I have a 1986 300SDL and the fuel gauge has started pulsating back and forth. Any suggestions on what is causing the problem and how to correct it would be greatly appreciated. Johnny

The float system is not the best design. Pull your first aid kit out to expose the sender, just unplug it and unscrew it and pull straight up without bumping the sides. Perhaps you will see something that is binding. Be careful as it is delicate.
Perhaps the problem is algae. If it is quite black, it means it is coated with algae-and so is your tank. Sterilize the sender with Clorox, rinse, dry and reinstall. This may cure the pulsating. Use a diesel algaecide for the tank.