So I bought this 1986 z28 and the fuel gauge only goes to quarter tank when the tank is actually full. From quarter tank it slowly goes to empty. If I buy an aftermarket fuel gauge will this fix the problem or is it the device on the actual fuel pump? How can I check this?

 

Pull the connector under the car (above the rear end)and take a look at the gauge. It should read full. Then jump the connection between the pink and black wires on the body side. It should read empty. If that checks out, fill the tank and pull the connector again and measure the resistance on the sender side of the harness between the pink and black wire. It should read around 90 ohms. If it is low, around 20 or so, measure the resistance between the pink wire and a ground. If there is little resistance (or same reading)the wire is most likely grounded somewhere. If there is no continuity, post back your results, but the sender is most likely bad.