I have a 1990 F-700 with a 429 carb motor. It has a tank on each side with electric fuel pumps in the tank.

-I can't get the fuel pumps to run on their own. I cut the wire on the driver side tank pump and put 12V to it and the pump will run so I know it isn't locked up. I hear the pump running and it sounds like it is pumping but no fuel comes out. If you take the cap off it sounds like someone is pouring gas into the tank when the tank is running. I shot some air into the line to see if it was plugged but it is open. When the air pressurised the tank it actually started a siphen and I had fuel coming out on its own without the pump running so I know the line in the tank is getting to the gas.

-The driver tank line goes to the passenger side where it connects to what I think is a selector vavle. The valve has an in from each tank and an out that goes to the carb. It has a single wire plug on it that comes from the passenger fuel pump. I put 12V to this wire and it made the passenger pump run.

-How are the individual tanks slected? I have no fuel tank switch in the cab that I can find.

-Anyone know of a cutoff or something somewhere that is keeping my pumps from working?

-I am thinking of putting a universal electric pump in line and wiring it up to get gas to the carb but would like to actually fix what is wrong if I can.

-There is a relay under the dash by the fuse box that I know has something to do with the starter. Is this just a starter relay or is the starter and fuel pump relay all in one? I change the relay with a known good one so I don't think that is the problem.

I can get the motor to run if I can only get the fuel out of the tank to the carb. Any help you could give me would be great.

 

That fuel system seems to be very different from the smaller F-Series trucks. So I am only guessing.
It would seem that the selector valve is fed 12v from the pass side pump so that when the pump is on in that tank the valve blocks off the pipe from the other tank.
The sound you hear in the drivers side tank when the pump is running would tend to indicate that the fuel is being pumped thru some sort of bypass valve and being returned to the tank. I wonder is there a pressure regulator in ther that is stuck open?
I would think if you fit an electric lift pump on the frame rail you would have to bypass the selector valve and also either fit one that is self regulated to around 5psi fuel pressure or fit a pressure regulator in the line.
I would also think there is a relay (possibly on the firewall) for the stock fuel pumps. Maybe one relay for each pump.
Just my thoughts.

 

Well I fixed my fuel delivery problems. Here is what I did, hope it can help someone else in the same boat.

-I replaced my fuel tank selector switch. I had a heck of a time finding it then POW I looked up and there it was on the heater pannel. Boy did I feel like an idiot after looking for a long time. I got one out of an F-150 at the pick and pull for $3. Be careful when you are looking though as there are different switches. First I got one from a trucks heater pannel that looked the same as mine but the plug on the back was a different shape so after I looked at a few more I found the right one. This cured my fuel pump power issues.

-I could hear the pump run when I hard wired them but sounded like it was just stirring the fuel in the tank. Well it turns out it was. I pulled the tank off the driver's side and found that there is a rubber line between the pump at the bottom of the hanger and the fitting at the top that comes out of the tank. This line was FUBAR and in pieces. I replaced it and put it all back together. I also found the fuel float was rusted away so I ordered one to make my fuel gauge work.

Now the truck will start right up. I still need to tune it up, set the timing and maybe rebuild the carb but I feel better that it is all small stuff from here. I also need to pull the exhaust manifolds off and weld up some cracks. All downhill from here.

Now I just have to fix the brakes and I will be all set.

 

Glad to hear. Alot of the meduim duty trucks form 87-97 use parts from the 80-86 f100-f350 trucks

 

I wish the brakes were the same. Mine are something called Lucas Girling. Guess I need to bite the bullet and just pull the wheels off and see what makes them tick. I have a leak and I think that they need blead

 

EW! they are hydraulic brakes?

 

Yes they are.

 

good luck. they suck in a truck that heavy w no jake brake