My check engine light has been coming on and I've been re-tightening the gas cap; sometimes it works and sometimes it doesn't. I noticed that when I look at the hole, there is no flap to block the hole. Is there supposed to be a flap that blocks the hole until you insert the gas nozzle when filling gas?
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This is no answer, but I know what you mean. I live in Oregon where gas is not self serve. But my Honda Fit had a gas "flap" just inside the opening after removing the cap. It was spring loaded. I know this because of times I added a treatment of Techron to the tank. I had to be sure to push the "nozzle" of the bottle past the spring loaded flap. I've ran a treatment or two of techron through the Prius but for the life of me, can't remember if a similar flap exists. I'd just make a quick stop at the dealership, because you either need a new flap...or a new gas cap. So either way the trip wouldn't be waste. Edit: Just saw above posted picture. So I guess you need a new gas cap.
Mine has no flap. It surprised me when I first filled up because all the other fuel injected cars I've ever owned have had a flap. Only owned one carb'd car, have had a few diesels and none of them had a flap; assumed it was a vapour emissions thing. I much prefer it without one, means you can really properly brim it.