1993 F150 5.0 w/ factory dual tanks. Just replaced BOTH pumps with new units as the original problem was the front tank would fill when using the rear tank (bad check valve on front pump confirmed by lots of cranks to start). While I had the bed off, replaced both pumps with high quality Bosch units.

Started driving today w/ FULL front tank and 1/4 full rear tank. After about 30 min driving, rear tank has 1/2 full and front tank 3/4 full. So somehow I've reversed the issue. Truck runs fine with both selected, but now front pump is filling the back tank when running.

I don't see a selector valve on my truck... it appears the system is controlled by power to the pumps and check valves on the pumps. The one question I have is how is the fuel return line controlled? How does the unspent fuel returning to tanks under light load know which tank to go to?

Any help appreciated... about to throw a brick through the windshield after wrangling the bed off to do both pumps only to have the issue again (only reversed).

Thanks,
Phil

 

Problem is you have bad check valves in your new pumps, now before you replace them again, they do make kits to correct this problem. Ford designed some in like check valves that attach between the pump and the fuel line and are much more reliable than the pump check valves. So might be a better investment as last I recall a pair of valves was 20-30$. You can do the front pump without removing the bed but you may have to for the rear.

 

Thanks... Kinda frustrating that a brand new pump would have a bad check valve. Any idea what the part name/number is for an inline check valve?

 

I don't have a part number, I only found out about the kits after talking to a dealership parts guy.They should look like this if you can find them. There is a half a number in the picture but i cant make it out.

 

Thanks for the photo - much appreciated. The only thing is I'm not entirely sure it is a check valve issue... the fuel system is retaining pressure. Before pump replacement, I had to prime the pumps twice (ignition on, off, on, start) in order to avoid hard cranking. Now the truck starts right up with either tank selected even after sitting for some time. I think the issue is the return line is somehow being diverted always to the rear tank. What controls which tank the return line goes into? I can't seem to find anything on this in the factory manual.

Thanks,
Phil

 

The switch that turns the pumps on and off when you switch them is the only thing that diverts the fuel. When you turn on the front, the back pump shuts off and there it a check valve that prevents the fuel from coming back. Whats happening is that both supply lines are connected together with a ''T'' fitting. Now if either of the check valves don't work, the front pump is pumping half of the fuel to the fuel rail, and the rest back to the other tank.

Edit: It could also be the shuttle valve on the return side, but either way the assembly should have to be replaced, you cant put an in-line check valve on the return.

 

So it seems that either the Bosch pumps were designed without shuttle valves (not designed for dual-tank use) or BOTH of my pumps have defective shuttle valves... as I found today that the return line is not going to the correct tank in any circumstance... One tank runs to empty until other tank is full and I gotta hit the dash switch, then eventually the other tank fills and I have to switch again.

Take this as advice AGAINST Bosch pumps for this application (I normally love them so that's hard for me to say).

I guess I'll go with the Spectra Premium (Motorcraft prices are a bit exorbitant).

-Phil

 

I believe the check valve system in the pumps controls both the fuel out and return fuel. Not sure if it is a single dual action valve or 2 separate valves in each pump. Anyone dissected one?

Just some ideas:
Possible that a fuel or return line is plugged or kinked?
Maybe there is debris or muck in the tanks that have fouled the check valves in the new pumps?
The switch is broken and running both pumps at the same time?

 

Possible that a fuel or return line is plugged or kinked?
- I'll check this when I pull the bed again to swap the pumps, but I doubt it.

Maybe there is debris or muck in the tanks that have fouled the check valves in the new pumps?
Tanks were really clean when I looked in them and new filter socks were installed with pumps. Also highly unlikely that BOTH would fail immediately.

The switch is broken and running both pumps at the same time?
I don't t think this is the issue as the gauge switches, indicating that the switch works fine.

Will locate and attach a diagram of pump internals for further discussion...

 

See below for diagram: