A potato in the tailpipe, and other automotive party tricks.

Dear Car Talk

Dear Car Talk | Feb 01, 2008

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.

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