I suspect I might have a problem with the selector valve. I switched my truck to the rear tank, which has never seemed to work properly, and of course it died. Now the truck will not start at all even with the front tank selected. Now I have no fuel pressure.

I have the 6 port selector valve, but I'd like to at least test it before I buy one. I'd hate to buy one only to find it's not the problem. Anyone have a method to test it? I'd be willing to bypass it but I'd want to keep the stock fuel lines and fittings intact.

1988 F350 with 351

 

If your truck has the dual tank reservoir with no power to it, it's up to the low pressure in-tank fuel pumps to operate, opening their respective pathways to the high-pressure pump and return line.


If so, concentrate on the forward tank, take the top fuel line loose, that's toward the transmission, on the side toward the tank, and tape it into a quart container.


The procedure would normally entail jumping the fuel pump test relay port at the diagnostic link, and see if you have 5-6 oz. of fuel for a 5 second test with the key in the run position.


The factory procedure for the reservoir is done with all lines disconnected, at air pressures below 5psi, with fittings mimicking fuel lines-type connections to not damage reservoir.


Pressures over 5 psi will rupture diaphragm.

 

Thanks, and yes the reservoir has no power connection.

Any info on on jumping the relay for the in tank pumps? I'd like to test both front and rear since my rear tank hasn't ever worked either.

 

On the diagnostic link under the hood, the longest surface of that four-sided plug, when that surface is facing up, the input at the left is the fuel pump test connection.


That connection to a ground will turn on the three (3) fuel pumps continuously.


Don't run them long. Just long enough to perform whatever fuel system test you focusing on.


The rear tank supply line is directly below the forward tank supply line.


They are both inboard, furthest from the frame rail, facing their tanks.

 

Thanks a ton. I will give it a go as soon as possible.

 

I pulled the fuel line off going into the valve from the front tank and jumped the fuel pump and got a nice steady stream of fuel, did the same with the rear tank and got nothing. Guess that pretty much confirms why my rear tank never worked, not really surprised.

I put the fuel lines back on and then took off the line coming out of the valve going to the fuel pump on the frame rail and got no fuel out when jumping the pumps. I guess that pretty much confirms the valve is bad correct?

Is it worth it to pick one of these up from the junkyard or is new the only way to go?

 

As long as you toggled the switch on the dash to isolate the circuits between the tanks.


It looks like you have an issue with the rear tank pump.


Then, if you had the switch in "F", and had no flow to the high pressure pump after confirming the front tanks flow to the selector valve, it seems that the valve is bad.


Selector valves are $160.00 to $200.00 on E-bay.

 

Yup, I set the switch to the appropriate tank for each test.

I'll be going to the junkyard tomorrow anyway for some other stuff so I'll pick up a valve there if they have one. If it works then great, if not I guess I'll have to bite the bullet and shell out for a new one.

 

Got one at the junkyard today for a few bucks and it works so the truck is running again. Also got me a cluster with a tach. Anyway, thanks for the help!

 

Glad it worked out!