I have a 1987 Honda Civic wagon. Over the last few years I've had the valves and rings replaced, oil seals replaced, head checked for cracks, and maybe more. I can't remember. But the same problem keeps popping up. The number four spark plug gets fouled with oil, and the car starts to run like a choo-choo train. If I take out the plug, clean it or replace it, the car will run fine for a few days, then it happens again. What's your diagnosis? -- Cynthia

Tom: Gee, Cynthia. If I had to guess, I'd say you have a bad number-four intake valve guide.

Ray: The valve guide is the sleeve through which the valve stem slides when the valve opens and closes.

Tom: And that valve guide (and the seal that surrounds it) is supposed to keep the oil at the top of the engine from getting inside the cylinders. If the oil does get inside a cylinder, it can do. . . what? Foul the spark plug! And that's what I think is happening.

Ray: This assumes, of course, that the ring job you had was done correctly -- or done at all. Because a car in need of a ring job could also foul the plugs.

Tom: But if the rings are good, then I'd look for a bad valve guide (or if you're really lucky, just a bad valve guide seal), Cynthia. The guide will cost you several hundred dollars to replace. Or, alternatively, pick yourself up a good spark-plug wrench and a family-size bucket of spark plugs.