Got myself a new sending unit for $19. Will try it out soon.
The old one was only threaded in about halfway, and looks like they used some kind of pipe threading sealant or something on the threads; looked like a really brittle washer, but I bet it was some type of sealant. The shop said to use Teflon tape on the threading when I replace it.
I do have another issue. It just runs too hot, and now it's causing me new problems. It leaked around the thermostat housing, so I started to take it apart. When it's running for awhile, the hoses are hard as rocks, nearly too hot to touch. It's really, really belching heat. Run great; no smoke, etc. So, I let it cool, and pulled the upper hose off to get some room to torque down the housing bolts. The fluid was orangish, and smelled fuelly/alcoholly. The fluid in the rad is bright green. So, it looks like it's not moving fluid, and I may have fuel in the block (I use red diesel).
The thermostat is brand new. Maybe it's bad. I'm also starting to think the water pump might be bad? But, it's not leaking there. My uncle, who is a mechanic, said it's rare to a water pump impeller blade to fail, but it's possible. He said to yank the thermostat, test it in hot water, and only replace the pump as a last resort. He said it's more common for the actual pump to blow out its' bearings, or seal, not a total impeller failure... He also said the symptoms just really don't add up, and it's probably something simple, but not obvious yet. It's also very strange that the radiator is NOT blowing off any antifreeze from the bypass tube at the rad neck, either.
The only other thing I can think of that may be contributing, is that I'm using a non-Mitsubishi Rad cap from Pep Boys. It's only a couple pounds higher than the old one (its gaskets were pretty rotten), and seems to fit real good. Wouldn't explain why the fluid is two different color, though.
So far, I don't think I have engine damage, but if I can't get it to cool correctly, I'm sure I will.