1st Gen R53 Cooper S Most liked posts in thread: Trying to diagnose a problem

We will all replace the expansion tank sooner or later, at ~$30 just go ahead and do it.

The thermostat housing O-ring is also known to fail. I think you can buy a kit with the O-ring and thermostat, might as well replace it too. Also, some have had leaks from cracks in the housing itself, as it is a composite material.

This is all pretty easy DIY stuff, just take your time.

Good luck with the fix.

 

Hopefully it is just the coolant tank but if not:

Look for puddled coolant or white residue (dried coolant) near the thermostat housing to tell if it is leaking. Going dry every other dry is probably a bigger leak than the theromstat housing.

Use a flashlight to look around the engine compartment for white residue indicating dried coolant and try to trace up to initial leaking spot.