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So I bought a new gas cap for my MTD 10HP Tecumseh blower. The old cap fell apart. My machine would stall because the cap didn't vent properly. The new cap appears to be identical. Same colour (red) and everything. Home Depot stocks them for $6.34 CDN. They appear to be an exact match to the old ones which Tecumseh does not sell any more. I was wondering how these devices work. Inside the cap there is a little metal disc which sits on top of a rubber cap on a small piece of sponge like material which all are held together on a plastic stem that runs through the center of the cap. I can't figure out how those pieces work together to vent the fuel. No big deal, just curious how things work.

https://www.homedepot.ca/en/home/p.gas-cap---fits-snow-king-engines.1000750074.html#
 

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What I remember of taking one apart some time ago is that the multi layers of foam and rubber and metal is to prevent gas escaping through the vent on underside of the cap when the tank is full and sloshing around. It allows air in but not gas out. Remove the parts and you get air in and lots of gas out.
 

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In days past a fuel cap had the small hole in the center and would vent continuously to atmosphere and allow air in.

EPA regs do not allow fuel vapors to be released into the air anymore and is covered under SHED evaporative emissions. Getting a vacuum in a fuel tank above the fuel is not desirable so a cap now lets air in through a one-way valve when needed such as when fuel level goes down while keeping fuel vapor pressure contained to a certain degree.
 

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It sounds like a mushroom (check) valve with a foam diffuser. Vents to admit air as needed and splash stays in.

The old caps were baffled to try to prevent splash and vented each way allowing evaporation.
 

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EPA regs do not allow fuel vapors to be released into the air anymore and is covered under SHED evaporative emissions.
Ever drive by a gas station when the truck is filling the tanks? No regulations there, the smell of gas fumes can be smelled a couple of blocks away.
 
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