Bought truck a few months ago and had rear tank issue so replaced tank and pump and running again. Now front tank is acting up . I got truck ready for trip to California with enclosed 6x12 trailer and headed out. Not 20 miles into trip and seemed like truck running out of fuel and slowing down. Flipped switch to rear for about 10 seconds then back to front and it was running again. Another 50-60 miles and front tank acted up again. Switch to rear again then back to front and it kept going. 900 miles of this. It's like it was starving for fuel but could go 10 miles or a 100 but eventually I made it. It never did this til the day I put a trailer to it. Once in Cali. Trailer unhooked but problem persisted although would got through about half tank before acting up without trailer on it.
Dirty filter maybe on pump in tank? I left cap loose whole trip thinking maybe vacuum lock? Any thoughts? Thanks

 

What year ? what engine ?
I'm thinking fuel pressure. Use a gauge on the schrader valve on the fuel rail on the motor.
Tell us which motor/year, and we can tell you what it should be.

 

It's a 94 f150 with the 300 six, dual tanks

 

Alrighty well... that's the transition year on I6. Technically they weren't supposed to be changed over till 95 but some 94's were. Had to do with a recall on the 93's because they were blowing check valves.
The specs, however show 87 to 94 I6 45 to 60 psi. 95 & 96 I6 30 to 45 psi. (all the 302's and 351's are 30 to 45 psi).
All that's important because quite often people will end up putting pumps for a v8 in a truck with an earlier I6
So you should actually be looking for something in the neighbourhood of 55 psi. That would be a healthy pump.
Based on your symptoms, I'm wondering if maybe your front pump is not putting out quite enough pressure (for whatever reason) and can't keep up, till you switch to the back, boost the pressure, and then switch back and it'll go for a little while more.
That's just a wild guess and could be way off the mark for your particular problem, but it fits. Put a fuel pressure gauge on it and do a little testing between tanks and you'll know pretty fast if it's pressure related.

 

Thanks for the reply. I'm thinking pump as well. Rear pump DOA when I bought the truck. It had only been driven 7k in 10 years and a third of that when moving 10 years ago by inlaws. Rear pump all black and ugly when I pulled it, tank okay but replaced that as well. May do the same here. He only put 10-20 $ of gas anytime and rarely used it more than once a month. Unfortunately had a family medical issue so truck is in california. now and me back in tucson. Won't get to it now til March or so. I appreciate help. I'm just gonna replace pump I think when I get back to it and go from there