75 k10 problem getting fuel from tanks

[Hillbillydrums]

Hey y'all. You were a good help with my last problem so  I came back 
I seem to not be getting gas from my tanks. But to be honest I don't even know how these work.
It has 2 20gallon tanks. How do they even switch over from one to the other? My 78 has a switch in the dash to choose. But nothing here.
I know the fuel pump works. I'm currently running line into a 5gallon tank. Just to get her running.

Maybe if I figure out how these work I can find my problem.

Thanks.

[sbx22]

I'd drop the tank and put a new fuel sending unit in there with a new sock filter. Sounds like you've done the work and narrowed it down that.

[rich weyand]

Are both tanks full, or just one?  Reason I ask is that the fuel tank selector valve is solenoid activated.  Unpowered, the tank selector valve connects the fuel line to the default tank.  Passenger side, I think.  When powered, the solenoid pulls the valve over to the extra tank.  The solenoid has to hold the valve over against the spring continuously to use the other tank.  When the ignition is shut off, the fuel selector valve falls back to the default position, then when the ignition is turned on again, the valve re-connects to the extra tank (assuming the switch on the dash is left alone).  If the solenoid fails, you will always be drawing from the default tank no matter what position the dash switch is in.  If the valve gets gummed up, such as when the truck is not run for a long time, such that the solenoid cannot pull the valve over, you will always be drawing from the default tank no matter what position the dash switch is in.

So if one tank has gas in it and the other doesn't, and you're not getting gas, you may just have a faulty tank selector valve, which is a very easy part to change out.

For myself, when I re-did the dual tanks on mine, I went to the other style of tank selector valve, which uses a motor to power the valve over, then shuts off.  When you flip the switch, the motor powers the valve back.  But the motor does not run, in either selected position, unless the tank selector switch is changed.  Slightly different wiring, and the valve is quite a bit more expensive.

Anyway, you should check and see if the solenoid in the valve is working.  If the ignition is on, engine not running, and you have someone flip the fuel tank selector switch on the dash back and forth, standing at the side of the truck on the passenger side with your head down below the fuel filler, you should be able to hear the solenoid click over in one direction, and the spring click the valve over in the other.  No clicks, bad solenoid or stuck valve.  Bad solenoid is a common failure.  They wear out.

This is the replacement part you need if your valve is bad.  Listed in the GM parts manual for 78 K10.  Check yours by looking inside the frame rail in front of the rear wheels, about at the back of the cab.  It should have six hoses, one wire coming back from the engine bay, and maybe one wire to ground.

http://www.amazon.com/ACDelco-467513-Fuel-Selector-Valve/dp/B0016HT92G

If you have gas in BOTH tanks and no fuel, then you have other issues.  The solenoid may be bad as well, but that isn't the only problem, because you should be able to draw fuel from the default tank even if the valve is bad.

[Irish_Alley]

rich i was kind of thinking maybe the valve could be jammed and clogged with rust. preventing it from switching and pulling fuel. but he says he doesnt have a switch how does this system work. i think i seen a handle under the seat but idk

[bake74]

     75 could have a switch under the seat. 
     Hillbillydrums, what have you done so far to troubleshoot your problem ?
     Here is a wiring diagram for a dual fuel tank set up, I have looked through the manuals and wiring diagrams in the tech section and have not found dual tank setups before.  Check out your wiring and figure out what exactly is not working, is it an electrical problem or mechanical problem.



     **This might be slightly different than your trucks wiring, but it will be close enough for you to troubleshoot.**

     Do you still have the mechanical fuel pump installed off the engine ?  Are you sure it is good or even pumping ?  Just some idea's.

[rich weyand]

Oops.  He also mentions the 78 in the OP, but the question is about the 75.

Re-checking the GM Light Truck parts manual, it lists 1973-1974 as having a mechanical valve, but 1975-1978 as having the electrical valve.  For 1975-1977 K trucks, the valve listed is #334557 or #348729.  334557 is a three-port valve, and I found one place where NOS is available for $189! I can't find anything on the net for #348729.  In 1978 the listed part is #467513, which is the six-port valve.

Of course, he could use the six-port valve replacement for the three-port valve, and simply cap off the extra three ports.

Still have to find where the switch is, though!  The same electrical switch is listed for 1975-1978 C and K trucks.  It looks like this:

[bake74]

It has 2 20gallon tanks. How do they even switch over from one to the other? My 78 has a switch in the dash to choose. But nothing here.
Thanks.

      This is why I questioned the switch being under the seat.  If you do not have a switch in the dash.

I know the fuel pump works. I'm currently running line into a 5gallon tank. Just to get her running.

     OK, seems I missed this originally.   
     Hillbillydrums, do you have any updates for us ?