My gas cap failed the leak test: is there any hope for it?


My gas cap (CST 5501 safety-vent locking fuel cap, bought 2006 April 30 after somebody siphoned some gas back when it was $4/gallon) failed the leak test. Fortunately the spare, the older one, passed. The leaky cap has a rubber gasket outside the threads. Does its seal matter? A lot of neighbors have noted that their gas caps failed at the same place recently. If he were scamming, as some suspect, he would have failed my old cap.

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Does its seal matter ? I would vote in the “Yes” column on this one.


@RandomTroll. The gas cap you bought in 2006 is 13 years old and hence a teenage gas cap. Anything that is teen age will cause problems


Gas tanks have been pressurized since at least 1989. My parent’s ‘89 Nissan Maxima was the first car that I saw make a hissss sound when I opened the fuel cap.

On my ‘98 Civic, I noticed the gas cap wasn’t hissing when I removed it, so I knew it was time to replace it.


The working gas cap is 32 years old. It worked all through its teens.

I have an '87 Toyota Pickup. Are they really pressurized, or just sealed? In the really old days (before the '60s?) they vented straight to the atmosphere. Gas was so cheap we didn’t care about the cost. Caring about clean air stopped that. Then they started having vapor recovery systems: carbon canisters that condensed vapor to return to the tank. Pressurizing them would make that work harder. I don’t get a hiss with either cap.


If your fuel tank isn’t pressurized, what kind of leak test did it fail? What was leaking from the cap?


The EVAP system performs a leak test on startup usually only when tank is between 1/4 and 3/4 full. It closes the valves and pulls vacuum measuring if any leakage. Sets OBD code for either gross or fine leak depending on leak rate. Modern limits are very restrictive something like 0.01" pinhole will fail. Could be even tighter now.

Some emissions testing will check the cap separately for older vehicles without sophisticated EVAP systems or simply as part of testing. The cap screws onto a pressure/vacuum tester and leakage measured.

A pressurized gas tank is usually a sign the EVAP is not working right…


By this train of thought, the advise would be not to toss it, but save and wait until it matures into mid-20s.

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I find it hard to believe that the guy testing the cars is making as living by selling everyone a new gas cap. He’d have to sell quite a few every hour to keep in business.

As @Triedaq says it’s 13 years old. Things wear out.


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The air the testing device pumped into it. An emissions test includes removing the cap, attaching it to a hose, pumping up air pressure, seeing if any escapes.

What happens to me.

He doesn’t sell gas caps. Others on NextDoor called it a scam. I didn’t.

The 32-year-old cap works. My question was whether the gasket on the outer part has to seal. I suspect the threads that screw into the nozzle provide all the sealing.


RT, if the old cap seals, you’re good to go, right? If you want to keep the new(er) cap to keep the locking feature, you might smear a little petroleum jelly or grease on the rubber gasket. Maybe it’s dried out a bit and the rubber will sort of rehydrate/swell. Or you could search for a new flat rubber gasket or o-ring (whichever it has). Or you could buy a new locking gas cap. But yes, I believe the gasket is supposed to seal.

I thought most of those Toyota trucks had a keyed lock on the gas filler flap anyway. My 1999 Tacoma did, if I remember correctly. Does yours?


Now that gas is cheap again, probably.

I have no problem buying a new gasket. I would have thought that the sealing happened in the threads, that the outer gasket didn’t matter.

Testing it is the drag. I have to go to AutoZone, borrow the tester. I asked here in case someone knew.



My Corolla’s gas cap failed one time. After that I’ve always applied a little motor oil to the cap’s gasket-seal before going to the testing place and it has always passed since. Still using the original 27 year old gas cap. If you want to junk your gas cap, they are fun to take apart to see how they work. The inner workings are more complicated than you might imagine.


A new cap for a 30 year old Toyota Tacoma is well less than $5.00 at Rock Auto.

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Non-locking. I like to fix things.


can you take the seal off your non locking one and put it on your locking one?

and yes- the seal matters. without it you will fail your test every year.


I assume you drive a Yugo? Because the only reasonable reason for you to be this reticent to replace the gas cap is if it’s worth more than the car…


Perhaps it can start a new career as a paperweight.

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Yes, the gasket needs to seal and no, the treads won’t seal well enough. The proof is in the pudding, the cap failed their test…


I really don’t understand why Troll is tearing his hair out about this

The effort he’s putting into all this is worth far more than a lousy fuel filler cap

There’s saving money . . . and there’s being stubborn