Atwood portable fuel tank with pressure relief valve in cap, not manual vent

[eaglecreeksailor22]

Hello all,
Been a while since I posted. Crazy spring in central Indiana, had 80* and frost in the same ten day span. Today May 25, 53* and furnace is cycling on heat!

Main question, does any one have experience with an Atwood tank with the pressure relief built into the cap. No manual screw vent. I have one and upon returning to the boat one sunny day with the tank in the fuel locker of my Precision 21 the tank was expanded quite a bit. I read Atwood's web site and it said this is normal. I am not real comfortable with this, don't know how much pressure this might be putting on the fuel system. Is my concern justified?

Thanks in advance for your thoughts.

Have a safe Holiday all.

Rich

[Riggerdood]

Good question Rich. Back in 2020, when I was trying to do everything and anything I could to resuscitate my old Merc 9.9 (never did, sold it for scrap, and bought a reliable Nissan 6), I bought a new tank from Worst Marine. While it supposedly has a cap vent, it swelled up so much in our 100F+ Summer heat here that I thought it was going to explode. It became permanently deformed, but continued to function normally, and I'm still using it. I'd still rather have the manual screw vent though.

[Charles Brennan]

Rich, I have been using Atwood outboard fuel tanks for about the past 35 years.  I've had 6 gallon tanks and currently, multiple 3 gallon tanks.  They all expand when it gets hot, for the simple reason that gasoline also expands; in fact, 1% for every 15º of increased heat.  But it's the heated air that causes the most dramatic changes in the tank volume.

I have never had a tank failure in all those years.
One of the more annoying problems is that on my boat, an expanded fuel tank cannot be removed easily from a storage locker.  To this end, I got some nylon tie-down straps and encircled the middle of the tank and tightened the strap.
That certainly worked to solve my problem, but what I've found works even better, is to push down firmly on the tank just before locking the fuel cap down.  You are essentially creating a partial vacuum, so that when the tank expands with temperature, it only expands half as much, since you had already reduced part of the tank volume, when you pushed the tank in, to attach the fuel cap.  I first got the idea when I noticed a "sucked-in" looking fuel tank, after a sudden cold snap.

Try it, it works well, for me.

Hope this helps,
Charles Brennan

[Roland of Macatawa]

Hi, Rich:

Your concern about over-pressurizing the fuel delivery line is valid. It can flood the carburetor.
The newer (>2011?) EPA-compliant Attwood fuel tanks recommend a pressure reducing valve between the tank and the primer bulb.
Attwood calls it a Fuel Demand Valve (FDV). Do you have one in your system?

That valve does not prevent the tank from swelling.
It limits the excessive flow from the over-pressurized swollen tank to the outboard.

BTW, my 2012 SunDayCat is just old enough to have come with the prior style gas tank.
So, I don't have any direct experience with the newer tanks.
Yet even with my older tank which has no FDV, I have experienced flooding due to excessive pressure in the tank.

Best Regards, Roland

[pgandw]

Our EPA in its infinite wisdom did away with manual venting on portable gas tanks.  The result has been far more spillage trying to pour from unvented gas cans, bulging outboard tanks, etc.  The worst that happened to me was gradual fuel starvation because the West Marine semi-permeable design didn't allow enough air in while the motor was running and I didn't think to crack the gas cap.

I finally caved, sold my 5hp Honda, and bought an Epropulsion Spirit Plus.  The Spirit turned out to be a far better auxiliary than I could have imagined when I made the purchase.

Fred W
Stuart Mariner 19 #4133 Sweet P

[Riggerdood]

I've always been irrationally leery of cracking the gas cap to vent, for fear that might allow moisture to get in, but I suppose it wouldn't be any more of a source of moisture than a manual vent? My Honda 2 has a vent on top of its cap that doesn't seem to be a problem, especially since it's only open when actually running.

[eaglecreeksailor22]

Hi, Rich:

Your concern about over-pressurizing the fuel delivery line is valid. It can flood the carburetor.
The newer (>2011?) EPA-compliant Attwood fuel tanks recommend a pressure reducing valve between the tank and the primer bulb.
Attwood calls it a Fuel Demand Valve (FDV). Do you have one in your system?

That valve does not prevent the tank from swelling.
It limits the excessive flow from the over-pressurized swollen tank to the outboard.

BTW, my 2012 SunDayCat is just old enough to have come with the prior style gas tank.
So, I don't have any direct experience with the newer tanks.
Yet even with my older tank which has no FDV, I have experienced flooding due to excessive pressure in the tank.

Best Regards, Roland

Roland, I have seen pictures of the FDV you mention. I believe my tank and hose is newer than 2011, but it came to me shortened so no valve. I may disconnect the fuel line at the tank at the end of the day to eliminate the threat of over pressurizing.

For all who has followed my post regarding removing and cleaning the carb on the 5 hp Merc sail power all is good. It has proven to be reliable and easy to start. This is why I am now putting on a portable tank for extended run time instead of just using the built in tank. I plan to do some trolling for wipers this summer on calm days to get in more boat time. Any excuse to go to the lake!

[Roland of Macatawa]

Hi, Rich:

Because your new style fuel tank has no pressure relief, I encourage you to consider getting a FDV for your fuel line.
It should be installed between the tank and the primer bulb. Note the proper orientation. Its ports are not interchangeable.

It apparently acts as a pressure reduction valve/regulator to limit the pressure and flow to the outboard.
It does this while the outboard is running and compensates for the fuel tank being over-pressurized due to heat-expansion.

Regards, Roland