You are probably referring to a statement I made in this thread: Need professional help! 190Ecooling System specifically, "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)."

There is not a bleed valve as such, simply several tapped holes into the water jacket in the top of the head. Some of the holes are used for sensors etc and one is usually fitted with a blank plug. It is easy to remove this plug (don't lose the washer) when filling with coolant. Being at the highest point of the water jacket on the engine ensures that no air is trapped in the water jacket. I generally fill mine with this plug removed but obviously others find it unecessary to remove the plug. As to why I do it this way, I seem to recall reading it as a recommendation in a workshop manual.

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