gas tank vent

[bgd73]

I was seeking a pro answer on this....

a high pressure and low pressure system..both using the same tank. For manufacturing cheapness, the low pressure system has a cheap tank.

Respecting vents, charcoal canister.. I noticed a problem unfolding, related to venting.. in the back, an odor.
as it turns out I have a flawed tank, no rupture, simply made badly (if you have a carbed subaru, you may have one too...)

The gas cap is vacuum only no vent..

Today, missing an appointment to get the tank done (remove replace), I saw the 90 degree mark coming and finally just stuck a hose between the seam of the car and the tank and let it have it with some metal silicone I had in the back.. (I have possibly more than a week to go now)

the hole was so small, that light gasses escaped, rather than gasoline.This soemhow dropped my mileage by ten or more...a venting pinhole...

I am curious as to when putting new tank in, if having a vent up tup, the very top, real small, like the errored hole with the current setup would help it, or hurt it...be it from my own lesson, there can be no venting at all, to gather the light gas for mileage, else, it escapes.

[mwebb]

so
you do not want to have hi pressure in a fuel tank .

newer systems vent to atmosphere through a charcoal canister which absorbs the HC as the pressure vents

when the engine runs
a valve opens that draws air through the canister and the absorbed HC gets burned in the engine
in closed loop so every thing stays at stoich the valve is duty cycled to control
PURGE FLOW ,

the system can close the EVAP VENT VALVE and open the PURGE VALVE to put a little vacuum on the tank , then with the FUEL TANK PRESSURE SENSOR monitor the rate of vacuum decay to see if the tank leaks
vacuum will decay faster in hotter temperatures as the fuel evaporates and the vapor pressure helps to decay the vacuum .

that is one common way to detect fuel tank leaks
there are other ways.
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what was the question ?