Shugster

Hi all having issues with my 2005 mini one. So I'm quite sure I've got fumes getting into my coolant. So I decided to try a gasket seal. Removed the thermostat and added where stated on the install instructions. So put everything back in place and when I turn the key the temp gauge goes to max car hasn't run in days. I replaced the coolant sensor incase this was at fault no change. I was out again today removed the thermostat completely in the hope to run for a bit without restricted coolant movement. After removing the battery box and all and replacing the car started with normal temp reading. However after running for 15mins to allow the sealer to do its thing I replaced the thermostat and all the rest its now back to high temp and wont start its turning over but no start battery is good starter is good 👌 I'm thinking loose cable ? Due to the freak start then back to not starting has anyone got any ideas here ? Thanks in advance