I think the fuel selector value is my issue. My question is what does the fuel selector valve do and can I bypass it and only use one tank.

Forgot to say thank you. I removed the fuel selector value and replaced with a straight fuel line. Works great now. Only running rear tank. The front tank never worked anyway. Thanks again

where was that fuel selector valve at? I'd like to have use of both tanks again, as I please, with gas going right back where it came from. it's dripping into the back tank for now. Front tank works great!! along with mpg

That selector valve in located on the frame rail, driver's side, just behind the cab. It may not be easy to see if there is a skid plate over the front tank. You will probably want to remove that first. Four bolts. Front passenger side bolt is really a bear, but doable. To bypass, all you need is a piece of smooth 3/8" tube, about 2" long. If you have the bundy type fuel lines, I would not use a barbed fitting. You may damage the fuel line seal. Smooth tubing is what you need, but it must be EXACTLY 3/8" OD, not 1/100th over or under. The replacement valves are no longer available new. Different kind of ports and will not fit. So, before you disconnect, make sure you have plenty of buckets to catch the fuel. You can't pinch off the rigid nylon fuel lines like you can rubber hose. The rear tank is higher and better to go to anyway, especially if you have a diesel truck. Diesels do not have pumps in the tanks, so you are depending on gravity flow. Difficult to get the front tank to flow, but once it's started, it's good. A little air pressure in the tank will push the fuel into the line and get it going. I modified a gas cap.. drilled out the center and put a barb in there with JB Kwik and I use a small compressor for only a couple of seconds to get the fuel flowing. I actually made a T out of hollow fiberglass rod, to join both tanks and it works well so far. Even though the rear tank is higher, you don't need to worry about it overflowing the front tank because both fill tubes are the same height. Yeah.. I just went through this, myself. The valve uses O rings and when those get old and start to leak, it will take out the electronic board and then you start to lose tank readings. If you want to get your tank reading back, bridge the lead going to the fuel gauge (next to last on one side, usually) and figure out which wire goes to the tank you are running on. It should be to one side or the other. Bridge those two with a blue wire clip and you will be able to read that tank, only. If you bridge both tanks (I had to make my own T, as nothing suitable was available), I put the gauge on the front tank. If you run on only one tank, make it the rear one. It will flow better and the bundy fittings will butt up together just fine. They are both at an angle and can face each other. The front tank is a straight fitting and you won't be able to join it without a T. I am talking specifically about a 1986 F250 diesel 4 x 4, but this will no doubt apply to many other models and years. Good luck!

I forgot to add that you really don't have to worry about bridging the fuel return lines. You can block one off with a 5/16" plug, home made if necessary, from tube with silicone shot into it, or JB Kwik.. Same deal.. 1/100th of an inch too much or too little either won't fit or won't seal. But, the fuel return line can go to the one tank, only, and once again, if you run only off the rear tank, you can putt the angled ends up against each other with a 2" or slightly longer piece of tube. It should be a snug fit and can be secured with wire or zip ties, but there should be no pressure normally, so it's not urgent to fasten the fitting ends together. If you do use air pressure in the tank to get it flowing, keep the pressure very low. It doesn't take much. In fact, I know a guy who would put a radiator hose over the fuel fill neck port and blow into that. If you over pressurize your tank, it will not be good. And, when I said to bridge the wires to get a fuel gauge working, I meant that you need to bridge them right at the end of the wire harness you pulled off the old valve. There should be five wires.. two on one side are hot leads for the tank selector valve operation (only one would be hot at a time and they are fused) and three on the other side are to the fuel gauge and the two tank sending units. The gauge lead is in the middle. Figure out which tank you are running from (I recommend the rear tank) and bridge the sending unit wire for that tank. Since I put a T in my lines, I now get my reading off the front tank, as fuel will flow from both tanks at once, but when also mainly from the back to the front, but you have to make sure that front tank is primed and flowing. The lift pump will not really be able to suck the fuel past that air in the lines without a LOT of cranking, and that is hard on your starter.