New gas cap, tank won’t vent

[Mthirsch14]

6 gallon tank, seems fine when engine is running but the next day the tank will be inflated like a balloon and rounded on top. Has been going on for as long as I had the boat but never seemed to effect the engine performance, I would usually just manually push the button on the cap to deflate it before starting the engine. Finally decided to get a new gas cap but still have the same issue of inflating. The knob is turned to the right position so it should let the pressure out at some point but it never does. Any ideas why?

 

[boobie]

Doesn't turning the knob to the right close the vent ??

 

[Mthirsch14]

I meant right as in correct. It is turned left to the open position

 

[Newyota]

I thought they only vented at a certain pressure??

 

[Vic.S]

6 gallon tank, seems fine when engine is running but the next day the tank will be inflated like a balloon and rounded on top. ......................
. Any ideas why?

The vent on my 22 litre plastic tank cap is either open ( turned counter clockwise ) or closed ( turned clockwise)
Its purpose is to allow air to enter the tank so that the engine can draw fuel when it is running.

When I run the engine I open the vent. When I am not using the engine I close it. When it gets warm the tank swells , when it gets cold it sucks its sides in.
When the tank is on deck I usually crack the vent screw open very slightly so to avoid the changes in pressure that would otherwise occur.but I close it when I stow the tank away in cockpit locker

 

[F_R]

6 gallon tank, seems fine when engine is running but the next day the tank will be inflated like a balloon and rounded on top. Has been going on for as long as I had the boat but never seemed to effect the engine performance, I would usually just manually push the button on the cap to deflate it before starting the engine. Finally decided to get a new gas cap but still have the same issue of inflating. The knob is turned to the right position so it should let the pressure out at some point but it never does. Any ideas why?

I am totally unfamiliar with your tank but I can tell you that for a number of years now, the Law says tanks cannot emit fuel vapors (neither can cars). This means they are vented one-way. They can admit air to replace the gas that was drawn out, but they cannot let air out. The tanks that I am familiar with also have a pressure relief valve that lets air out if the pressure gets so high as to be a danger. I'm assuming your tank is made this way.

Today's cars can even tell if you leave the gas cap loose, and light up a warning light.

 

[oldboat1]

but no warning light on your tank -- crack the cap slightly as long as you are storing in an open area (like in a boat, in the driveway, in the sun....)

 

[flyingscott]

Absolutely normal for the tank to do that. As stated it cannot vent to the atmosphere. What you need to be careful of is a bad needle seat in the carb or bad connector seal. Because the pressure in the tank can sometimes push past them and then you have a fuel leak. I always disconnect the connector from the motor on those tanks when not in use.