I have a 81 Ford F100, 5.0, AOD SWB,
My problem is I have pressure building up inside my tank even when the truck is not on and it's been sitting, I bought a new vented gas cap thinking that would help but no success.
Is it possible there is a vent tube on the tank that could be bad or clogged, it's building up alot of pressure causing my fuel pump to leak if I don't loosen the gas cap

 

You did not say if 1 tank or 2 so not know what picture to show so will do both.
Side tank

Rear tank

On the side tank just above the filler hose you can see a white plastic thing with a small hose on it, it is a roll over vent.
On the rear tank it is in the center with a small hose on it.


With duel tanks like mine the 2 hoses go to metal line, join together by the right rear cab and 1 metal line goes up to the engine bay.
IIRC this line hooks into the charcoal canister / vacuum system to pull vapors from the tanks. This I think is up near the batt on the right side of the engine bay.


Now the roll over valve(s) could be plugged or nipple broken off and something made a nest and plugged it.
Hose(s) bad and is blocking it.
Metal line is rusted inside and blocking it. 1 of my metal lines rust in 2 pieces behind the cab.


I would start to see if the valve(s) work and them follow the lines to the front to make sure they are open.
I do not know if you can get the roll over valves or gaskets. Hose is just small fuel hose. Same for the metal lines just normal metal line.


BTW vented fuel caps have a PSI rating think 5 hg/psi so they will build up HG & PSI.
Dave ----

 

Thanks for the info, I have a side tank will check lines today

 

I'm not sure the tank vent system is the entire problem here. I think your leaky pump itself is the primary problem. I don't know the exact specs for the vent system, and my cat refuses to let me up from my recliner by the woodstove and head out to that frosty garage to dig through the manuals. Heck, I'm having a tough enough time typing on my tablet, as she keeps reminding me her ears won't scratch themselves...

The point, and I do have one, is that your tank is not vented to ambient air pressure. On a warm day, a slight pressure will build up as any air in the tank expands. On a cold day, or as fuel is consumed, a slight vacuum will develop. It's perfectly normal to have pressure or vacuum in the tank, anywhere within a specified range.

Totally guessing at a value, let's say the vent system would allow 2 psi of positive pressure to build up in the tank before venting. That's going to pressurize the line feeding the pump, which is perfectly normal. The pump's inlet check valve opens, and lightly pressurized fuel enters the diaphragm chamber. After that, the pump's outlet check valve opens, and that same lightly pressurized fuel reaches the float valve at the carb inlet. If the fuel level in the float chamber is low, it will refill until the float rises and closes the float valve.

So if the vent system failed closed and tank were to become pressurized beyond the normal range, the only place fuel would go is into the carb float chamber. If the pressure was high enough, eventually the design limit for the float valve would be exceeded, and fuel would overflow the float chamber and flood into the intake manifold. Normal pressure at the carb inlet (in other words, normal fuel pump output) is around 6-8 psi, so the pressure would have to go a lot higher before that happened.

Fuel should not be leaking anywhere from the pump. Even if the tank vent failed shut on a hot day, the pump should be able to safely handle the pressure. The leaks sounds like a bad pump to me. Where exactly is it leaking from the pump? Any time a mechanical pump fails, check the crankcase for fuel in the oil. The pump has a secondary seal to prevent any fuel leaks from reaching the crankcase, but this seal can fail, too. If the pump hadn't been leaking any fuel, this secondary seal could have failed a long time ago and you'd never know.

For comparison, I used to run an electric boost pump back by the tanks on my '84 F250. All it did was pressurize the line feeding the stock mechanical pump on the engine. (The electric pump was to help reduce vapor lock.) The electric pump acted exactly the same as if the tank had become pressurized, and never caused any leaks like you are experiencing.

 

The tank vents into the charcoal canister as was said in a previous post. People like to get rid of vacuum lines and stuff under the hood and put caps on lines. See if you can find that line up front that comes from the tanks, it might be capped off. It needs to be open, either to the atmosphere or the charcoal canister.