There are 3 valves involved in this system. The right and left fuel return valves and the center tank selection valve. All 3 get power when the fuel changeover switch is out, or selecting the right tank. The left return valve is normally open, the right return valve is normally closed, and the center tank selection valve is normally left. When the valves fail the left valve fails in the open position so when you run on the right tank it pumps fuel from the right tank into the left. When the right valve fails it stays closed so when running on the right fuel has to force it's way somewhere which may force it's way into the left tank, it may push open the right valve and go into the right tank anyway, or it might cause your fuel pressure go too high and cause your car to run rich. When the center valve fails you will be most likely be drawing fuel from the left tank and when trying to get it from the right tank and yet the left return valve still opens so you pump fuel right to left again. I've seen center valves get stuck on the right tank and pump left to right or stick in the middle and suck from both tanks while returning it to whichever tank return valve is open. The only other thing is that if all 3 valves are the same age.
Any variety of problems may be afoot. Especially if the car has been sitting for a while. Look for the other two valves in other listings.