I have a kind of double pronged issue.
I just got my grandpa's 91 RS

Brought it home, and the coolant reservoir looked like it was boiling. I read that it could be air in the lines and that a new radiator cap would fix it. I put a new cap on, and havent had that issue since.
However... On my initial drive home, the temp gauge didnt move, not even a little. Not sure what this is about. I replaced the thermostat, and the coolant temp sensor, and it still won' move. It moves when the engine is started, but that is it. I have tested the leads going to the gauge, the pins in the dash, and the sensor itself. Everything seems to be fine, except for the yellow wire off of the sensor. That one doesnt seem to read anything. Could this be causing it, or could it be a problem with the actual gauge?

This leads me to my next issue. The car seems to run fine, but I can't tell if it is overheating because of the gauge not working. I drove it arount the other day, and came out to a leak from the lower radiator hose. I heard someone say that sometimes the problem could be that hose needing to be replaced. If the thermostat and the sensor didn't do the trick, would a new hose work? It appears to be shriveled and wrinkly compared to the top one.

 

The sensor with a yellow wire has nothing to do with your temp gauge. The sensor that does control your gauge has a dark green wire, as indicated by this diagram.......