Ken, an entire cooling refresh including block drain should take 5 (working) hours with the right tools. If you are losing coolant you have a leak- either you can't see it or its evaporating off the block. Look for white/ gray crust on metal around and under cooling components. My 330i had a leak on the radiator end tank that took a complete refresh to finally track it down. Save time, do a refresh first, it ends up saving you money and time.

The thermostat in the first picture is the engine thermostat, the second is the automatic transmission thermostat I believe?

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2006 e53 4.8is Imola / Cremebeige
2005 e53 3.0 Black / Truffle - AT, Sport, Premium, ZCW
2001 e39 M5 Black / Caramel - Extended Leather, Premium Audio