1980 Diplomat, slant six. I started having an issue with my fuel gauge a couple months ago. I would be driving and the needle would plummet down to E, past the empty mark as if the key was off. Sometimes it would come back up to the accurate level where it was before. It did this on and off for awhile, then it stayed on E permanently. Without testing my sending unit, I replaced it and my dented fuel tank. The replacement sender came with a ground wire which I used, and bolted it to a clean bare metal spot on the body. With the new sending unit in, the needle does move now but does not go further than 1/4, even when completely full. A member of this site (and dippy.org) generously sent me a used fuel gauge for free. He knew it to be a good gauge, so I tried it. It does the same thing, only its needle doesn't go as far as 1/4 full. I even ditched the replacement ground in favor of the factory two wire connector, the style used on my distributor. Still no luck. How can I test this problem? I know the Ohms range of a properly working fuel sender, so I will check that out. However, I doubt that is my problem since the old and new both haven't solved it. How about the instrument cluster voltage regulator? I noticed the temp gauge also reads a bit low, and it's light is almost always on. I did install a 160 thermostat, and that may be why.