Dual tank problem: appears to switch side but only seems to draw off left tank
 

1981 GMC longbed 2wd. Just got home from a 2500 mile trip to Cruise to the Woods in Iowa. The switch and valve have worked fine for years and for 1800 of those miles. I was running on the left tank and getting low, so I filled up both tanks and selected the right tank. I drove 1.5 hours and noticed the fuel gauge had not dropped. I switched to the left tank and saw that it was down below half. I switched back to the right and gauge goes up. My next fuel stop I see that the right tank still selected, still shows full. I filled the left tank and it took 14 gallons yet I had selected the right tank. Tried topping off right tank and it was completely full. I drove 3 hours with the right tank selected and gauge never dropped. Switched to left and sure enough, it's just above empty. I got home stopping every 2 hours to fill the left tank and quit trying to switch.

I haven't got any tools out yet but I did turn the key on/engine off, tried the switch and I can hear the motor in the selector valve move. Is it possible for the valve to fail internally so it continues to only draw from the left tank despite the motor moving?

Re: Dual tank problem: appears to switch side but only seems to draw off left tank
 

Is the motor actually moving or is it just making noise? I would pull the lines off the valve and using a hose, select one of the ports and see if you can blow through the valve then switch to the other and try to do the same

Re: Dual tank problem: appears to switch side but only seems to draw off left tank
 

It makes noise, don't know if it actually moves anything inside the valve yet.

Re: Dual tank problem: appears to switch side but only seems to draw off left tank
 

A lot of the times it's the switch.
Sounds like you may actually need a Pollack valve.
I would sanity check the power at the valve with a meter to be sure

The part you need is Pollack 42-159