F550 Fuel fill issue

[sbbtmatt]

I'v got a F550 when we were building it we put a 70 gallon tank on top of the bed. To feed it to the factory aft axle tank I ran a hose from the top tank to the factory one. On top of the factory tank was a fitting that appeared to be for exactly my need (filling the tank) it didn't have a tube so I knew it wasn't to supply a generator or anything. So that is how I plumbed the tank. It worked, well sorta, on a long trip the truck burns it faster than it feeds back in. In the hotter weather it would often not feed at all. In the heat when you would open the factory cap air would hiss out. I recently installed a pump with the hopes it would help.

Amazon.com: Mr. Gasket 12D Micro Electric Diesel Fuel Pump: Automotive

Amazon.com: Mr. Gasket 12D Micro Electric Diesel Fuel Pump: Automotive

. I have not had it long enough to see how it works but it is SLOW! and with it even in this cold air hisses out when the factory cap is removed.
The fitting I tied in to is the one on the right that kinda looks like a crop circle.


So my questions...
1 Is the fitting I used on the factory tank made for what I used it for?
2 Is this pump okay for my purpose?
3 What is the air hissing out of the tank? I believe the cap is supposed to vent, is it not venting?
4 Any suggestions?

Thanks guys!

[sargar]

In your picture the small round part to the right of the tank sending unit is what I believe to be either a vent or a roll over valve. I think there should have been a black rubber cap on it that you would have removed. You should be able to remove this part with a quarter turn to the left, I think.

My suggestion is to put the top of the tank back like you found it then cut the rubber vent hose between the tank and the filler neck and install a tee. Run your on the bed fuel pump to the tee. I suspect that will cure all of your problems. The valve in the top of the factory tank may need to be replaced. Maybe someone else will chime in on this.

The hiss is built up pressure that is not being vented.