Today: The Case of the Missing Fuel Cap Flap

Dear Car Talk

Dear Car Talk | Dec 20, 2011

Dear Tom and Ray:

I drive a 2005 Honda Accord. Recently, I started getting a "check fuel cap" message on my dash. When I checked it, it was properly tightened, but the message kept appearing. I went so far as to remove the cap completely to check and make sure everything looked normal, at which time I noticed that the metal flap that normally covers the opening of the tank was missing. How this happened is beyond me. How serious a problem is this? Am I going to explode driving down the highway, get 2 mpg, contribute even more to global warming? Is this going to cost my life savings to fix?

-- Kate

TOM: The answers to your questions are no, no, maybe and no.

RAY: I've never seen the little flap fall off, but I suppose it's possible. It's just a flimsy, hinged piece of sheet metal to prevent backsplash when you remove the gas cap to add fuel. And I had no idea the flap had any connection to the emissions system or the "check engine" light.

TOM: I don't think it DOES have any connection. Nor would the flap do any harm if it happened to fall into the gas tank. It would just sit at the bottom of the tank forever. But we DO know that if your gas cap is faulty or not tightened completely, that WILL make the "check engine" light come on (or the "check gas cap" message in your car, Kate).

RAY: That's because a poorly fitting or loose gas cap allows fuel vapors to escape into the atmosphere, which creates smog, which turns Los Angeles into Beijing.

TOM: And despite the mystery of the missing flap, you may just have a bad fuel cap. So here's what I'd do. Go to the Honda dealer and buy a brand-new gas cap for $25. Install it and see if the light goes off after a few days. If it does, you just needed a gas cap, and the whole flap "flap" was just a distraction.

RAY: If the new cap doesn't solve the problem, then you'll need to have a mechanic look at it for you. Perhaps the top of the fuel filler neck got damaged somehow, and that's preventing the cap from making a proper seal. Or maybe there's something else preventing the fuel system from maintaining proper pressure.

TOM: But rather than start by paying $150 for the dealer to scan your car and do a bunch of tests, start with a gas cap. It's cheap. It's easy to install yourself. And it may be all you need. Good luck, Kate.

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