OK, I have an 1984 F350 crew cab 4x4 460 4spd. I have replaced intake, coil, distributer, fuel return valve, all rubber fuel lines, fuel pump relay, tank selector valve, both sending units and pumps, ignition switch, installed mechanical oil, voltage and temp gauges. It will idle fine all day, run strong for 15 + miles and then loose fuel, switch tanks and it comes right back for awhile and goes away again, switch tanks and fine again. interval gets shorter each time. Doesn't matter if caps are loose or tight. Mechanical gauges show no drop in oil pressure when it happens.
Noticed today after test run that glass fuel filter between carb and return valve showed air bubbles entering at lest 15 minutes after stopping.
Any ideas????

 

It sounds like you may be sucking a vacuum in the tanks, but that can't be if it doesn't matter if the caps are on or not.

Did it do this before you replaced all of those parts, or after?

 

Most of the replacements have been an attempt to remedy this.

 

Ok then.... when you say you replaced all the rubber fuel lines, does that include those from the tanks to the hard lines? And hard lines to the tank selector valve? And the valve to the hard lines going forward?

If so, I would put a pressure gauge in the line. I think I'd put it ahead of the selector valve and see if the pressure there drops when the problem arises. If so, then you have to figure out why, and if that happens I'd put the gauge on the other tank's line. If they are both doing it then it is something common to them, and the only thing I can think of is power. So I'd check the voltage at the tank's connector. And, I'd check the grounds for the tanks.

Edit: I forgot - WELCOME TO FTE! Where's home?

 

Thanks, I'm in Marblemount WA.
Yes I replaced every section of rubber line, it was in bad shape. I also have a glass filter just past the selector valve, if I pull over without switching tanks both filters will be empty. I'm thinking electrical but very intermittent. I believe the only other component is the inertia switch, I may replace it next. I plan on keeping this truck until I die so I don't mind having new parts.

 

I think Page 105 here represents your wiring: Electric Fuel Pump Control - ???Gary's Garagemahal.

 

And then it goes back to Page 97 here: Fuel Tank Selector & Gauges - ???Gary's Garagemahal.

 

Yep, appears to be from the same book I have.

 

Be careful as that's from the 1986 EVTM, and in '86 they did away with the dot and hash marks on the wiring. Yours being an '84 it will have some dots and hash marks where the 1986 EVTM shows stripes. But, maybe you have the 1984 version?

 

yes I have the 84 version and it has been a lifesaver as some of the wiring has been a nightmare, i had never seen so many household wire nuts before.