Dear Tom and Ray:

True or false? In our clubhouse parking lot, I came across a car that was left with the engine running, keys inside and doors locked. Security was not able to help find the owner. I mentioned this incident to friends, and someone said that if I had put something, like a potato, in the tailpipe, the engine eventually would have shut off. Would this really have happened? Would it have caused any damage? -- Mary

TOM: It certainly would have happened, Mary. Assuming the exhaust system is intact and not leaking, you can stop an engine by plugging up its exhaust outlet.

RAY: Here's why. If the exhaust gases can't escape from the cylinders, then there's no room for the fresh gasoline and air to get in. So the engine gets starved for fuel.

TOM: Think about it this way: If someone stopped you from breathing out, you wouldn't be able to breathe in, would you? No room! And your engine would stop, too.

RAY: Now, a potato is the time-honored vegetable of choice in the fraternity community because of its wide availability, appropriate selection of sizes and tendency to stay put once placed firmly in a tailpipe. But the truth is, holding a thick rag over the end of the tailpipe would cause the engine to stall, too.

TOM: In fact, we do that as a test in the garage, to see if a car has an exhaust leak. If you plug up the end of the tailpipe and the car keeps running, you know the exhaust is escaping from somewhere else (i.e., leaking).

RAY: There's one other consideration in your case, Mary. When the engine dies, the key will still be in the "run" or "on" position. That means that any electrical accessories that are turned on (blower motor, lights, CD player) will continue to run off the battery. So there's a chance that the battery will be dead if the car is left all day like that.

TOM: Of course, if you leave it running, it could run out of gas and THEN run the battery dead.

RAY: So it's a judgment call, Mary. A car with a properly functioning cooling system won't be harmed if it sits and idles for hours.

TOM: But if the car is in a place where it's unsafe to let it idle -- if it's indoors, near people, in a place where it might be stolen -- or if it's clearly overheating, then plug up the tailpipe and kill the engine. The owner may have to get a jump-start later, but the tow truck presumably will be there anyway to help him break back into his car and get his keys.

Dear Tom and Ray:

There is a fluffy white/blue deposit at the base of the positive-terminal post of my battery. The service manager's explanation is: This is perfectly normal for a sealed battery. When a battery is charged, gas escapes from a small gap between the post and the battery case and forms the deposit. The service manager recommended "corrosion treatment" for about $20. Are all batteries designed this way? Is this corrosion treatment worth it? -- Art

RAY: We see a lot of batteries with a little bit of corrosion at the terminals. But if there's a lot of buildup there, it could signal a problem.

TOM: Right. Batteries and charging systems that are functioning properly don't create mountains of whitish-blue crud on battery terminals.

RAY: So you could pay this guy $20 to take care of it for you. Or, if you prefer, you can clean it up yourself. You just mix up a cupful of baking soda and water, to the consistency of watery pancake batter.

TOM: Then, slather it all over the terminals. It attacks the corrosion, and then you wash it all off with a hose or a bucket of water.

RAY: We also use a spray you can buy at an auto-parts store that coats the clean terminals and prevents the corrosion from coming back -- kind of.

TOM: If there's something wrong with your battery or charging system, the corrosion will come back despite the spray. So, then you'll need to investigate.

RAY: The most likely culprit is an alternator that's overcharging -- like we do all the time at the shop -- and causing your battery to out-gas.

TOM: So, Art, I'd clean off the terminals, re-secure the connections and then keep an eye on it. And if the stuff comes back in a couple of months, put the 20 bucks you saved toward a complete charging-system check.

Opinions expressed solely are those of the writer. Got a question about cars? Write to Click and Clack in care of this newspaper, or e-mail them by visiting the Car Talk Web site at www.cartalk.com.