I have an old 86 E-350 cab chassis bus with 460 and carb that is used for Boy Scout trips once a month. There are multiple problems:

The tank selector relay is bad.
The tank selector switch is bad.
The fuel pump power relay is bad.
The tank selector valve is bad, at the moment it is stuck in an intermediate position.
The rear in-tank pump is bad.
The rear tank has an external electric pump that works, it draws thru the pump fine. It has been in operation several years. It is wired directly.
The front in-tank pump is intermittent.
The front tank sender does not work.
The rear tank sender works.

The wiring has been hacked several times by previous owners but it tests good.

I do not want to spend a lot of money fixing this thing and replacing all of those parts. The scouts don't have the money for a lot of repairs. I don't want to drop the tanks to remove the bad pumps either. Both tanks are about 3/4 full.

I wired the rear tank sender directly to the gage bypassing the selector valve.

The rear tank is connected directly to the pump now and the engine will run off that tank.

Since both tanks are on the same level (OEM tanks) can I just connect the two lines from the tanks with a T before the pump and draw off both tanks? Would whichever tank had the highest fuel level push fuel into the pump?

Will both tanks fill each other so the rear tank level sensor would report the "common" level in the tanks? Or are there check valves in those in-tank low pressure pumps that would prevent fuel transfer?

Filling one tank may fill the other thru that 3/8 line by gravity, if there are no check valves, but it should not be a problem to fill one until the pump stops then top off the other.

I know this wouldn't work if the tanks were on different levels.

 

I would think if you unhooked the front intank pump so that it doesn't pump fuel into the rear tank, I don't see why a T in the line wouldn't work as long as the external pump is strong enough to suck from both tanks.
I would try to make the lines from the tanks to the T as equal and short in length as possible to insure that they draw as close to the same as possible.

 

Thanks for the info.

The front tank pump is disconnected electrically but still in the tank just like the rear tank pump.

The system has one of those 6-port tank selector valves for the hot fuel return but somewhere down the line the PO disconnected that system, maybe a carb replacement... All of the hot fuel return lines are plugged. I am thinking about just connecting those return lines for both tanks together also as a way to equalize the tanks a little. From the diagrams I have of the system the return line does not go all the way to the bottom of the tank. It looks as if they may have some sort of filter/valve/something on the ends of the tube in the tank. I have never had one of those units apart so I don't knoiw what it might be.

One thing about pumps tho. It is the air pressure in the tank or the head pressure from the fuel level that actually pushes fuel into the pump inlet. I would think that a difference in fuel level from one tank to the other would force fuel into the pump and also back into the other tank if there is no check valve.

Someday I am going to tap a fitting into the side of my radiator cap tester so I can use that nice hand pump to presurize other things besides radiators and caps.

Both tanks are vented.

 

I know that my Mustang has the return line (EFI) and on the end of the tube in the tank is a rubber check valve. I just keeps fuel from siphoning back out. It is similiar to ones that I have seen on some small fish tanks the closes up when not under pressure to keep water from running back into the air pump.

 

That is kind of what it looked like from the drawings in the manual but I could not quite make it out. They must not work real well because I had a lot of fuel in those lines. Of course these are OLD...

Right now the bus is running with a tee in the pump inlet. I will have to drive it a hundred miles or so after filling both tanks to see if it is drawing from both tanks.

Theoretically any difference in fuel level between tanks will make it easier to draw fuel from the tank with the highest level. That assumes that the check valve springs, if any, are equal also.

If both tanks draw down evenly then the rear tank fuel sender will give an overall fuel level reading for a 40 gallon combined fuel tank.

The one problem I see is whichever tank goes empty first will stop the pump from drawing from the other tank. Air will flow into the line easier than gas...

Time will tell I guess. I will post the results when I get to drive it next time.

If it works then I won't have to worry about switching tanks on my 78 or worry about the hot fuel return line filling one tank either. I won't have any in-tank pumps or check valves to worry about on it at all.

I have advocated a hot fuel return line to prevent vapor lock to a lot of people for many years. Come to find out that Ford has been doing the very same thing, the very same way with a small orifice, on the 460 in certain applications for many years. I wonder who was first -hehe! Probably some guy in Germany in 1930. The EFI systems have a bypass regulator that does the same thing.

 

Yeah, I worked on a 85 F250 with a 460 and it was carb'd but used an electric pump and a return line. Some had that option, some didn't.

 

This bus was set up with that 6 port switch and there is tubing leading to the engine compartment for that system but it has all been disconnected forward of the switch. The steel line and some short sections of rubber hose is all that is left.

My 85 OEM manual has diagrams for systems with and without that hot fuel return.

 

As per LXman, I would make sure the tank lines were connected to the horizontal bar of the tee, otherwise it might tend to draw straight through, once the fuel was moving at full speed.

That points to other possible problem. If the vehicle is on an incline, one tank might run slop dry, which might either produce temporary vacuum or allow air bubbles. If you hooked a separate equilization line between the tanks, they'd certainly tend to the same level, but again, on an incline, all the gas would tend to rush to one tank or the other. Implying that you've really got one *30* gallon tank in two parts.

What occurs to me would be to try a little home fluids experiment. You could get to clear milk jugs, and fill one up halfway, and the other near full, run a line to the bottom of each jug, hook those to a tee and another line and then start a siphon on the third line and see what happens. For extra fun you can try varying the relative height of the jugs.

ash
['All purely theoretical, I've got no experience with it.']

 

That small 3/8 line with restrictions will take a long time to produce a significant difference in the tanks with just a difference in level due to a grade. But it is something to keep in mind.

To make the experiment valid I would need some 1/16" ID line and tee to match.

Since one tank is in the rear of the bus I ran the rear tank to the run side of the tee and the shorter hose to the front tank to the branch. A crude attempt at "balance".

 

Only for informative purposes,

Ford had a recall on the fuel rail switching valve for dual tanks. The problem: Fuel from rear tank going into front tank. This recall has been recently dropped.

Fuel sending unit info: 90% of the time the malfunction is due to either the sending unit grounds for each tank or the float (made from an inferior plastic) has a crack which allows it to become fuel logged and therefore read incorrectly.
The in dash unit almost never goes bad.

Not that this helps you out with your sliced and diced problem, just info as to the possible original causes (except for the fuel pumps, cause when they go, they are gone and there is no getting around that!).

Good luck with your experiment!

 

Thanks for the info. This is an old carb system not EFI. Stone fence technology here. I have checked the fuel tank grounds and they look good. The front tank sender unit seems to always read ~2/3 full but varies a small amount but that could be ambient temp effects on the gage.

 

We have an '85 F-150 351 H.O. dual tanks with similar problems....
Figured I'd pass it along.

Have a good day.

 

Keep the old beasts running

If you have EFI you have more problems.