Gas caps loosen when tank is pressurized?

2005 PONTIAC VIBE
276,650 MILES • 1.8L • 4 CYL • 2WD • AUTOMATIC
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05GOODVIBESONLY
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I've tried 3 different gas caps on the filler neck. Two normal ones and a locking one. The caps all click in place like they should but once the car is started and the gas tank is under pressure, the caps pop back open.
The newest thing I've done to try troubleshooting is I actually replaced the entire filler neck from a 2005 Toyota Matrix at the salvage yard. Great condition, no holes in the soft hose and no corrosion or rust on the rest. I changed it because I figured the issue was my old one just being worn down over the years but it's having the same exact problem. The caps click but loosen under the tanks pressure once the car is started.
I'm so confused on what to do next it's really unmotivating lol.

Any ideas or advice are appreciated!
Aug 16, 2022 at 12:45 PM
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STEVE W.
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The tank should not be under any pressure. If it is, you have a canister or vent valve problem that is preventing the tank from venting properly. Do you have any issues when you fill the tank with fuel? Anything other than the cap issue?
Aug 16, 2022 at 11:01 PM
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05GOODVIBESONLY
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No, no issues filling up. Nothing else seems amiss. Burning through gas faster for sure. Just worried the injector will clog up if dirt and debris make their way into the neck.
Aug 17, 2022 at 1:31 PM
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STEVE W.
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Okay, reduced mileage and pressure in the tank are related. I would check both the EVAP vent valve and the canister. Do you ever top up the tank? IE add gas after the pump clicks off.
If yes suspect the canister, if not then the vent line has a problem. The canister on these is known for failing and dumping the charcoal into the lines causing them to get blocked.
I would start at the vent valve, it's under the car and has two lines on it that connect between the charcoal canister and the filter. If you remove the line and find small black pellets the canister has failed. Replacement isn't hard but it's not a cheap part. You go to the canister under the rear driver's side, remove the hoses and electrical connector. Then use compressed air to blow out the lines and clear the charcoal. Also clear the line from the air filter. Then unbolt the canister and bolt in the replacement. Reconnect the lines and electrics and drive it.
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Aug 17, 2022 at 6:01 PM
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05GOODVIBESONLY
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Which lines go to the vent valve? I can't tell when I actually get under the car and look. Everything is so close together.
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Aug 29, 2022 at 10:53 AM
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STEVE W.
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This is the vent line.
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Aug 29, 2022 at 11:00 PM