1985 F250 Duel Tanks

[Dean_L]

Ive had an issue with my front tank for a while and have just got by with my rear tank.  When I switch tanks, the fuel gauge registers but the truck dies from lack of fuel in a few seconds.  Ive swapped out the fuel relay, but it did nothing.

My 1st question is, if the fuel sending unit is getting power would the pump also be getting power?  If so my guess is the fuel pump is out.

Could I also be getting power to the sending unit and pump, but the selector valve is not switching over?

Looking at the wiring diagram (thanks for the resource!) it appears to me that if the fuel gauge is registering with the front sending unit, the front fuel pump must be getting power, thus my issue is likely the pump.  Just looking for confirmation.

Truth be had, Ive put off dealing with it because the tank is full and I really dont want to deal with disposing of 10 year old gas

[Gary Lewis]

It isn't safe to say that if the pump gets power the sending unit will.  Those are sourced by two different wires and different contacts in the switch.  So the pump could be bad.

But it is very possible the selector valve isn't working.  Those things are notorious for that.  If it were me, I'd pull the line from the front tank where it goes into the selector valve and see if the pump is working.  If so, it is the selector valve.

[ArdWrknTrk]

If it's the midship tank you can likely reach up and unplug the harness from the tank bung.

You will see fluctuating 5V on the sender pin and should see close to battery voltage at the pump positive.
This wire should have a resistor, but the meter is not drawing enough to make it evident.

Then you can tell if it's the switch valve or the pump itself at fault.

[Nothing Special]

If you have in-tank pump you have a different fuel system than my '85 did so this might not apply at all.  My '85 F-250HD had the 351 with a mechanical fuel pump.  At one point it had an issue where it would run on one tank but would often lose power, or even stall out entirely on one tank but always ran fine on the other.  It turned out that the plastic fuel line was cracked, but rather than cracking all the way out there were just cracks in it (if that makes sense).  So it didn't leak, but there was a flap inside the line that was acting like a reed valve, choking off the fuel flow.

That was a long time ago (I sold that truck in '98).  But it was a weird enough issue I still remember it.

[grumpin]

Take the cap off, switch tanks and listen for the pump.