On my 90 Ford E250 it appears the tank valve has an issue. When I run off the rear tank it appears to return the fuel to the front tank.
This in and of itself isn't an issue but last month I noticed the charcoal canister was dripping with fuel.
This is also an intermittent problem.
My short term solution is to simply switch between the tanks more often.
So I am curious of what the flow rate is for the return? The same rate as the flow to the injectors or is the return a pressure release and or pop off valve of sorts?
On a truck of that age, it would return a pretty significant percentage of the total flow. I'm thinking you need a new switching valve. I'm pretty sure they switch both pressure and return. If it has 6 hoses on it, that's likely the trouble.
Thought this was a question about Formula One.
The Ford switching valve I am familiar with switches the return line, the pump power, and the sending unit wiring, so the fuel gauge reads the active tank. The feed lines are unswitched since the fuel pumps have check valves. An unpowered pump is just a closed pipe. They do route the fuel feeds through it but that portion is just a tee, no valve.
Hummer H1s use these valves. I do not know their flow capacity but it is less than an Aeromotive 340lph pump 
. (aka "why does this thing have 90psi at idle?")
To answer your question, except for modulated units, the pump runs full bore all the time and whatever the engine does not use goes back to the tank.
If the valve is returning fuel to the wrong tank, it will overflow and fill the vapor canister with fuel. If you have a 190lph pump, this can probably fill an empty tank in the span of a short drive.
It is also possible for a leaky check valve in one of the pumps to do this, too.
I can't speak to an E-250 in particular, but in general the way that fuel pressure regulators work is that there is an input line running at a high flow rate and they regulate a valve that bleeds off flow to keep the measured pressure in front of that valve at the desired value. If the pressure is too low they close the valve, if it's too high then they open the valve, with the fuel that goes through the valve entering the return line and going back to the tank. In vehicles from the 90s this regulator normally sits on the fuel rail and is paired with a "dumb" pump that sends full flow all the time, with all of the excess fuel going into the return line and back to the tank.
In a situation like this, the flow through the return line is equal to the flow through the supply line minus the flow through the injectors. It's highest at idle, probably being 97-98% of the flow through the supply line, and lowest when the engine is making max power.
Pete is right, I spoke before putting my brain in gear.
Pete thanks this at least gives me a place to start.
I had to try to familiarize myself with it because something in my memory seemed off.
If it works the way I think it works, the job of switching pumps and fuel sending units is the actual switch on the dash. The switching valve switches on when, IIRC, the rear tank's pump gets power.
