fuel tank sender and gauge

[unoleisu]

Ok, on my 87 BII, 2.9L, M50D, My fuel gauge has been messed up. It reads fine when full, all the way down to half full. Below that it drops to empty. I got brave and continued driving, knowing that there is still half a tank of gas. I noticed it started to rise slowly, to about 1/4, then dropped again. I figured the sender in the tank was bad (the float reastat) so I just installed a whole new sender unit. The original didn't look bad and the reastat showed mild wear. After the new one was installed, it's still doing it. (yay, I have a good spare, now) I did clean the plug terminals, they showed mild corrosion.
Any ideas what would cause it to act like this?

 

[BONES]

Mine wasn't doing that exactly.

Turns out the float had a small crack in it and would fill and empty on it's own, because sometimes the gage would read full directly after filling it but once I started driving it, it would slowly drop down to 1/4 tank and after a while to empty and then stay there until I ran down the tank.

I was relying on mileage numbers to figure out how much fuel was left in the tank until I accidentally found out that the float was faulty. I replaced the sender and the gage and the problem was still present. The float was cracked, but along the seam so it wasn't noticeable. I had it apart for a few days at that point and the float had dried off, when I picked it up to put it back together I tipped it over and fuel leaked out of it, that explained my erratic fuel readings. Replaced the float and now my gage works great. I too now have a spare sender and gage, lol.

Don't know if this helps but I thought I would share my fuel reading problems.

 

[unoleisu]

I'm starting to wonder if my Instrument Voltage Regulator is going out. I had to run an aftermarket coolant temp gauge not long ago, because the original wasn't working. I haven't lost oil pressure, so IDK if it's working up to par, other than the fact the light comes on for a moment at startup, like it's supposed to. All three gauges receive power from the IVR.
It just confuses me that the fuel gauge reads fine above half full, but bottoms out at half full or below. I guess I should research how to test the IVR. hmmmmm.....anyone know?