My back tank is full and only registers at half. Put $40 in front tank and nothing registering. Can anyone tell me where to start looking. All lines to the tanks are hooked up if that helps. Also with the $40 in the front tank, whenever I switch it to that tank the pickup runs for 5-8 seconds and dies so thinking have bigger problems with the front tank.

 

Pull the wire from the sending unit at the tank and ground it. Then get in and turn the key on. With the tank switch in the proper position to select that tank the gauge should go to Full within a few seconds. If it does the wiring is good and the sender is bad. If it doesn't work, try it on the other tank. If either tank shows full scale then you know the gauge and Instrument Cluster Voltage Regulator are good.

 

Second to what Gary said, you probably have a issue with the fuel selector valve located just behind the fuel filter. I can only speak for my 85 EFI, not sure about the carb units produced in early 85. My selector switch was acting up and it would run about 30 seconds then die. When I was switching tanks the valve was sticking. I found the wires were in bad shape going to the valve. I got another plug from the wrecking yard and spliced it in place of the bad plug and it worked just fine. I also, had to replace both sending units to get the gauge to register right. You may also, have a defective fuel pump in the front tank and/or the rubber hose that is supporting the fuel pump has softened and the pump is laying in the bottom of the tank. I know this is a lot of "ifs" but this is what I ran into. There are a lot of people more knowledgeable than I on here that can be more specific than I.

I found it easier to remove the bed rather than dropping the tanks. Just unbolted it and a friend and I lifted it off. Then you have access to both tanks and valve and it was a easy job. I got my sending units from NAPA, they were the only people that I could find that had the front sending unit in stock. The front and rear tank sending units are different and are not interchangeable. Hope I shed some light on your problem. I might add this is a good time to replace your rear shocks as well. Richard

 

I've had good luck lowering the tank using the all-thread or long bolts the factory used to pull the ends of the bands together. They have been long enough to let me get to the top of the tank without removing it. But, they sometimes need PBBlaster or similar penetrating oil to keep from twisting them off.

You need to test both front and rear to see if the problems are with both tanks or with just one. That will let you divide and conquer, which is the way to troubleshoot.