I read somewhere that 1/32 in of scale in a radiator will reduce thermal transfer something like 50%. Its bad stuff.
I really think that this flushing bit is overrated, it is probably good practice periodically but if you have a radiator that is really gunked up, just a single flush is probably not going to get all the stuff out of the inside of those thin tubes. Now, realizing that the 603 is a hair trigger thing what with #14 heads on them and it has been said that this car has a cooling system that is marginal ~~~ I just recommend if your radiator is over say 10 years old it is quite justifiable to replace it w/ a new Behr then you don't worry on a hot day while driving along in slow traffic, uphill etc. That's proven to be the solution in my experience.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!