Oh, but you have to give Troll credit for his efforts. In my 77 years of life, I have never put in as much time reading about gas caps until I read this thread.
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It has no separate gasket. Thatās part of what made me suspicious. This cap has a āsafety ventā, which the older cap lacks: maybe thatās whatās leaking.
I donāt know why it failed.
Iām not tearing my hair out. I just asked a question. I didnāt expect a kind of Spanish Inquisition!!!
Nobody expects the Spanish Inquisition!!!
ā¦and I donāt like Spam!!
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So the replacement cap had a vent that the OEM cap did not?
You asked if the gasket matters. It does.
The fact your replacement cap could be leaking from the safety vent has no bearing on whether or not the seal on the gas cap is necessary. If the seal wasnāt needed, it wouldnāt be there. On this particular cap, the leak can be from at least two different sealing mechanisms. But for $10 who really cares which one it is? It doesnāt change the situation does it? You need a new cap. Iād suggest an OEM equivalent this time aroundā¦
Iām advocating for OP trying to fix it myself ā¦ whereās the fun in just buying a new one?
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How much does an emission test cost there George? Around here, we get one free re-test. After that, you pay again. Sorry, Iām as frugal as the next but Iām not risking my free re-test on a guess. Then your $10 cap becomes $55+. Not to mention, the rest of the non-OEM compliant cap is probably 20 years old. Replace the vent seal and then the neck o ring fails. Having fun now? Over $10ā¦
If the gas-cap is truly failing it should/would generate an error codeā¦something like a P0442.
Weāre talking about an 87 Toyota, long before ODBII codes
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It costs about $60. Usually you get a free test, but you have to get that in writing before the first test I discovered. One time I had been using a shop for years that even had a sign on the wall saying second test is free, but when I pointed that sign out the tech after he billed me for a rare second test he just ripped the sign off the wall and threw it in the garbage can ā¦ lol ā¦ no argument common sense says to buy a new cap. But I think its fixable and the fix (if all appeared well on disassembly and re-assembly would be a pretty low risk to fail a cap test). Another possibility is to buy a new cap just for testing, and use the repaired old cap otherwise. Then youād always be using a new cap for the test.
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I would just cap things off by purchasing a new cap.
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You canāt be serious. When was the last time you had a gas cap fail? They last dozens of years. This isnāt a precious treasure that needs protecting. The term frugal to a fault seems appropriate hereā¦
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Iāve had to replace the cap on my wifes 07 Lexus once. Replaced cap on my 05 4runner. But both generated codes.
Keep in mind that a bad purge valve can generate the code as well.
But as @It_s_Me pointed out, those codes arenāt gonna be generated on an '87. 
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Did you ever have to replace them again in the time you owned those vehicles?
Thatās the point. Buying a new cap to use only at the test and then running around on the old one that doesnāt work properly anymore- does that make any sense to you?
Once. For the 4runner I went to ADAP (They were open on Sunday). 1 week later getting code again. This time I went to dealer. Never had the error again.
One option on the 1949 Nash was a gas cap that flipped open when one pushed a button on the dashboard. This was an option. It operated electrically. It wasnāt a door with a gas cap behind it. It was an exterior gas cap. That, to me was the ultimate in gas caps.
Did it seal? In 1949, gas tanks werenāt sealed, so the caps didnāt need to be.
Probably not. I just remember the our neighbors 1949 Nash Ambassador with this electric gas cap.
What do you think of the modern capless designs? I have yet to use one but it seems like a good improvement. Honda even includes a special funnel for filling from a can if thatās necessary at some point.