Fuel tank venting problem

wmk0002 · Jun 20, 2024

I probably already know the answer to this, but anyone else had problems with this new style EPA compliant cap? Its what came on the Moeller 3 gal tank that was included with my Yamaha F25. Motor randomly starves for fuel at high rpms, which is why I'm not really concerned it's anything motor related. On this style cap you have to press down and vent the tank every once in a while even in the run position which is obviously extremely annoying.

I just bought ones of these manual vent caps to replace it with. I'm pretty confident that will solve my problem. It's just weird to me it seemed to work ok for a few years and is just now doing this. Anyone had any luck modifying the cap it came with it so that it vents properly? For the 3 gal Tohatsu tank I have, the dealer drilled a hole in it somewhere which allowed it to breathe and work like it should, however, it will seep fuel out now from sloshing. Would be nice if the modification would allow the red piece would still close it off in the closed position and then when it the open/run position vent normally and not act like a check valve.

MDCrappie · Jun 20, 2024

Without modification the EPA caps are designed to always allow air in to the tank, just not back out. This in itself would NOT cause your motor to starve for fuel. Sounds like you may have air leaking at one of your fittings. When you squeeze the primer bulb does it get hard all the time? Not doing so is a sign of air leaking in the fuel supply.

wmk0002 · Jun 20, 2024

MDCrappie said:
Without modification the EPA caps are designed to always allow air in to the tank, just not back out. This in itself would NOT cause your motor to starve for fuel. Sounds like you may have air leaking at one of your fittings. When you squeeze the primer bulb does it get hard all the time? Not doing so is a sign of air leaking in the fuel supply.

I feel like it's not allowing air into the tank but I haven't tested it. I've tried squeezing the primer bulb when it acts up and it does get hard once the motor side fuel filter fills back up. So you dont think when I press the pressure release button it is actually drawing in air vs expelling gas vapor pressure. I know that's not how it is supposed to work but I don't have too much faith in these caps either. Maybe I could rig up a way to connect my MityVac vacuum pump to the tank (empty) and put a vacuum on it to see if if it holds one or if it draws in air like it should.

MDCrappie · Jun 20, 2024

wmk0002 said:
I feel like it's not allowing air into the tank but I haven't tested it. I've tried squeezing the primer bulb when it acts up and it does get hard once the motor side fuel filter fills back up. So you dont think when I press the pressure release button it is actually drawing in air vs expelling gas vapor pressure. I know that's not how it is supposed to work but I don't have too much faith in these caps either. Maybe I could rig up a way to connect my MityVac vacuum pump to the tank (empty) and put a vacuum on it to see if if it holds one or if it draws in air like it should.

Classic result with an air leak. Do you have push on fitting? Or is the hose clamped to tank and motor? My guess is you have fittings and the orings are leaking. Once you have the fuel filter filled and and the bulb hard, wiggle the fitting and see if gas or air tries to escape when you do.
The easy way to tell if it's the cap is to run it with the cap not screwed down all the way.

Pappy · Jun 21, 2024

Not sure you are on the right track with this yet but we dont have enough information at this point.
Main reason is that these plastic tanks are well known for building pressure in them as has been stated already. They are typically designed to hold up to 5psi. With any pressure in the tank there is no need for a vent system until that pressure goes away and a vacuum takes over. Once a vacuum is present in the tank the tank should vent air in it automatically. Is there a set of instructions that came with this tank about having to manually vent it? Something tells me that would be a huge legal liability to take on as a manufacturer.
Now....on the air leak theory. The air leak would have to be between the liquid level in the tank and the fuel pump on the engine or the vacuum side.
If there was an air leak on the pressurized side of the pump it would also show up as a visible fuel leak.
Two things you can do. Crack the tank cap while running and see if the issue goes away. Secondly pump the primer bulb when the issue occurs and see if the issue goes away. This will tell us more about where to go from here.

wmk0002 · Jun 21, 2024

Appreciate the input fellas. I should have cracked the cap first before posting on here. Its somewhat intermittent so it may take a while before it happens again. For example last time out ran fine no issues. Time before that ran fine for a couple miles and then starved out. I pushed the vent and pumped the bulb a couple times and it was good to go again. I'll get out this weekend and run around for a while and try to get it to do it again and then crack the cap and be more deliberate in noting my steps taken and the results of each one.