I found if you install the thermostat before you fill it with coolant you can get an air bubble under the thermostat, which will mean it won't open. On my car I filled it near to the top of the block so there wouldn't be any air, and then installed the thermostat. If you have a heater it needs to be turned on so it can fill with coolant too.

There should be the little "jiggle pin" on the thermostat that allows the air to get out, but mine was either a bit slow or didn't work all together..

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I used to find that if you put a jack under the Rad side of the mini and raised it up 2-3" then run and top up fluids, this can help any air bubbles trapped in the system flow to the Rad.
(bring car up to thermostat temp with the Rad cap off)
You'll see the water flowing in the Rad.
WARNING can sometimes burp hot water out the system, so no looking to close down the filler spout.
Then top up Rad, Drop the mini back down to the ground, and check levels again.

Just my 2 cents worth

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Maelgwn No Mini Yet