1979 F150. Thought it was a bad fuel pump and bought a new one but it turns out it isn't. Runs fine when a fuel line is ran from the pump to a gas can. Any ideas? I'm guessing it's a problem with the line or the tank but they're difficult to get a good look at and I don't know much about cars/trucks. Worried I'm going to have to drop the gas tank. Is it going to be as bad as it looks?

 

Check ALL your rubber lines . They can suck air if bad . Sock or pick up tube in the tank could be bad also . My 78 had a problem with the hose close to the tank . It was rotted and sucked air , not fuel . Just a thought !

 

The fuel line is mostly hidden but it looks like it's a metal line and the last 3-4" that goes into the top of the tank is rubber.

 

With MUCH caution , and the aid of an assistant , you can apply regulated air pressure to the fuel tank . No more than 10 psi . That said , you can then listen for leaks . Just an Idea .... Best of luck !

 

Probably a 2" layer of rust in the tank.. I had to clean my 79 78 and 77 because all the gunk wouldn't let it pull fuel.. as said above all it takes is a pinhole on one of the rubber lines as well

 

Back flush the complete fuel line with LOW air pressure from the end of the fuel supply line where it connects to the pump. Have a helper listen for air leaks and then bubbles in the tank.

And any in line fuel filters along the way maybe done on a frame rail? Replace all together.

Dropping a tank is not that bad as long as its empty. Post up the 1st suggestion results then more help will follow. Ah heck here it is now.

This first part is trouble shooting the gas gauge....Disconnect the wire that goes to the fuel sending unit at the tank. Ground that wire to the frame, making sure the frame is clean. Turn the ignition to the "run" position and watch the fuel gauge. If the gauge swings all the way to full, either the sending unit itself is bad or it has a bad ground. If the gauge stays on empty then either the gauge is bad or there is a wiring problem.
To pop the plug off for the test you can get up in there with your head and see the plug. While looking at it, VERY CAREFULLY use a screwdriver to pry it off. AND I MEAN VERY CAREFULLY. It is only pushed on like a 1/8". This is only if you can not get it with your hands.
Once you have done the test (above) and decide it’s the sending unit. And you have cleaned the ground and that did not work. Then disconnect the ground again and fuel line and read on.
If you can't get either one disconnected, no worries you will have a little more room once the tank is dropped down a little. But make sure you have some wire slack in the plug connector.
Run the tank dry or almost or siphon it empty, I would not do it with ½ a tank if you just have to, use a big floor jack and a piece of 2x2 plywood. It’s not that heavy, unless you have it full or 1/2 full of gas.
Loosen the filler and vent hose clamps at the tank end and maybe you can pop the lines off? But most of the time you have to just loosen the clamps and wait till the tank is about out.
With the jack in place, just loosen the hdwr (2 bolts) on the front straps, and remove tank straps aft hdwr all together, then swing the straps out of your way.
Slide the tank to the dvrs side about 1/2" and then pull out or let the passenger side drop out of the frame. Then disconnect the elec connector and eng feed fuel line if you could not get to them earlier.
Then slide it as far as you can to the right and pop off the filler/vent lines. Sometime you need to carefully put a flat tip screwdriver in between the rubber hose and the tank to break it loose. The filler neck and vent lines get stuck to the metal pretty good sometimes.
Once out there is a lock that comes off with a screwdriver or punch, look for a small tab that has been bent over to lock it. Un bend it, tap off the lock ring, remove the sending unit.
Insp the underside of the tank where the straps rest, lots of dirt and moisture build up there, and that causes the corners to rust out. You can also come reinstall time replace the anti-chaff material. Old 2" ratchet strap work great, glued to the support straps.
Insp the inside of the tank, might be a great time to replace it all together? Before you reinstall anything connect the electric connection and ground to the sending unit. Wire it up under the truck so it is not hanging by the connection. And with the key in the 1st (on position) you should be able to move the float arm and see a difference in your gauge?? It is always easier to do this with a helper.....
Also before install, paint the complete tank with some good paint. Even spray on bed liner or undercoating??

 

Could the fuel sending unit have anything to do with it? I know it measures fuel level but does it do anything else? I know in the past I've had problems with my fuel gauge not working.

 

Yes the sending unit has a fuel pick up tube and on the end there is a filter "sock" and it can be clogged up.

 

okay thanks probably gonna check that out and make sure no holes in any of the lines. Also have one more question. How do you switch between fuel tanks? Guy I bought it from told me he didn't know the condition of the inside of the rear tank so I've never bothered with it.

 

Like the owners manual say's, if you have the dash switch over by the heater controls, unless you have the valve in the floor?

Also look for this valve under the frame back by the tank.

78/79 duel fuel tanks frame switch-Duralast/Universal 3 port fuel selector valve used with SW48 toggle switch Part Number: FSV4

That is what the switch in the dash is tied into.