Looks like my 78' has learned a new trick from my 91'...

Two weeks ago I decided to start using the rear tank in my 78 (F150/460/dual tanks). I filled up both tanks all the way at that point. Today a full front tank got even fuller and overflowed.

What I have: Dual tanks, with the electric selector valve. After the selector valve there is a carter electric fuel pump. Then there is about 10' of line or so to the engine compartment. In the engine compartment I have a glass fuel filter and then a 5.5lb pressure regulator in front of the carburator.

I haven't used any fuel out of the front tank, as I've run the rear tank to about 1/4 full. I've been running it on and off for about two weeks. All was well until today at work when I shut the truck off and then fuel started to come out of the front tank filler hole (over filled). If I had to guess pressure built up by my electric fuel pump was causing fuel to go from the line back into the front tank???

I've suspected something *like* this was going on for a while now, as when the truck is off the clear glass fuel filter in front of the carb drains after a while. The truck starts fine, as there is still fuel in the carb bowl. It never fades or loads up when driving it either.

The 1997-1992 generation is famous for this overflowing fuel into the wrong tank. My 1991 does it and there have been recalls. On the 1991 I can understand it because there are tons of hoses going into the selector valve.

I can't figure out how the 1978 would have this issue though unless positive pressure from my electric fuel pump is causing it. I also couldn't figuire out why it would happen now that the tank is about empty in the back. Seems to me that it would've happeneed when both tanks were full and there was less volume for air.

I didn't install the electric pump, it was on the truck when I got it.
Before I started building the engine I rinsed both tanks and replaced the lines and filters in each.

Also, with the selector valve on these trucks, does it switch between tanks and stay based on the position of the switch or does it default to the main tank and only switch to aux when the truck is on?

Also, the selector switch is messing up on me in that if I put it all the way up it doesn't switch the gauage. I have to have it all the way down or 3/4 of the way up. I haven't seen these switches at LMC truck or Bronco Graveyard. Maybe I need a switch too, but I was going to look at the switch and see if I could fix it. Anyone ever replaced the switch? Where did you buy it?

I'll probably just replace the selector valve. Just wondered if anyone has any input or if anyone's heard of this. Has anyone ever heard of this fuel overflowing happening with a 78/79?

 

I had a similar problem on my 79 f250. dual tanks, usually used my rear/aux tank, since the guage doesn't read properly on the front one. I happened to run out of gas in my rear tank, and randomly tried switching to the front one, only to find it was full to the brim. Helped me out in a pinch, although before that I never could figure why I was only getting 5 mpg . All I did was replace the selector switch, and problem was solved. I got one from the local kragen, they had a universal 3-port selector switch that worked perfectly.

I, however, don't have an electric fuel pump. The only other thing I would check, besides the valve, which I understand to default to the main tank position unless power is applied to it, is to make sure the gas tank ventilation is still there. There is the return line, considered part of the emission system, that will prevent a vacuum effect from building up in the tank. Not sure about how it handles pressure, though. But something I would check. Good luck

 

As long as the pump is inbetween the selector valve and the carb, it shouldn't be any different than a mechanical pump. I too would suspect the selector valve.