Dear Tom and Ray:

Our 2001 Subaru Forester (93,000 miles, we're the only owner) has been an excellent, essentially maintenance-free vehicle. Recently, when a front axle was replaced for a torn CV joint boot, the garage owner recommended a pre-emptive replacement of the timing belt. He says we're entering a period of likely failure, which can be problematic when on the road (especially in remote places). He also may have been suggesting that this failure can cause attendant engine damage. I am of the "if it ain't broke, don't fix it" school, especially on car repairs that will run several hundred dollars. Predictably, my wife took the mechanic's word as gospel. What do you think?

-- Terry

RAY: I think if your wife always takes the mechanic's word as gospel, it's probably because you've given her plenty of reasons to do so, Terry. And you're about to give her one more.

TOM: Here's the story. There are two types of engines: interference engines and non-interference engines.

RAY: An interference engine is an engine whose pistons and valves share the same space inside the cylinders, but at different times. So when the valves are open, the piston is down and out of the way. And when the piston comes up, the valves are closed and out of the way. This is an efficient use of space, and gives the engine more power and better mileage.

TOM: But. And this is a big but -- bigger than JLo's. If the timing belt breaks on an interference engine, then the pistons and valves can collide. What does that mean? It means you need at least a new cylinder head, and perhaps a new engine. In either case, you've got a repair bill in the thousands.

RAY: And guess what, Terry? Your '01 Forester has an interference engine. Congratulations!

TOM: So if you opt for the "if it ain't broke, don't fix it" approach in this case, when it IS broke, it could cost you 25,000-dollars for a new car instead of 400 for a timing belt.

RAY: And statistically speaking, based on the age and mileage of your car, your timing belt is ready to break any day now.

TOM: If your car happened to have a non-interference engine (where the pistons and valves don't cross over into each other's space), a broken timing belt would not be such a disaster. Your engine simply would stop running, and would leave you stranded. Although I'm not sure that would be popular with your wife, either.

RAY: But in this case, because of the type of engine you have (and many other cars have interference engines, too), it's actually a much more urgent matter. In fact, we recommend that you not only change the timing belt now, but also change the timing-belt tensioner and the water pump, because if either of those fails, it could break the timing belt, with the same disastrous results.

TOM: So that's a lot of stuff that "ain't broke" that you're going to be fixing, Terry. But it's the wise -- and the cheaper -- thing to do in this case.