Gas Tank pressure too high

[countrypyhysician]

74 Bronco. 302 with Holley Sniper EFI.

I had issues with a gas smell. When the Bronco has been running a while and the tank is filled, when stopped, it will start to push fumes and even gas up through the vent line. Let me illustrate my fuel and vent line routing. I have a Tom's Offroad 23 gallon tank which comes with an in the tank fuel pump for my EFI. This tank has openings for: a fuel filler hose, fuel vent hose (that attaches to the fuel filler up near the gas cap), and 2 EVAP lines. The in the tank fuel pump has a fuel line, return line, and vent opening. See the pictures below for more details. Originally I had no EVAP canister. I hooked the fuel line to the Sniper, the return line to the Sniper, and 5/16 line to the vent opening on the fuel pump and routed it above the gas cap with no cap on it. I capped off the EVAP lines on the tank (they seemed redundant given the vent line on the fuel pump). 62 PSI reading on the pressure gauge. When at operating temp and the tank if filled, when stopped a strong gas smell and pressure was high enough to push gas out of the vent line I routed above the gas filler opening. I added a Tanks inc rollover valve to the vent hose, but the pressure would get high enough to push the metal ball inside up and prevent any venting leading to very high pressure in the tank.

I decided to change gears and try one of the Delorean rollover valves in hopes that it would have a higher tolerance for pressure. At the same time, I bought an EVAP canister for a Mustang. I routed the 5/16th line from the fuel pump vent line to the firewall where the Delorean rollover valve is mounted as high as possible, then from there to the EVAP canister (also on the firewall as high as possible under the front hood), and from there to a fitting on the air filter. When I filled the gas tank and let the bronco get to 190F, gas pressure is building up because the rollover valve is closing off. The vent hose at the fuel pump leaked gas (I did not have it clamped just a snug fit). The vent hose at the rollover valve leaked gas (I did not have it clamped just a snug fit). I have since clamped the rollover valve hose.

Also, I have routed all the fuel lines on the opposite side of the frame as the exhaust. I also wrapped the fuel lines where they come over the top of the engine/manifolds with heat wrap.

I am at a loss of where to go from here. I am planning on dropping the tank, capping the vent opening out of the fuel pump and using one of the EVAP lines on the tank for my vent and running it to my current Delorean rollover valve and then EVAP canister. My thought is it being further from the fuel return line could decrease temps. It also may be slightly higher (though only slightly).

Any suggestions or thoughts on how to improve these high pressures?

 

[Broncobowsher]

What you are calling a vent hose, isn't really for venting. It is for filling the gas tank. As gas goes in, air comes out. When you put the gas cap in place, it is just a parallel interconnected path with the filler tube.

The return style fuel system uses the mass of the fuel in the tank as a heat sink to keep the fuel lines cool enough to not have issues. Starting around the early '00s OEM went away from the return style to a single line fuel system. I remember doing testing on this when it was coming out. The main reason was evaporative emissions. The heat being dumped into the fuel tank was causing higher levels of evaporative emissions. With careful fuel line routing and better computer controls that don't require mechanical manifold pressure compensation of fuel pressure delta across the injector, no return fuel system. Even the OEMs had issues with hot fuel in the tank.

What do you have to allow for the expansion of fuel in the tank when it warms up? Typically you will find an air pocket designed into the top of the tank. Inside the tank if there is a turn down on the fill and vent, it will prevent the tank from filling completely, giving that air space. And room for heat expansion. If you look at the original Bronco aux tank over the years, the advertised capacity dropped even through the physical size stayed the same. They were making air space as the evaporative emissions came onto scene. When the gas caps went from vented to non-vented. (Which do you have?) Non-vented pushed the evaporative emissions to the charcoal canister to be sucked into the engine and burned. Instead of just leaking out the vented gas cap.

 

[countrypyhysician]

What you are calling a vent hose, isn't really for venting. It is for filling the gas tank. As gas goes in, air comes out. When you put the gas cap in place, it is just a parallel interconnected path with the filler tube.

The return style fuel system uses the mass of the fuel in the tank as a heat sink to keep the fuel lines cool enough to not have issues. Starting around the early '00s OEM went away from the return style to a single line fuel system. I remember doing testing on this when it was coming out. The main reason was evaporative emissions. The heat being dumped into the fuel tank was causing higher levels of evaporative emissions. With careful fuel line routing and better computer controls that don't require mechanical manifold pressure compensation of fuel pressure delta across the injector, no return fuel system. Even the OEMs had issues with hot fuel in the tank.

What do you have to allow for the expansion of fuel in the tank when it warms up? Typically you will find an air pocket designed into the top of the tank. Inside the tank if there is a turn down on the fill and vent, it will prevent the tank from filling completely, giving that air space. And room for heat expansion. If you look at the original Bronco aux tank over the years, the advertised capacity dropped even through the physical size stayed the same. They were making air space as the evaporative emissions came onto scene. When the gas caps went from vented to non-vented. (Which do you have?) Non-vented pushed the evaporative emissions to the charcoal canister to be sucked into the engine and burned. Instead of just leaking out the vented gas cap.

I have a non vented gas cap.

In the middle picture above, I have the fuel vent hose opening routed to the filler tube.

So do you think I should cap off the current opening I am using on the top of the fuel pump (middle opening on picture 3) and use one of the evap lines at the rear of the tank in picture 2 for my venting/evap?

 

[73azbronco]

try a vented evap canister, ensure fumes go out. Fuel vent hooks to filler neck evap lines, one of those goes to your evap system.

 

[Oldtimer]

This may give you some help?

Gas Tank vent

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