The truck I'm working on is an 1979 F-350 with 400 and a 670cfm holley truck avenger. It has two fuel tanks, with a switch in the hvac controls (all the way to the right) that when switched changes the fuel gauge between tanks. There is also a toggle switch mounted at the bottom of the dash that when switched changes what tank is used to fuel the engine. For the last couple years (infrequently working on it) I've had fuel issues. I've swapped the holley truck avenger with a new unit, and I've replaced the mechanical fuel pump and after each of those things seemed to work better but I'm back with a fuel issue. When I changed the carb I added a small pressure gauge to the fuel hoses on the supply side of the carb. When everything is hooked up and working the fuel pump was registering around 6-7 psi and it was running as recent as last weekend when I got a call from my son that the truck cut out on him. When I got there to help we could see no pressure on the gauge. Then when I disconnected hose from carb and plugged it with my thumb, we got about 1psi and no fuel came out or built up behind my thumb. I had him crank the engine a fair amount with the hose just disconnected and me holding it to watch if any fuel came out. None did. We have a fuel filter inline and I when everything is connected and we crank the engine no fuel enters the filter with one caveat. If I spray carb cleaner in the carb when trying to crank, the engine will start and run as long as I keep spraying, and I do see fuel build up in the fuel filter. I assume this is coming from the tank and not coming backwards out of whatever could be in the float compartment of the carb. (This may be a ridiculously stupid statement about backwards flow and if so, it gives you and idea of my comprehension of how everything really works.). Anyway, my next guess is to replace the tank/sending-unit/tank-selector-device and clean the lines with some carb cleaner and air... or some combo of these four. I'd really appreciate anyone else's thoughts on what might be a better starting point to reduce time or money spent.

Just for my own knowledge, if everything were working as expected, and I disconnected my fuel hose from the front of the carb and crank the engine, should I see gas flow out of the hose or does the carb add suction that is required to get gas to flow?

Thank you in advance.

 

in a factory configuration, the switch in the hvac panel switches both the gauge and the tank...so you are going to have to figure out why it was altered...and yes, you should have fuel coming out when cranking (assuming you still have the factory mechanical fuel pump)...

 

When you crank the engine the mechanical pump should push gas out of the disconnect fuel hose (and at ~6psi) - carb doesn't add anything to that. Is the filter up by the carb (top of engine) or before the fuel pump?

I'm wondering if your multi-tank plumbing is causing an issue - like depressurizing the line between the fuel pump and tank so the fuel pump can't create suction to pull fuel from tank.

Anyway you could bypass the multi-tank setup and run a direct line from one of the tanks to fuel pump? (that would rule out the multi-tank and leave you with either suspect fuel pump*, vapor lock or issue with tank your testing with)

*fuel pumps bought these days can be unreliable

 

You definitely have a fuel supply problem. Be sure there is fuel in the tank. It is possibly a defective pump (even if it is new). A place in the line between the pump and the tank with a leak would allow the pump to suck air. Or it could be a congested filter (or a sock strainer in the tank).

 

I bet all the rubber hoses at the tanks& switching valve are from 1979 right? Don't look at them replace them as the break down inside and block flow.

While replacing ALL RUBBER HOSES check why the HVAC switch dose not switch the valve?
I think that valve is a 3 port valve.
1 port from each tank and 1 to motor like om my 81 F100.
That valve defaults to 1 tank with no power. In my case the rear tank but have heard of some being front tank.

I think replacing the hoses it will be ok but if not you could bypass the valve and run it off 1 tank to help pin point the issue.
BTW when this happened was it on front or rear tank?
Dave. ----

 

X2 the HVAC panel far R/H switch SHOULD normally change the fuel gauge AND make the switch back by the cross member change what tank fuels the engine. Sounds like you possibly have an elec/wiring issue that the PO corrected/did a wire around repair by adding that lower dash toggle to activate the tank selector switch. In tank sending units pick up tubes (filter sock) can get clogged, even fall off over time. I would trouble shoot the sys and figure out why the PO added the toggle switch.

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

 

Thank you all very much. It turned out to be one or a number of things and we wouldn't have figured it out without all of the tips. The first definite issue was a faulty tank selector valve that we found allowed fuel to be pulled from both sources simultaneously and didn't change its behavior if it received 12v or not. We also found a (electrically disconnected) electric fuel pump between the faulty fuel selector valve and the new mechanical fuel pump we installed. I'm not sure if the electric fuel pump caused any issues or not, but we took it out of line and didn't think to blow through it after we took it off to check if it was passing fuel. Also of note, we knew we had a leak in the filler neck of the rear tank and as such we haven't put fuel in it for some time. All that said, I've had two mechanics, one has since died, who have been in to the fuel system and don't know when the addition of the electric fuel pump was made or when the tank selector valve went bad. I just know my son was driving it and it stopped running... Now that we have a functioning tank selector valve in (electrically disconnected for now) the fuel is only pulled from the one tank (the one without a filler leak) and the mechanical fuel pump pushes the gas in between 5-7 psi. We have so many other issues with the truck (will be posting about another shortly) but one day we will get the filler hose leak resolved, then fill the rear tank, then hook the A/C dash switch back up to the tank selector valve so that we can use both tanks. As a side note, I believe the gentlemen who restored the truck about 10 years ago added the lower dash switch to be able to get the current fill level without changing where the gas was coming from. If we can get through our other issues we'll put it back that way. Thank you again for all of the responses.