I've checked the gauge, it's pretty accurate. In fact last month, my fan belt snapped, and because I had a faulty alternator, I ignored the light until I saw on my gauge, the water went past 100 deg C so I pulled over... the thermostat may have stuck open since then. I'll renew it for piece of mind. I just hate green liquid...

I did the drill out of the thermo ring with three holes to allow coolant to flow around the thermostat before it opened. Never had one fail in a mini and I do it with all my vehicles now as in the patrol(nearly cooked it once and replaced it with a drilled stat…never happened again). Flushing for me is remove thermostat, connect a brass fitting to the block drain plug and reverse flush it. All the sediment settles behind this drain plug and the hose blows it all out the stat housing. Block is clean as a whistle and made a huge difference to the cooling also. Change it every year and is a good cheap investment.

The other thing to look at is making sure the soft rubber shroud is on the radiator as this will lead to overheating too. I lost mine on a drive from Canberra to Sydney and motor got to 3/4 to hot. Went to a Clark rubber in western Sydney and bought some rubber strip and fixed it…car never made it to a 1/4 hot, even in Sydney traffic.

_________________
Rob Forsyth
Miniot!!

I suspect that this is your issue.
It’s not uncommon for a thermostat to jam open after an overheat situation. From memory some of the better brands are designed to do exactly that. Can’t remember what brands though. The red coolant is worse than the green... Toyota use red and it oozes everywhere
Neil