Did you flush the engine?
There is a small nipple on the passenger side of the engine block. Attach a hose to it and loosen the nipple.
Once you flush everything you need to look for a breather hole on the engine head. Depends on the engine. If you have a 300E it's a small bolt 12mm, next to the valve cover.
Take it off. Make sure to tighten engine block nipple and radiator drain.
Now you are ready to pour in the coolant thru the reservoir.
Pour in until the coolant starts coming out of the breather hole on the head. Put back bolt on breather hole and start engine. Observe reservoir and add coolant if the level drops slightly.
That's it, you've done a successful coolant flush.