plugs are under tremendous pressure from the explosions inside the cylinders. Once they start to get loose, it's just a matter of time before they take off like a North Korean rocket.
The solution is what your mechanic did. You "tap" a new spark plug hole. There's a kit we buy that comes with an insert. It's a sleeve that's slightly bigger than the existing spark plug hole and has threads on the outside and the inside. We drill out the new hole, which is a little bigger than the old one. Then, we screw this sleeve in there and epoxy it in place. The spark plug threads inside that new sleeve.
They work. Your mechanic is right that the insert should not fail again. So, I'm guessing you've had three different plugs blow out. The good news is you only have five more inserts to pay for!
The bad news is that because each spark plug in this engine has a coil built on top of it, and that coil gets ruined when the spark plug blows out, each insert is going to cost you about $400 a pop.
You can do them prophylactically and replace them all now, so you won't have a problem again. But since this truck is going on 20 years old, you might want to take it a plug at a time. Who knows what else might go in the truck before you get through five more inserts.
You might even be able to delay future problems by checking and tightening your plugs on a regular basis. Like once a week. Or twice an hour. Good luck, Frank.