Dear Car Talk:

I have a 1991 Volvo 240 with 220,000 miles. I had the original clutch replaced at 174,000. When I park on a steep incline, the car rolls. The parking brake never works, but I should be able to park it in gear to prevent it from rolling, right?

My mechanic is mystified, and says nothing is wrong with the transmission. Given that I have no plans to move to San Francisco, it's a minor issue. But I'd like to know what is causing it and get it fixed, if possible. Thanks. -- Bill



This is your most serious problem, Bill?

When you park your car in gear to keep it from rolling, you need two things: You need good engine compression, to prevent the pistons from moving. And you need a good clutch, to keep the engine solidly connected to the wheels. If either the clutch or the compression is weak, the weight of the car (and this is a heavy car) combined with the slope of the hill can overcome the holding power of the engine.

Let's assume, for the moment, that your clutch is still good since it was replaced 45,000 miles ago. That leaves engine compression. And with 220,000 miles on this old beast, I'd say your compression is the prime suspect. What keeps the car still is that it's very hard for the pistons to compress the air in the cylinders. If they can't compress the air, they can't move, and the car stays still. But if your piston rings are old and worn out and leaky, and can't hold pressure, it's a lot easier for the pistons to move. And that's when you look out the window and see your Volvo rolling down to the 7-Eleven for a Slurpee.

So have your mechanic test your compression. That's easy to do. And if your engine compression is weak, then you have a few choices. You can rebuild the engine, which I would be hesitant to recommend due to the age and mileage of the car. You can avoid parking on hills. Or avoid parking on hills steep enough to allow the rolling. You can toss a couple of six-by-six blocks of wood in the car and chock your wheels when you have to park on a hill. Or you can try fixing the parking brake.

And if you manage to make it work, you may have the only Volvo 240 in captivity with a working parking brake. But start by getting a compression test. If the compression is good, test the clutch. It's one of those. My money's on compression.