I am working on an 87 f-150 5.0 efi larriat that sat for over a year. It has dual tanks. Truck initially started and ran enough to move it. Then switched from the front tank to rear tank and will not restart on either tank now. I can hear the main fuel pump on the frame rail come on when the key is turned. However, there is no fuel coming out of the line between the pump and filter as I turn the key. It got dark before I could check fuel in the line going to the dual tank valve before the main pump which I plan to do in the morning. I'm thinking maybe something to do with the dual tank valve or switch. Just wondering if anyone has delt with this, and if I could get some suggestions on diagnosing this issue.

 

these trucks have 3 pumps one on the frame one in each tank the ones in the tank are basicly lift pumps-to lift the fuel to get to the main pump on the frame-see if the lift pumps are running-making noise when key is on-the rear front switch could be bad they are known to crap out on ya there is also a relay under the hood for those pumps-get a manual it will help locating the pieces to check

 

If the frame pump runs the relays are OK.

 

I was aware of the 3 pumps. Would I be able to hear the in-tank pumps from the cab as well as the main while turning the key, or would I need to be underneath while someone else turns the key? And thank u subford for the input on the relay. However is it possible its working for the main pump and not the in-tank pumps.

 

You may need some one to turn the key for you. Note that the fuel pumps will only run for one second each time the key is turned to run.
You may be able to hear the boost pump run by taking the cap off at the filler tube. Is not it is time to go under.

No, there is only one wire leaving the Fuel Pump relay and this one wire will run the frame HP pump and the selected in tank boost pump.

 

so that rules out the relay. There are 4 lines going into the dual tank valve/ reservoir. 2 on the top which seem to go to the front tank and 2 on the bottom which I assume go to the rear. By removing these can I test the in-tank pumps by turning the key and checking for fuel coming out of the line?

 

I tried this with the top 2 lines that i believe go to the front tank with the switch on front tank and got nothing as i turned the key. I was not able to check the bottom lines that i believe go to the rear due to it starting to rain. Could this be a sign that the front in tank pump is bad or could that still be something to do with the switch even though its behind the vavle/reservoir

 

I tried this again with the lower 2 lines, from the rear tank and again got nothing. I tried listening for the in-tank fuel pumps while underneath the truck and someone else turning the key, and honestly couldn't distinguish weather or not I was hearing them or just the frame pump. If I was just hearing the frame pump then I don't believe either in-tank pumps are working. Which I find highly unlikely for both to go out at once. If the switch is bad, could this prevent the in-tank pumps from running while the frame pump still works?

 

It can be the switch on the dash as they have been the problem.





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so I looked for the switch and only place I could find it was the dealership for $50. I am on a budget, so I'm wondering what else I could do to diagnose what's wrong without replacing the switch first. Can't afford to waste money on something that may not be wrong. Any suggestions?

 

As for the switch on the dash I did not say to replace it. You need to take a loading test light and check the wires going into and out of it going to the fuel tanks. Do not test the gauge wiring with the test light.