Could letting the engine run idle for 36 hours do any damage?

Dear Car Talk

Dear Car Talk | Jan 01, 1996

Dear Tom and Ray:

While I was away on business, my wife locked her keys inside her car. She decided to wait until I got home to unlock it, but unfortunately, she didn't realize the car was running. Approximately 36 hours later, when I got home and went to retrieve the car, the engine was still running. I have two questions: 1) Did she damage the engine? 2) In case this happens again (and she realizes the car is running), is there a way to kill the engine without getting under the hood?

TOM: Not to worry, Frank. I can tell you're annoyed at your wife, but you don't have to be. The engine can take it. There are lots of cars that idle for hours and hours at a time. Just look at the police cars in front of your local donut shop.

RAY: As long as the cooling system is working properly, the engine should be able to idle indefinitely. So no damage at all was done to the car.

TOM: As for future occurences, there IS a simple way to kill the engine without opening the hood. You simply plug up the exhaust pipe. Take a good, thick rag (because the exhaust is hot) and cover the end of the tailpipe with it. As long as you don't have an exhaust leak somewhere, the engine should die within ten or fifteen seconds.

RAY: The reason it dies is this: In order for the engine to run, a fresh mixture of gasoline and air has to get into the cylinders. But it can't come in unless the old, burned-up stuff is able to get out. If you plug up the tailpipe, none of the used-up fuel can escape, so no new fuel comes in, and the engine goes kaput.

TOM; But just remember, when you do that, you'll be leaving the ignition on, so you'll be running down the battery. And if you leave it like that for more than a couple of hours, you may need to get your battery recharged to get the car started.

RAY: So when you consider that at idle, the car only burns a few bucks worth of fuel in 36 hours, what your wife did may actually have been cheaper than plugging up the tail pipe. You may want to give her a pat on the back, Frank. Her resourcefulness saved you the cost of a battery recharge and a locksmith call!

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One