I have a 1980 300SD. The fuel gauge was stuck in the empty position. I added some Biobor to the fuel when I filled the tank. After a short while, the fuel gauge went to the full indication. I removed the sending unit from the fuel tank and cleaned it with carborator cleaner. It was not clogged with slime, but it had some slime on it. Now it seems to be stuck on the full indication.

Do I need to replace the sending unit.