To ensure the correct 50/50 coolant ratio, I always thoroughly flush the cooling system (including heater) with water which I then allow to drain. I then add the required amount of coolant concentrate (eg. for 9 litres total I add 4.5 litres of concentrate). I finally add water to bring it to the correct level. In this way you are sure to have the correct coolant ratio. It does not matter if the system was not completely empty when you started so long as what remains is clean water. For example, if you started with say 2 litres of water remaining in the system and added the necessary 4.5 litres of concentrate, you would then add another 2.5 litres of water to fill it and the correct ratio is achieved. If you use pre-mixed coolant, it is impossible to ensure the correct 50/50 ratio unless the system is absolutely empty before you fill it (which is highly unlikely).

As to bleeding the air from the M103 engine, if you want to be fussy, you can remove one of the blank plugs next to the temperature sensor on top of the cylinder head towards the front on the inlet manifold side. You can then fill it until coolant appears at this opening. This way you know the block and head water jackets have been filled. It does not guarantee that there won't be air trapped elsewhere though (eg. heater). As Gilly suggests, run it with the heater operating and regularly check and top up as required. This will ensure that all air has been bled from the whole system.

If after having done all this, coolant is still disappearing, then you have a problem such as suggested by Steve.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
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