...and in a fascinating fun-packed fiasco today we took the mini's plumbing apart a little bit to confirm the total lack of a thermostat [shock-horror].
So the correct 88° thermostat was inserted in the housing and just because we already had the plumbing undone I chose to remove the "straight through" heat-out fitting and pipe from the cylinder head for one with an inline on/off valve. That's when things went sideways.
Some kind soul had fitted the "straight out" fitting with incorrect bolts that were wrong thread and wrong length so the correct heater on/off valve is hanging on (not by a thread [pun]) but by a wing and a prayer.
The correct bolts/studs/nuts to use would have been 1/4" UNF, but no, they had forced metric 6mm bolts in there instead.
So, I have put it all back together (with these incorrect bolts (that you dare NOT tighten). I started it up and let it run for a while.
Well blow me down with a feather...... As I had predicted the water temp gauge actually worked and went all the way from "C" to the midpoint "Normal" mark and then dipped when the thermostat opened and rose back up to normal after a few minutes.
So another problem resolved, and another one discovered. Correct thread bolts arriving Wednesday.
Running so cold may also be a reason why it has performed so poorly of course.