fuel tank vacum??

[leeg]

i have a 18 ft. alum. columbia with a fuel tank in the floor. pushed by a 115 EFI merc.my question is how does it not create a vacum in the fuel tank when running at high speed and sucking up gas when the only vents it has are 2 ( one way vents) which are run out on port side of boat i assume they mean by one way they mean vapor out only and no air or moisture in. or is there some sort of a vent in the eng. or else where to prevent a vacum. i looked and could not find any other vents. thanks Lee

 

[Another retired guy]

Re: fuel tank vacum??

Usually there is only one vent - and it is "open". By that I mean it does not have a one-way valve. It allows air in to replace the fuel as it is sucked out, and it allows vapor to escape, and pressure to equalize, when the motor is not running. Why do you believe your vent is one-way?

 

[leeg]

Re: fuel tank vacum??

i have two vents. side by side that are on the side of the boat. which are just under the fuel filler cap which is on top of the bow. and i could smell gas fumes coming out of them. And somewhere i was told that they were one way vents to the tank. to prevent expansion. so i assumed that both were one way vents. makes sense now that one maybe is two way. and vent caps are pointed down with 1/8 to 1/4 opening. could they possabnly take in water. the columbia river gets rough somtimes. P.S. thanks for responding

 

[Another retired guy]

Re: fuel tank vacum??

Interesting that there are two vents. Is the boat new? Maybe they are both one-way, with one venting out and the other venting in! I can see how that could cut down on moisture getting into the tank which is a problem with an open two way vent - especially when the boat is stored for a long period over the winter.
They should be pointing down - if they were up then rain water would get in. The best position would be for them to point slightly towards the stern so bow wave etc is less likely to go into them.

 

[leeg]

Re: fuel tank vacum??

checked for water in water/fuel seperator. was ok. the boat is a 2008. question came about from a high rpm miss in eng. wot. 5800/6400rpm. i can run allday long at 5500 rpm but go to 5600 and above and i get a miss. and thought poss. tank not venting quickly enough. thanks again for your response.

 

[Another retired guy]

Re: fuel tank vacum??

Suggest you start a new thread in the Merc outboard forum for " high rpm misfire". I'm sure you will get lots of help

 

[UncleWillie]

Re: fuel tank vacuum??

Re: fuel tank vacuum??

As we argued to death in another thread.
If your vent line loops up higher than the fill fitting, water will not get over the hump and fuel will not either.
Picture shallow inverted "U".

There is really no reason to vent a fuel tank OUT, it just evaporates your hard earned gas.

Interestingly enough. The Newest Portable OB style fuel tanks that must meet C.A.R.B. rules have one way vents in their caps.
You can no longer Open and Close them. They Vent IN only, to prevent a collapsed tank.
The Poly Tanks are designed to withstand at least 5 psi so you can put them out in the hot Sun and they will not vent.

This is really nothing new, your car fuel tanks have been pressurized for years.
I live in an area of the state that requires EPA Testing on all our cars every two years.
One of the tests is the blow off pressure of the fuel cap.
They typically do not vent until well into the 20 psi range!