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Dear Tom and Ray: I have a 2001 Dodge Neon with more than 150,000 miles on it. Until very recently, it’s been an extremely reliable vehicle, and I’ve loved it.

Lately, and at random times, however, the engine will not shut off – even with the key removed from the ignition and in my hand! I’ve already gotten a new key and replaced the entire ignition cylinder, but it happened again yesterday evening. Luckily, I was able to fiddle with the key a little bit and finally get the stupid thing to shut off. What could be causing this? At this point, the mechanics I’ve taken it to haven’t been able to identify or fix the problem, and I can’t find anything on the Internet. Each time it’s happened, my local mechanic has disconnected the battery, which made the car turn off.

After that, though, it runs perfectly for maybe a couple of months, until it happens again. Any idea what could be wrong? – Grace

Tom: You’re a lucky woman, Grace – you can’t get your Neon to stop. Most of our customers with Neons can’t get them to start!

Ray: You say you replaced the ignition cylinder. But that’s just the locking mechanism that the key fits into; it’s not the ignition switch itself. So you easily could have a bad ignition switch.

Tom: But before you replace the switch, try replacing the ignition relay. Rather than have a huge amount of current running through the steering column to the ignition switch at your fingers, the ignition switch signals a power relay under the hood. It’s that relay that actually sends current to the engine’s computer, which enables the engine to run. That relay could be faulty and sticking in the “on” position.

Ray: The relay probably costs 20 bucks, and it takes two minutes to replace. So start there, and see if that fixes your problem.

Tom: If not, then it’s almost certainly the ignition switch, because there’s really not much else that could cause this. But since replacing the ignition switch will cost you several hundred dollars, don’t do that until you’ve ruled out the $20 fix. Good luck, Grace.

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Dear Tom and Ray: I finally figured out what the sound is in my garage. Something on my 2009 Toyota Corolla is running when the car is turned off! I have heard it at various times of the day and night, and also when the car has been parked for hours. There is a noise coming from the driver’s-side rear – behind the tire. It sounds like a fan. It runs for four to five minutes at a time, then shuts off. I called Toyota; they have no idea what it is and want me to bring it in. After taking it several times, I really don’t want to give up the use of my car AGAIN. And it might not turn on until everyone has gone home for the night. Have any ideas? P.S. The car runs fine. – Elaina

Tom: Well, first we have to chide you for hanging around your car hours after you’ve shut it off, Elaina. What are you, some kind of Corolla stalker?

Ray: It’s probably the evaporative emissions system pump, Elaina.

Tom: Before we were concerned about pollution and smog and asthma and not being able to breathe and all that, all gas caps had pinholes in them.

Ray: That allowed air to enter the tank as the gasoline was drawn out. Otherwise, the gas tank would crumple in on itself, like a baggie that you sucked the air out of. But, of course, the hole in the gas cap not only let air in, it also let gasoline vapors out, and that created ... well, Los Angeles.

Tom: So, an evaporative emissions recycling system is now used on every car. It allows air to get into the tank but stops gasoline vapors from escaping. Instead it stores the vapors in a charcoal canister, and then purges them and sends them to the engine to be combusted when the car is started again.

Ray: What you’re hearing is the evaporative emissions pump pressurizing the system. It does that automatically to check the system for leaks. If it discovers a leak, it’ll eventually turn on your Check Engine light.

Tom: I’m guessing you have a sticky valve or some sort of small leak in the system. It should be covered under your emissions warranty.

Ray: So you can go back to your dealership and point them in the direction of the evaporative emissions pump, and ask them to check it out.

Tom: Or, alternatively, stop loitering in your garage, leave the car alone and don’t worry about it until the Check Engine light comes on. Good luck, Elaina.