Obviously, the fan works at some point. I just added the info about test grounding the switch as an aside, not a suggestion. The test is, touch a wire to the terminal on the switch, touch the other end to any ground, like the frame, engine block, etc.
Sounds like someone has possibly mixed dexcool and green antifreeze, which results in mud.
If there's no calcium/lime buildup in the radiator, I'd suggest a flush with dishwasher powder, dissolved in water before adding to the system. With straight water and no antifreeze, run engine then rinse until clear, repeat as needed. This will clean out the mud. Calgon brand is good, it even produces heat when mixed with water.
The new thermostat may cure it. In a system this small, the coolant probably never stayed in the radiator long enough for cooling to occur. So it just kept cycling increasingly hotter coolant. Can happen with a 160° t-stat too.
Best to get that mud out of it first. Use green antifreeze, not red. I take dexcool out of anything I find with it. They call it deathcool for this reason.
Last edited by Spockster; 09-13-2016 at 01:41 AM.