Hazen Arnold is right about the citric acid flush, but to really do the flush right you should first do a degreaser flush. Acid alone will not remove any oily deposits in the cooling system. The MB degreaser part no. is 001 986 21 71.
It cost about $11. You can also use TSP;available at hardware stores. Cheaper, and more readily available is just a plain non-foaming dish washing compound. It works just as good. As for the citric acid, you can get it at the super market. Look in the home canning section. Or get it from the MB dealer. Part no.000 989 10 25. Cost about $17.
The whole flushing procedure will take about an afternoon, but most of the time will be spent waiting for the chemicals to do their jobs.
Bill Lewallen Lexington,Ky. The Bluegrass State, where the grass is really green, and our coolant is Green.