VENTED GAS TANK VS AUTO VENTED QUESTION

[mike2587]

Hello!

I have an older pontoon boat with a 40hp outboard on it... I keep it parked in a marina at a slip I rent out for the summer and my gas tank is exposed and sits on the back pontoon due to the design of the boat and how it was when I bought it. Long story short, I had about 3/4 to a full tank of gas and the boat sat for about a week to two weeks without use. I have an older plastic 6 gallon tank which has a manual vent and a make shift pneumatic adapter fitting which is a quick disconnect from my motor gas line. When I came out a week and a half to two weeks later, my tank was just about empty, maybe a gallon was in it. Now I guess I won't rule out the possibility that someone took the gas, but that just seems odd to me and my slip is in a bad location where they would have to have gotten it from the back of my boat and you can't get on it unless you uncover the entire thing and go from the front all the way to the back and then recover it. I closed my vent a little more than normal before I left last time I filled it up since I knew it was going to sit for a bit before I could use it again and we've been getting a good amount of rain. What are the odds that pressure built up and gas leaked out? Can that happen? There were real sunny days during the week where it got pretty warm. I'd like to try and make myself feel better in assuming it leaked vs was siphoned out. But for someone to siphon out gas (50:1) mixture and to know that and not touch my tank or take it for that matter seems odd as well.

My main question though is... I'm looking to buy a new MAIN tank that I might put in the boat when not in use where then I can put on and leave my current one that's on the back just as a dummy tank to leave my fuel line connected at all times and to not have to re-run the fuel line into the boat for when I take the good/new tank off of it. I see that most new tanks are attwood ones with an "automatic" vent which is apparently in the cap? I do like the sound of not having a manual vent just on the odd event rain can get into my gas tank if the manual vent was open too far by mistake or somehow water got in. Are the manual vented tanks better? Do you think I'll have any problems with one of those auto vented tanks?

Any insight would be helpful.

Thanks,
Mike

 

[Chris1956]

If the gas leaked out of the tank, you should see an oil residue, as it does not evaporate like gasoline. Look under the motor as well for the oil residue. Maybe you mis-remembered how much fuel was in the tank?

As for the auto vented portable tanks, I can not comment as I have never used one.

 

[H20Rat]

If the gas leaked out of the tank, you should see an oil residue, as it does not evaporate like gasoline. Look under the motor as well for the oil residue. Maybe you mis-remembered how much fuel was in the tank?

As for the auto vented portable tanks, I can not comment as I have never used one.

To go along with that, the fuel that is left in the tank would be DARK, as in you can't see through it at all. It wouldn't burn in an engine either without fowling the plug in a couple minutes.

 

[greenbush future]

I would not believe that much gas could evaporate, just not gonna happen, even with the gas cap off, it wouldn't lose that much. Now leaking out, or unplanned siphoning could occur, and I'd be more inclined to that or possible theft. Just my opinion though.
The decoy idea is a good one, but heck if I was gonna steal, I would have taken the entire tank, not just "some" of the fuel. It just doesn't add up to me what happened here. Good luck trying to figure it out.

 

[mike2587]

I actually want to say I did see some residual that was floating around the boat and was on the back of the toon... I didn't realize it might have been coming from mine at the time I saw it since there was about 5-6 other boats at the same location and I've never heard of that happening. That's why I'm thinking it leaked, but I'm not sure how/if that's possible? The tank cover was on tight, but I closed the vent tighter than normal due to the worry rain was going to enter it. We had some hot days, it's an older plastic tank. Is there anyway possible that the heat could have caused the tank to swell up a bit and it leaked through part of the tank where the quick disconnect goes in? The quick disconnect part that plugs onto my gas line isn't solid plastic like the newer tanks are and is rubber with a gasket around it which allows me to rotate and swivel where I want the adapter piece to go - maybe there's a leak there? Perhaps the vent was too tight and somehow that happened? Or is there just NO way?

I agree on someone taking the tank, I don't know why if they did that they wouldn't have just taken it all or the tank like you said, it seems pointless to go through that hassle for 4-5 gallons of 50:1 mixed gas especially if I had no clue if it was really mixed or not. They probably would have had to siphon it into another tank and check it before using it. I guess it's possible, but that basically leads me to believe maybe it's the tank. I never really know how tight I need the manual vent valve closed or how open to have it when running it on the water. I could spin it about 4-5 rotations to pull it out vs being fully tight, so there's some unknown thoughts about how much to have it open/closed. I wanted it closed more over the last few weeks because I was worried water was going to get in with all the rain we've been having. But every other day we have a sunny hot day. That's the problem with an outside tank is my concern about water entering the system and causing problems for me. That's where this all started with looking into a new tank to maybe hot swap but I keep hearing the automatic vents aren't as good as manual..... But I'm always so leery about leaving the vent open so much.

Thanks for the insight thus far.

 

[MH Hawker]

Mine sits on a slip in full sunlight all year and it may lose a few OZ in a months, I suspect you have some ones help.

 

[mike2587]

What type of tanks do you use? Do you have a manually vented tank?

 

[gddavid]

I would guess that the gas is leaking out at your outboard and into the water, either at the fuel line connection to the motor or the float valve is leaking slightly. This is less likely to happen on a non-pontoon boat because the tanks are rarely above the carburator height but it could happen very easily on a pontoon. Just disconnect the fuel line from the motor when your are leaving and it won't happen again.