Fuel crossover problem

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Dleslie212

Full Access Member
Joined
Sep 2, 2019
Posts
234
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139
Location
Phoenix
First Name
Dustin
Truck Year
1977
Truck Model
K10
Engine Size
5.7L 350
Hello all.

About three years ago, I replaced both my fuel tanks to convert to an EFI system. Both tanks, senders and pumps were replaced, along with the fuel selector valve and switch. Everything had worked great up until a few weeks ago

I just finished a complete rewire of my truck, including another fuel selector switch. Nothing on the valve, fuel tanks or plumbing was changed.

Two weeks ago, I took it out for a drive for the first time in over a year. I filled both tanks completely up. As I was cruising around, I kept getting strong scents of gas - so I pulled over and saw that gas was pouring out of the right hand fill port, as well as the breather under the hood. The vents on both tanks are teed together, with a single line running up under the hood with a breather

As soon as I saw that, I flipped the switch to start drawing on the opposite tabk. I sat there and watched it for a few minutes, at which point fuel started dumping out of the left hand fill port

My immediate thought was that I'd crossed the return lines at the selector valve. But nothing on the plumbing had changed, and I'd been running that way for years. I ran back home, luckily I wasn't far.

In the driveway, I did some quick troubleshooting and found that whichever tank is selected for use, the opposite tank starts spitting gas out of the fill port. I got under the truck and traced the lines out - the return lines are not crossed.

I replaced the fuel selector valve, just in case there was an issue with it allowing crossover - made no difference

I disconnected the left hand power wire from the selector switch, and started up running on the right tank. No difference - fuel still spitting out of the left fill port.

I disconnected both vent lines at the point they tee together and plagued them both - no difference

Finally, I swapped the return lines just in case. It does seem a little better, but still have fuel dribbling out of the left tank when the right is selected.

I'm positive that if I run the left tank, the right will start overflowing. Its just not making any sense to me. I think next step is to disconnect the return line and plug it off, and see if I can confirm that it is indeed an issue with the return

Does anyone have any ideas for me?
 

Dleslie212

Full Access Member
Joined
Sep 2, 2019
Posts
234
Reaction score
139
Location
Phoenix
First Name
Dustin
Truck Year
1977
Truck Model
K10
Engine Size
5.7L 350
Just adding - the only change that has been made since last time the fuel system operated properly was a complete wiring harness. It's always possible that this has been an issue the whole time and I either didn't notice, or never had both tanks completely filled at the same time before, but I'm 99% sure that's not the case
 

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